Keywords
COVID-19 modeling, Forecast, Africa, Meteorology, Stringency Policy, Human De-
velopment Index
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1 Introduction
The ongoing coronavirus disease 2019 (COVID-19) pandemic in Africa is threatening millions
of lives, a crisis compounded by the continentβs unique spectrum of disease and fragile health
care infrastructure (1). Essential to African countriesβ efforts to control the pandemic are effective
Methods
to track and predict new cases and their sources in real time. Time-critical interpretation
of daily case data is required to inform public health policy on mitigation strategies and resource
allocation. To address this need, we developed a data-driven disease surveillance framework to
track and predict country-level case incidence from internal and external sources. We chose a
spatiotemporal strategy to take advantage of and combine openly available data on coronavirus
epidemiology, social policy affecting human movement and public health, meteorological factors,
socioeconomic and demographic variables, seeking to inform rapid policy development.
The ο¬rst COVID-19 case on the continent was reported in Egypt on February 14, 2020. By
August 13, 2020, over 1 million new cases and over 20,000 deaths had been reported in all African
Union (AU) Member States according to the Africa Centres for Disease Control and Prevention
(CDC) (https://africacdc.org/covid-19/). Over 44 million cases and 190,000 deaths
in Africa are projected within the ο¬rst year of the pandemic (2). Although Africa has a younger age
distribution that could theoretically lead to fewer symptomatic or severe infections (3), modeling
predicts that the relatively low healthcare capacity in many parts of Africa, in combination with the
large, inter-generational households (4) could lead to infection fatality rates higher than those seen
in high income countries (1). In addition, the high prevalence of comorbidities such as HIV/AIDS
is predicted to lead to increased risk of severe COVID-19 in infected individuals (5). Moreover,
the co-existence of infectious diseases such as malaria (6), tuberculosis (7), dengue (8), Ebola (9),
and others (10) pose additional signiο¬cant medical and infrastructure challenges in controlling the
COVID-19 epidemic in Africa.
Meteorological variables have been linked to the transmission of and survival from seasonal
inο¬uenza (11β14), severe acute respiratory syndrome coronavirus (SARS-CoV) (15β17) and Mid-
dle East respiratory syndrome coronavirus (MERS-CoV) (18, 19). It is therefore unsurprising that
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there are many recent studies exploring the link between temperature, humidity, and COVID-19.
All studies to-date have focused on modeling, or on identifying a statistical link between meteoro-
logical variables against reported COVID-19 cases, without laboratory studies. A recent systematic
review reports agreement among published research, with cold and dry conditions contributing to
COVID-19 transmission (20). However, these results must be considered preliminary. Early stud-
ies of COVID-19 transmission focused on the emerging pandemic during the boreal spring (March
through May), when the majority of cases were found in China, the US and Europe. It is difο¬cult
therefore to extrapolate meteorological results to the very different climates found in the tropics,
for example from the recent outbreaks in India and Brazil. Many low and middle-income countries
(LMICs), as deο¬ned by the World Bank, are located in the tropics, where many potential con-
founding factors which could mimic a weather signal. These factors include median age, testing
and health capabilities, population density, access to sanitation and the number of new cases arriv-
ing in a country through global travel hubs (21). It is also difο¬cult to extract seasonality from a
single outbreak that began in the boreal spring, and considerable caution has been raised regarding
tropical case-load and confounding factors (22).
The human response to the pandemic can also drastically shape its timing and intensity. Where
data on social distancing is sparse, government testing and stringency policies (See Methods Sec-
tion 4.3) can be used as a common surrogate to compare countriesβ efforts to contain the spread of
the virus, bolster healthcare systems, enact rigorous testing policy, and provide economic support.
The Oxford Coronavirus Government Tracker (OxCGRT) standardizes these complex systems into
a set of policy metrics in each of these domains (23). More strict social policies identiο¬ed in the
OxCGRT have been associated with reductions in human mobility (24, 25). Across 161 countries,
some of these policies were signiο¬cantly associated with lower per capita mortality (26) includ-
ing: school closing, canceling public events, and restrictions on gatherings and international travel.
Likewise, others have found that strict policies are negatively associated with the growth of new
cases (27β29). The relationship between policy and observed changes in social distancing, case
numbers, and mortality is complicated by an unknown delay of effect. One estimate indicates a de-
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cline in growth of new cases within one week of enacting strict policy, and deceleration of growth
within 2 weeks (29). Although the implementation of containment policies can be and have been
used by many African nations (30), lockdown cannot be maintained in these countries without a
worsening of severe poverty and resultant loss of life (31, 32).
We have therefore developed a COVID-19 surveillance strategy that explores a growing spatio-
temporal database on coronavirus epidemiology, meteorology, and social policy interventions. To
model the spread of COVID-19 in Africa, we employ a data-driven endemic-epidemic model (33)
to 1) visualize the burden of cases including the proportion of cases arising from sources local
within-country and external between-country, 2) describe the factors which most correlate with
spread, and 3) enable short-term forecasting of new cases. This modeling framework has been
used previously to ο¬t space-time dynamics of COVID-19 in Italy (34), Germany (35) and the
United Kingdom (36) and to analyze other infectious diseases (37). The model is divided into
three main parts: two epidemic components that capture sources of infections coming from within
the country and from neighboring areas, and an endemic component that includes all contributions
to the reported number of cases that are not taken into account by the epidemic part. The epidemic
part of the model has an auto-regressive nature, this means that the past number of COVID-19 cases
reported both within a speciο¬c country and in the rest of the continent will be used to forecast the
present and future trend of COVID-19 cases. How much the past observations contribute to the the
future disease count depends on two parameters,Ξ» for the local transmission and Ο for the external
transmission and will be estimated from the data. In particular, the impact of cases reported in the
neighboring countries depends also on a set of weights that modulate the spatial connectivity of
the countries in the continent (see Method section). These two parameters are also functions of
social policy, testing availability, meteorological and demographic factors whose association with
transmission we aim to determine.
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2 Results
2.1 COVID-19 spread and response
As of August 13, 2020, the 55 AU Member States had reported over 1,000,000 cases and 20,000
deaths from COVID-19. The southern region had the most cases, reporting over 50 percent (over
560,000 cases and 11,000 deaths) of the total for the continent. North Africa carries the highest
regional case-fatality rate (4 per cent) but contributes 20 per cent of the continentβs cases, with
countries such as Egypt (102 cases per 100,000), Morocco (94 cases per 100,000) and Algeria (83
cases per 100,000) driving the overall numbers (Figure 1). As more countries conduct targeted
mass screening and testing, these ο¬gures continue to change. The spatial distribution of cases
per 100,000 displays no clear geographical pattern (Figure 1). South Africa, Djibouti, Equatorial
Guinea, Gabon and Egypt carry the largest burden of cases per capita, ranging from 100 to 500
per 100,000. The epidemiological curves for the African countries display varying shapes, mostly
driven by the frequency and intensity of testing. For example, the epidemiological curve of South
Africa is similar to those of the UK and the US (Supplementary Figure S1). An exception is
Tanzania, which stopped reporting new cases in late April (Supplementary Figure S1).
Time series for case incidence and temporally-varying model inputs are shown for selected
countries in the left panel of Figure 1. The full set of case incidence time series for all coun-
tries can be found in Supplementary Figure S1. A majority of the countries imposed containment
policies, including lockdowns and curfews, to prevent further COVID-19 transmission within their
borders in early March. These social policy interventions have remained in effect through August
for most countries (Supplementary Figure S2). Testing policies, which were restrictive at the be-
ginning of the pandemic due to inadequate testing infrastructure, have become more open as testing
is made widely available (Supplementary Figure S3). As expected, spatiotemporal distribution of
temperatures, rainfall and speciο¬c humidity are very heterogeneous across the continent (Figure
1, and Supplementary Figures S4, S5, & S6). Population weighted averages of these three mete-
orological variables were calculated for each country and day. This type of weighting prioritizes
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Cases per 100000Stringency IndexTestingTemperature (Co)Rain (mm)Humidity (gkg)
Mar 09 Mar 23 Apr 06 Apr 20 May 04 May 18 Jun 01 Jun 15 Jun 29 Jul 13 Jul 27 Aug 10
0
5
10
15
20
0
25
50
75
0
2
4
6
0
10
20
30
0
10
20
30
0
5
10
15
20
Day
A Egypt Senegal South Africa Uganda
0 500 1000 1500 2000 2500 km
N
0 β 5
5 β 10
10 β 18
18 β 24
24 β 35
35 β 56
56 β 69
69 β 97
97 β 159
159 β 960
Total cases reported per 100,000 up to 08β13β2020
B
Figure 1: Temporal distribution of reported cases, stringency index, testing policy and
weather factors. (A) Time series (February-August 2020) of (from top to bottom) daily reported
cases per 100,000, stringency index, testing policy, temperature, rainfall, and speciο¬c humidity for
representative countries Egypt, Senegal, South Africa and Uganda. These time-dependent covari-
ates were used as predictors (explanatory variables) in the best-ο¬tting model shown in Table 1.
The gray and orange shaded areas show the time window of data used to ο¬t the model and the data
held-out for model validation, respectively. (B) Country-speciο¬c distribution of cumulative cases
per 100,000 on August 13, 2020
the human-climate interaction over the land-climate interaction (Supplementary FiguresS7, S8, &
S9).
2.2 Optimal model
The model speciο¬cation reported in Equations 3, 4 and 5 representing the endemic, within- and
between-country component of the model, respectively, is the result of a model selection proce-
dure based on the Akaike Information Criterion (AIC) (38). A summary of model comparison and
selection process is presented in supplementary Table S1. We began with an intercept only model
(Model 1) with a population offset in the the endemic component of the model and countryβs mea-
sure of connectivity based on a power law. More complicated versions of the epidemic component
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were evaluated by sequentially adding weather, demographic, stringency index and testing policy
in the formula for the within (Ξ») and between-country (Ο) components of the model. We also tested
whether multiple lags for cases and covariates better described the observed patterns: considering
lags d = 1, . . . ,D where D = 14 days. Exploration of the higher order model with Poisson and geo-
metric lag weights, revealed that relative to the ο¬rst-order model, the largest improvement in AIC
(βAIC = -1560) was achieved with a model with D = 7 (Supplementary Figure S10). Therefore,
the model with lag 7 days for cases, testing policy, stringency index including human development
index (HDI), landlocked status and population in the between-country component, and meteo-
rology factors, HDI, landlocked status, stringency index and testing policy in the within-country
component, yielded the lowest AIC (from 48824.32 in model 1 to 48437.99 in model 5 without
random effects, Supplementary Table S1).
Table 1: Maximum likelihood estimates and corresponding 95% conο¬dence intervals for a
model with 7-day lag. For climatic variables, a 1 standard deviation increase in climatic variables
Results
in the shown relative risk. For stringency index, a 10% increase in stringency is associated
with the increased relative risk shown. HDI and testing policy are on ordinal scale 0 to 3 (HDI)
and 0 to 4 (testing policy). Bolded estimates are statistically signiο¬cant. The spatial weight decay,
Ο, reο¬ects the strength of inter-country connectivity, and overdispersion parameter, Ο.
.
Final
Model
P
arameter Relative Risk 95% CI p-value
Endemic
Intercept
11.071 (7.150, 17.142) -
W
ithin-country
Intercept 0.958 (0.550, 1.669) -
log(population) 1.036 (0.956, 1.123) 0.391
HDI 0.957 (0.819, 1.118) 0.580
Landlocked 0.674 (0.531, 0.857) 0.001
Stringencytβ7 1.872 (1.170, 3.000) 0.008
Testingtβ7 0.817 (0.729, 0.918) 0.001
Raintβ7 1.045 (0.981, 1.112) 0.175
Temperaturetβ7 1.106 (1.014, 1.206) 0.023
Humiditytβ7 0.856 (0.780, 0.940) 0.001
Between-country
Intercept
0.045 (0.004, 0.481) -
log(population) 1.428 (0.923, 2.210) 0.110
HDI 1.239 (0.584, 2.628) 0.576
Landlocked 0.844 (0.306, 2.323) 0.742
Stringencytβ7 1.630 (0.560, 4.749) 0.380
Testingtβ7 2.322 (1.676, 3.218) < 0.0001
Ο 2.186 (1.532, 2.839) -
Ο 1.703 (1.631, 1.775) -
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Due to the high spatiotemporal heterogeneity of reported cases across Africa and to better
capture country speciο¬c transmission dynamics and incidence levels not explained by observed
covariates, we allowed the intercept (mean levels of Ξ» and Ο) in the local (4) and neighbor-driven
(5) sources of infections to vary for each country. The relative risks for each explanatory variable
included in this ο¬nal model and the associated 95% conο¬dence intervals are reported in Table 1.
Landlocked status, stringency index, and testing policy were signiο¬cant contributing factors on the
local transmission of cases (Figure 2).
HDI
Humidity
LL
Population
Rain
Sindex
Temperature
Testing
Time constant
March to August
β0.6
β0.3
0.0
0.3
0.6
β0.6
β0.3
0.0
0.3
0.6
β0.6
β0.3
0.0
0.3
0.6
β0.6
β0.3
0.0
0.3
0.6
Contribution to log(Ξ»it)
A Time varying
EgyptSenegalSouth AfricaUganda
Apr May Jun Jul Aug
β0.6
β0.3
0.0
0.3
0.6
β0.6
β0.3
0.0
0.3
0.6
β0.6
β0.3
0.0
0.3
0.6
β0.6
β0.3
0.0
0.3
0.6
Time constant
March to August
β1
0
1
2
β1
0
1
2
β1
0
1
2
β1
0
1
2
Contribution to log(Οit)
B Time varying
EgyptSenegalSouth AfricaUganda
Apr May Jun Jul Aug
β1
0
1
2
β1
0
1
2
β1
0
1
2
β1
0
1
2
Figure 2: Magnitude and direction of the explanatory factors to the within and between coun-
try transmission. Time constant and time varying covariates from 4 selected countries showing
variations in the strength and the direction of the contributions to the local within-country (A) and
external between-country transmission of cases (B).
In addition, higher lagged mean temperature was a positive contributing factor but higher spe-
ciο¬c humidity had a negative effect on the transmission of cases. For example, a 1 standard de-
viation increase in the lagged mean temperature results in 11% higher contribution on the within-
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country transmission [p = 0.023, RR 1.11, 95% CI: 1.01 to 1.21]. However, a 1 standard deviation
increase in the 7-day lag mean speciο¬c humidity resulted in a 14% lower contribution on the local
transmission of cases [p = 0.001, RR 0.86, 95% CI: 0.78 to 0.94]. More accessible testing remained
the only signiο¬cant contributing factor explaining the numbers of cases from the neighboring coun-
tries. With each level increase in the openness of the testing policy from 0 to 4, the contribution
to the transmission of cases from neighboring countries was higher by 2-fold (p<0.0001). The
overdispersion parameter decreased from the ο¬xed effect model (1.95, 95% CI: 1.87 to 2.03) to
random effects model (1.70 95% CI: 1.63 to 1.78) as a sign that the random effects absorbed part
of the unexplained variability between countries.
Figure 3 shows countries speciο¬c (random) effects for the local and neighbor components of
the model. A value higher (lower) than 1 means that a country has an average transmission rate
that is higher (lower) than the rest of the continent. This may be interpreted as a country-speciο¬c
propensity to generate more or fewer cases given the past number of reported infected individuals.
South Africa and Djibouti are the only African countries with an effect signiο¬cantly higher than 1.
On the other hand, the Republic of Congo is the only country with a signiο¬cantly lower than the
continental-level mean caseload. With respect to the between-country contributions to transmission
of cases, Benin, Cameroon, Central African Republic, Ethiopia, Gabon, Ghana, Guinea, Malawi,
Republic of Congo and Senegal had a signiο¬cantly higher than continental-level mean number of
cases. Angola, Chad, Lesotho, Namibia and Tanzania had lower cases means compared to the
continental-level mean. Panels A and B of Figure 3 also show some interesting spatial clustering
of these effects. The estimated variation of these country-speciο¬c effects in the within-country
component of the model is small ( ΛΟ2
Ξ» = 0.07) compared with their variation in the neighborhood
component (ΛΟ2
Ο = 2.3). Although the between-area variability of transmission resulting from cases
reported outside of the country was larger, the between-country intercept (Table 1) is very small
and so the neighborhood component is in general a small contributor to the ο¬t.
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0 500 1000 1500 2000 2500 km
N
Mean relative risk
0.5 β 0.7
0.7 β 0.8
0.8 β 0.9
0.9 β 1.0
1.0 β 1.5
1.5 β 2.0
2.0 β 2.5
Not analyzed
Random Effects β Within country
0 500 1000 1500 2000 2500 km
N
Mean relative risk
0.01 β 0.29
0.29 β 0.50
0.50 β 0.84
0.84 β 1.33
1.33 β 2.07
2.07 β 4.25
4.25 β 7.68
Not analyzed
Random Effects β Between country
A B
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
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β
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β
Within country Between country
0.5 1.0 1.5 2.0 0 10 20
Zimbabwe
Zambia
Uganda
Tunisia
Togo
The Gambia
Tanzania
Swaziland
Sudan
South Sudan
South Africa
Somalia
Sierra Leone
Senegal
Rwanda
Republic of Congo
Nigeria
Niger
Namibia
Mozambique
Morocco
Mauritania
Mali
Malawi
Libya
Liberia
Lesotho
Kenya
Guinea
Ghana
Gabon
Ethiopia
Eritrea
Egypt
Djibouti
DRC
Cote d'Ivoire
Chad
Central African Republic
Cameroon
Burundi
Burkina Faso
Botswana
Benin
Angola
Algeria
Relative risk of random effects
Country
C D
Figure 3: Random effects for the local and neighbor components of the model. Country-
speciο¬c relative risks (RR) and their 95% CI for (A and C) within-country and (B and D) between-
country model contributions. The dashed blue line in the forest plots represent continent-level
average. RR greater than 1 indicates higher propensity for transmission as compared to the rest of
the continent.
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2.3 Contributions of within- and between-country transmission
We distinguish between endemic, within-country, and between-country contributions to the mean
number of cases. Fitted values for all components according to model formulations in equation 1
are shown in Figure 4, with a complete listing in Supplementary Figure S11. The number of cases
attributed to within- and between-country transmission of cases during the entire study period
varied greatly. Across countries, the contribution from the endemic component was found to be
minimal. Of the 46 countries analyzed, 16 of them are landlocked, and 13 (81%) of these had
a substantial contribution of cases from their neighboring countries: Botswana, Burkina Faso,
Burundi, Central African Republic, Ethiopia, Lesotho, Malawi, Rwanda, South Sudan, Swaziland,
Uganda, Zambia, and Zimbabwe.
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Apr May Jun Jul Aug
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B
Figure
4: Contributions of new cases from endemic, within-country, and between-country
model components. (A) Black dots and connecting lines are the observations and shaded colors
are the model predictions from the three contributing components of the model. (B) The relative
contribution of new cases from within-country sources, by country.
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ββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ
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South Africa Uganda
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Apr 06 Apr 20 May 04 May 18 Jun 01 Jun 15 Jun 29 Jul 13 Jul 27 Aug 10 Apr 06 Apr 20 May 04 May 18 Jun 01 Jun 15 Jun 29 Jul 13 Jul 27 Aug 10
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Reported cases β β Predicted mean retrospective forecast 50% β 95% CI retrospective forecast
Figure 5: Retrospective and forecast model ο¬t for selected countries. Retrospective model ο¬t
(blue shade) and forecast (orange shade). The 50% and 95% conο¬dence intervals are represented
by dark and light colors respectively. Filled circles in the blue shade are observed cases and the
solid blue line the predicted retrospective mean. Open circles in the orange shaded area represent
cases from model forecast and the solid orange line the predicted forecast case mean. Majority of
individual country case count data are captured well within model prediction intervals. For the full
list of countries see Figure S12.
2.4 Short-term forecast
We keep the last 7 days of data out of the ο¬tting procedure in order to use them as a forecast
validation data set. We produce one-week ahead predictions and compare them with the reported
data to check the quality of model forecast. The results show that the majority of individual country
case count data are captured well within model prediction intervals (Figure 5 and Figure S12).
Across countries, the model predictive performance was assessed with a calibration test based on
proper scoring rules as described in (39). A map of p-values for the calibration test is shown in
Figure S13. Overall model predictions are well calibrated and a misalignment between forecast
and observations was only detected for few countries (p < 0.05, Burundi, Cameroon, Somalia,
Botswana).
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3 Discussion
We present a model that can improve the ability of African countries to interpret the complex data
available to them during the COVID-19 pandemic. This approach balances the simplicity and
consequent robustness of an empirical model against the more complex, potentially more realistic
but also more strongly assumption-driven kind of compartmental mechanistic models (40). A
key feature of our approach is the ability to distinguish between case incidence arising from the
local within- or neighbor-driven transmission of infection. Distinguishing within- and between-
country transmission of cases allows us to identify potential strategies for social or health policy
intervention. The model further enables reproducing the history of the epidemic in relationship to
past policy, and producing short-term predictions of the dynamic evolution of the epidemic.
We ο¬nd that a countryβs testing capacity, social policy, landlocked status, temperature and hu-
midity are important contributing factors explaining the within and between-country transmission
of cases. The availability of more testing to a wider swath of the populace is a potent contributor
for reduced case transmission within country, while having the opposite effect on case transmis-
sion from neighboring countries. Testing policy, another surrogate for healthcare capability and
preparedness to handle the pandemic, demonstrates this unique opposing effect on the two model
components. Countries in northern and southern Africa that have relatively high HDI demon-
strated comparatively higher numbers of cases per population. On the other hand, even in the face
of border closures, landlocked countries depend on open borders for trade. For such countries,
strict border closure measures are difο¬cult to impose, enabling a constant inο¬ux of cases from the
neighboring countries.
The observed association of temperature and speciο¬c humidity with the case numbers, although
small, points to the possible biological and behavioral responses to weather patterns, which in
turn drive the dynamics of SARS-CoV-2 infection. Temperature and humidity are known fac-
tors in SARS-CoV , MERS-CoV , and inο¬uenza virus survival (41β43). Lower humidity has been
consistently associated with higher cases. Besides potentially prolonging half-life and viability
of the virus, other potential mechanisms associated with low humidity include stabilization of
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the aerosol droplet, enhanced propagation in nasal mucosa, and impaired localized innate immu-
nity (44). Whether the observed association is driven by the change in social behavioral patterns or
the effect on the survival of SARS-CoV-2 remains to be explored (45). It is also possible that the
observed contribution of meteorological factors to case transmission might be an artifact of spatial
averaging and assigning one meteorological value to an entire country. It will be important to ex-
plore such associations in more detail before any policy relevant conclusions can be drawn. Thus,
at present, policy makers must focus on social-behavioral interventions such as reducing physical
contact within communities, while COVID-19 risk predictions based on climate information alone
should be interpreted with caution (46).
Our infection surveillance tool adds to the public health capacity already in place on the conti-
nent to better understand transmission patterns between and within African countries. Containment
and mitigation strategies to limit the spread of the virus, including restrictions on movement, public
gatherings, and schools, were implemented very early in the pandemic. In a resource-limited set-
ting such as Africa, containment and mitigation strategies remain the most robust defense against
high infection rates and mortality. However, it is anticipated that physical-distancing measures en-
forced to limit transmission will also restrict access to essential non-COVID-19 healthcare services,
such as disruptions in the existing programs for tuberculosis, HIV/AIDS, malaria, and vaccine-
preventable diseases, causing long-lasting collateral damage on the continent (32). Although, be-
tween 29 million to 44 million individuals (2) in Africa are projected to become infected in the ο¬rst
year of the pandemic if containment measures fail, these numbers may be underestimated since the
proportion of asymptomatic infections is not well established. Since detection is biased towards
clinically severe disease, the attack rate of the infection is probably substantially higher than what
is reported. At the beginning of the pandemic, it was estimated that up to 86% of all infections
were undocumented and were the source of 79% of the documented cases (47). Such observa-
tions explain the rapid geographic spread of the infections and challenging efforts at containment.
The number of asymptomatic cases is best determined by population-based seroepidemiology data.
However, due to the fragile healthcare systems of Africa countries, this type of disease surveillance
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remains limited. On the other hand, it is also plausible that the lower incidence rate of the virus in
Africa is because of the investment in preparedness and response efforts toward various outbreaks
on the continent (such as Ebola virus disease, Lassa fever, polio, measles, tuberculosis, and human
immunodeο¬ciency virus) (32). This technical know-how has been swiftly adapted to COVID-19.
An additional strength of our modeling strategy is the ability to incorporate the disease-speciο¬c
serial interval between sequential infections in the autoregressive model. We attempted to mimic
the longer (greater than 1 day) serial interval (48, 49), infectiousness (48, 50), and latency (48)
of COVID-19 transmission, by extending the observational interval of the infectious process to
several days. The Poisson autoregressive weighting method used in our modeling strategy also
captures an initial increase in infectiousness and may thus be more appropriate for longer serial
intervals or daily data. In their recent work, Bracher and Held show that moving beyond one day
lags to higher order time lags improves predictive performance of these endemic-epidemic models
(51). For our optimization scheme we tested lags up to 14 days, and found that a lag at 7 days
provided the best model ο¬t. Short-term predictions enable the monitoring of case incidence trends
but are limited by high levels of uncertainty. This is the result of the non-negligible overdispersion
detected in the data and due to the several sources of unmodeled spatial and temporal heterogeneity
across the continent.
3.1 Limitations
A number of assumptions were made in our analysis. Contact patterns across countries were
assumed to be constant over time. Current patterns, such as inter-country air travel and border
crossing by land might not follow the weights we have assumed, and actual population contact
probabilities might not be constant over time. Nevertheless, the use of higher order neighborhood
(beyond sharing a border) contact patterns led to improved model ο¬t compared to an assumption
of ο¬rst order neighbors (bordering countries) only. Other assumptions related to the testing and
stringency policies. These are coarse approximations of governmental response to the surveillance
and control disease transmission. The absence of quantiο¬able tests per capita is a limitation of this
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approach.
We are aware that SARS-COV-2 is often carried by apparently healthy individuals who might
unknowingly transmit the pathogen. In the present analysis, we could not disentangle asymp-
tomatic and symptomatic disease. Under-reporting can introduce artifacts in the autocorrelation
structure and may confound the estimation of lag weights of the underlying serial interval distri-
bution (52). Additionally, we assumed that model coefο¬cients were constant over time. This is
not the optimal ο¬t for SARS-COV-2 transmission if there is seasonal variation, and implies that the
interaction with weather is the same in summer and winter. We avoided adding sinusoidal (smooth,
repetitive oscillation) functions in the endemic component because of the short interval of the cur-
rent pandemic not spanning a whole year. Lastly, we excluded Equatorial Guinea, Guinea-Bissau
and Western Sahara due to missing stringency index and the six island nations (Madagascar, Co-
moros, Mauritius, Seychelles, Cape Verde, SΛao TomΒ΄e and PrΒ΄Δ±ncipe) due to the lack of connectivity
to mainland Africa which prevented model convergence. This modeling strategy is limited by the
quality of the data and the lack of non-linear dynamics in the model.
3.2 Conclusions
We present a pan-African COVID-19 surveillance tool to track and perform short-term forecast
COVID-19 cases and to quantify between- and within-country sources. Our analyses give insight
into the sociodemographic, geodemographic, testing, mitigation/containment and meteorological
factors that inο¬uence the spread of the SARS-CoV-2 infection. Although our strategy can be used
for short-term predictions of cases, their accuracy heavily depends on the quality of testing and
reporting data. In settings with fragile health systems, coupled with the vulnerability of lower
HDI economies, the capacity to effectively track the pandemic is especially challenging. Such
challenges point to the potential advantages in regional efforts to coordinate resources to test and
report cases. Seeking equitable behavioral and social interventions, balanced with coordinated
country-speciο¬c strategies in infection suppression, should be a continental priority to control the
COVID-19 pandemic in Africa.
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4 Materials and Methods
4.1 Overview
Our analyses included 46 countries of mainland Africa. We do not provide estimates for Equatorial
Guinea, Guinea-Bissau and Western Sahara due to the missing data on stringency index, or the six
island nations (Madagascar, Comoros, Mauritius, Seychelles, Cape Verde, SΛao TomΒ΄e and PrΒ΄Δ±ncipe)
due to the lack of spatial connectivity. Modeling the spread of COVID-19 over the African con-
tinent poses challenges, given the extensive cultural, political and environmental heterogeneity
between countries. Indeed, this heterogeneity results in substantial variability of reported case
counts across countries. It is this variability in case counts that motivates our choice of a relatively
simple data-driven autoregressive modeling approach. Such a modeling approach focuses on the
interaction of cases reported in time and space without hidden variables to be estimated.
4.2 Meteorology/Weather Factors
The seasonality of inο¬uenza transmission has been associated with cycles of temperature, rainfall,
and speciο¬c humidity, although in different regions of the world transmission may peak during the
βcold-dryβ season (temperate climates) or during βhumid-rainyβ season (tropical climates) (53).
We estimated the inο¬uence of meteorological factors on the transmission dynamics of COVID-
19 in Africa. Real-time, daily in-situ synoptic weather observations are sparse across much of
Africa. Therefore daily, 10km spatial resolution mean temperature, rainfall and speciο¬c humid-
ity data were obtained from UK Met Ofο¬ce numerical weather prediction model output (54, 55).
These data are extracted from the early time steps of the model following data assimilation, to more
closely approximate an observational dataset. This approach also has the advantage that future
studies have access to the same coherent dataset at a global scale for applications outside of conti-
nental Africa. The weather product that generates these data closely approximates an observational
dataset at locations that have dense observation coverage, whereas in observation-sparse areas the
dataset relies more heavily upon the numerical weather prediction model (a physics-based, rather
than statistical model). The dataset contained no missing data.
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A population density-weighted spatial average (supplementary Figures: S7, S8, S9) was then
applied for each day and country using the R package exactextractr (56). Population density
was obtained from the Gridded Population of the World version 4 (GPWv4) from the Centre for
International Earth Science Information Network (CIESIN) (57). Weighting climate variables by
population gives a closer approximation to the weather conditions faced by humans living in that
country compared to an unweighted average over total land area. For example, the country of
Algeria, in which much of the population resides along the coast, demonstrates a cooler, wetter,
and more humid climate when weighting by population (Supplementary Figures S7E, S8E, &
S9E).
4.3 Stringency index and testing policy
To include an aggregate measure of countriesβ social policies, the stringency index sourced from
the OxCGRT data set was used. This composite measure reο¬ects government policies related
to school and workplace closures, restrictions on public gatherings, events, public transportation,
Limitations
of local and global travel, stay-at-home orders, and public education campaigns (23).
A full description of these variables is provided in Supplementary Table S2. The stringency index
is calculated from these categorical variables using a weighted average, with a range of 0 to 100
indicating weak to strict stringency measures, respectively. A time-dependent metric of testing
policy was also extracted from this dataset. Ranging from 0 to 4 this categorical metric increases
with more open and comprehensive testing policy.
4.4 Human Development Index, Demography, United Nations Geographic
Regions and Coastline Access
In our modeling strategy we incorporate key socioeconomic and sociodemographic epidemiolog-
ical data including human development index (HDI), population, United Nations geographic re-
gions and coastline access (supplementary Figure S14). HDI represents the national data on key
aspects of development, namely education, economy and health (58). The HDI is the geomet-
ric mean of normalized indices for each of the three dimensions. The education dimension is
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measured by average years of schooling for adults aged 25 years and more and expected years
of schooling for children of school entering age. The economy dimension is measured by gross
national income per capita, and the health dimension is assessed by life expectancy at birth. In
Africa, majority of the countries fall in the low HDI category (supplemental Figure S14). The
northern part of Africa and South Africa has a considerably higher HDI compared to the rest of
the continent. Country-speciο¬c median age was correlated with HDI Pearsonβs correlation coef-
ο¬cient (R=0.71, p<0.0001), Supplemental Figure S15, therefore we excluded this covariate from
the model. We include in the model the 2020 population obtained from the Population Division of
the Department of Economic and Social Affairs of the United Nations Secretariat (59). The cat-
egorization of sub-Saharan and Northern Africa was based on the United Nations geoscheme for
Africa (60). This regional factor captures the human genetics (61), environment and climate (62),
and sociocultural and sociodemographic variations of the African population ( 63). Finally, lack
of direct access to the coastline may inο¬uence the ο¬ow of infections from neighboring countries
as border trade remains an essential operation. For example, Uganda introduced border closures
and tighter preventive measures on truck driversβ movements during the epidemic; despite this, a
substantial number of new infections have been imported from truck drivers crossing the border for
trade (64). Such cross-border commerce remains a crucial part of the supply chain for landlocked
African countries such as Uganda and Rwanda.
4.5 Model formulation
We chose a class of multivariate time series models for case count data introduced by Held et
al (65), and further extended by Bracher and Held (51) with the addition of higher-order distributed
lags.
New COVID-19 cases Yit from country i at time t are assumed to be conditionally independent
given past observations Yi,tβd, i = 1, . . . , N, d = 1, . . . , D, and distributed according to a negative
binomial distribution with mean Β΅it and overdispersion parameter Ο as
[Yit| Ytβ1, . . . , YtβD]βΌ NegBin(Β΅it, Ο).
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The conditional variance isΒ΅it+ΟΒ΅2
it, which demonstrates the role of the overdispersion parameter
to capture variability greater than the mean. The conditional mean Β΅it is decomposed into three
additive components,
Β΅it = Ο΅i + Ξ»it
Dβ
d=1
udYi,tβd + Οit
Dβ
d=1
β
jΜΈ=i
udwjiYj,tβd, (1)
where Ο΅i, Ξ»it, and Οit represent three contributions to case incidence. The ο¬rst term, Ο΅it, is the so-
called endemic component and captures infections arising from sources other than past observed
cases (e.g. contributions from areas that are not included in the neighbor set). The two other
terms in (1), Ξ»it and Οit, constitute the epidemic part of the model and modulate how infective
individuals reported in the past d days both locally and from neighboring countries will contribute
to the average future number of reported cases. The strength of connection between countries is
described by spatial weights wji. This inter-country transmission susceptibility is deο¬ned using a
power-law formulation proposed by Meyer and Held (66),
wji = oβΟ
ji , (2)
where oji is the path distance between countries j and i (with oii = 0, oji = 1 for direct neighbors
i and j and so on) and Ο is a decay parameter to be estimated from the data. The spatial weights
are normalized such that β
k wjk = 1 for all rows j of the weight matrix (Supplementary Figure
S16).
The normalized autoregressive weights ud are shared between the local and global epidemic
components, and represent the probability for a serial interval of up toD days - which is the average
time in days between symptom onset in an infectious individual (or primary case) and symptoms
appearing in a newly infected individual (or secondary case) when both are in close contact (67).
The parameters Ο΅it, Ξ»it, and Οit are constrained to be non-negative and modeled as the natural
log-transformed linear combination of different country-speciο¬c covariates. The endemic compo-
nent,
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log(Ο΅it) = Ξ±(Ο΅) + log(Ni), (3)
is decomposed as a constant Ξ±(Ο΅) speciο¬c to the baseline endemic and a term proportional to the
country-level population Ni. In the epidemic part of the model, we expect new cases to also be
driven by country-speciο¬c factors: population (Ni), Human Development Index classiο¬cations of
low, medium, or high (HDI i ={0, 1, 2}) and land-locked (LLi) status for each country are as-
sumed constant over the time scale of analysis. Other forces driving new cases vary over time,
either as a response to policy changes or natural ο¬uctuation in environmental or societal patterns.
Time-dependent covariates include mean daily temperature (Ti,tβΟ), rainfall (Ri,tβΟ), speciο¬c hu-
midity (Hi,tβΟ), testing policy (Xi,tβΟ) and government stringency index (Si,tβΟ), lagged at Ο days.
The full set of explanatory variables that contribute to the model from both internal and external
epidemic components are formalized in (4) and (5) as
log(Ξ»it) = Ξ±(Ξ»)
i + Ξ²(Ξ») log(Ni) + Ξ³(Ξ»)HDIi + Ξ΄(Ξ»)LLi + Ο(Ξ»)Si,tβΟ + Ο(Ξ»)Xi,tβΟ + (4)
ΞΈ(Ξ»)Ti,tβΟ + Ο(Ξ»)Ri,tβΟ + Ξ½(Ξ»)Hi,tβΟ ,
and
log(Οit) = Ξ±(Ο)
i + Ξ²(Ο) log(Ni) + Ξ³(Ο)HDIi + Ξ΄(Ο)LLiΟ(Ο)Si,tβΟ + Ο(Ο)Xi,tβΟ , (5)
where Ξ±(Ξ»)
i βΌ N(Ξ±(Ξ»)
0 , Ο2
Ξ») and Ξ±(Ο)
i βΌ N(Ξ±(Ο)
0 , Ο2
Ο) are a set of independent country-level random
effects. This modeling framework is implemented in the R packagesurveillance (68). A complete
table of data sources for model input is found in Supplementary Table S3.
Model ο¬tting
We selected models based on Akaikeβs Information Criteria (69) (AIC) if random-effects were
not present (Supplementary Table S1). To compare models that included random effects we used
proper scoring rules for count data (70). Scoring rules are functions S(P, y) that evaluate the ac-
curacy of a predictive distribution P against an outcome y that was observed. We chose the model
22
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with the lowest AIC or with the lowest logarithmic score computed as minus the logarithm of the
predictive distribution evaluated at the observed count. We began with the ο¬rst-order autoregres-
sive modeling (D = 1 in (1)) of daily COVID-19 incidence using intercept only model population
offset and country connectivity. In a mechanistic interpretation of such a ο¬rst-order model, the
time between the appearance of symptoms in successive generations is assumed to be ο¬xed to the
observation interval at which the data are collected, here as one day (52).
After the estimation and illustration of this basic model, we expand the model by sequen-
tially adding the following additional covariates: country-speciο¬c HDI, population in the both the
within-country and neighboring countries, meteorology factors, stringency index, testing policy,
landlocked status and random effects to more fully account for unobserved heterogeneity of the
cases. Social policies and meteorological data were included in the model, testing for ο¬t at differ-
ent lags (for example, Ti,tβΟ , Οβ 0, 7, 14 days).
Model predictions
As in previous work by Held and Meyer, we use plug-in forecasts: forecast from the ο¬tted model
without carrying forward the uncertainty in the parameter estimates (71). We assess both the model
ο¬t and one-week-ahead forecast of the higher order autoregressive model with the logarithmic
score. The smaller the score, the better the predictive quality (72,73). Mean scores were generated
for each countryβs forecast, by averaging the log-score obtain for each day of the validation week.
23
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The copyright holder for thisthis version posted November 16, 2020. ; https://doi.org/10.1101/2020.11.13.20231241doi: medRxiv preprint
5 Acknowledgments
We thank the Ugandan Ministry of Health, and the National Planning Authority for providing
Uganda testing data.
6 Funding
This work was supported by a U.S. National Institutes of Health (NIH) Directorβs Transformative
Award 1R01AI145057 (SJS). MMN was supported in part by the Covid Virus Seed Fund from
the Institute for Computational and Data Sciences and the Huck Institute at the Pennsylvania State
University.
7 Author contributions:
Conception and design: PS, CF, AG, MMN and SJS. Data acquisition: PS, CF, and AG. Statis-
tical analysis and interpretation of the data: PS, CF, AG, HG, SJG, MMN, SJS. Drafting of the
manuscript: PS, CF, AG, SJG, AJW, MMN, SJS. Critical revision for important intellectual con-
tent, discussion, review and ο¬nal approval of the manuscript: all authors. Responsible for the
overall content as the guarantors: PS, CF, AG and SJS.
8 Competing interests
All authors declare no competing interests.
9 Data and materials availability
All code and data are available in the supplementary materials and posted online at https://
github.com/Schiff-Lab/COVID19-HHH4-Africa.
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10 Supplementary Materials
β’ Code: Github code repository
β’ Figure S1 - Time series of reported cases for all countries of Africa
β’ Figure S2 - Time series of stringency index for all countries of Africa
β’ Figure S3 - Time series of testing policy for all countries of Africa
β’ Figure S4 - Time series of population-weighted mean temperature for all countries of Africa
β’ Figure S5 - Time series of population-weighted mean rain for all countries of Africa
β’ Figure S6 - Time series of population-weighted mean speciο¬c humidity for all countries of
Africa
β’ Figure S7 - Population weighting of mean daily temperatures
β’ Figure S8 - Population weighting of mean daily rainfall
β’ Figure S9 - Population weighting of mean daily humidity
β’ Figure S10 - Distributed lags
β’ Figure S11 - Model contributions by country
β’ Figure S12 - Week-ahead predictions by country
β’ Figure S13 - Calibration test results by country
β’ Figure S14 - Time-constant covariates utilized in the model
β’ Figure S15 - Correlation of human development index of a country with median age
β’ Figure S16 - Spatial connectivity of countries
β’ Table S1 - Model selection for AR7
30
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β’ Table S2 - Codebook for the factors included in the OxCGRT stringency index
β’ Table S3 - Sources for data included in the model
31
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The copyright holder for thisthis version posted November 16, 2020. ; https://doi.org/10.1101/2020.11.13.20231241doi: medRxiv preprint
Togo Tunisia Uganda Zambia Zimbabwe
Somalia South Africa South Sudan Sudan Swaziland Tanzania The Gambia
Namibia Niger Nigeria Republic of Congo Rwanda Senegal Sierra Leone
Libya Madagascar Malawi Mali Mauritania Morocco Mozambique
Ethiopia Gabon Ghana Guinea Kenya Lesotho Liberia
Central African Republic Chad Cote d'Ivoire Democratic Republic of the Congo Djibouti Egypt Eritrea
Algeria Angola Benin Botswana Burkina Faso Burundi Cameroon
Mar Apr May Jun Jul Aug Mar Apr May Jun Jul Aug Mar Apr May Jun Jul Aug Mar Apr May Jun Jul Aug Mar Apr May Jun Jul Aug
Mar Apr May Jun Jul Aug Mar Apr May Jun Jul Aug
0.0
2.5
5.0
7.5
0.00
0.25
0.50
0.75
0.00
0.25
0.50
0.75
0.0
0.1
0.2
0.3
0.4
0.0
0.3
0.6
0.9
0
2
4
6
0.0
0.2
0.4
0.6
0.0
0.5
1.0
1.5
0
1
2
3
4
0
1
2
3
4
0.0
0.4
0.8
1.2
0.0
0.1
0.2
0.3
0.00
0.05
0.10
0.15
0.20
0
10
20
0.0
0.5
1.0
1.5
0
1
2
3
4
5
0.0
0.2
0.4
0.6
0.8
0
5
10
0
1
2
3
0
3
6
9
0.0
0.1
0.2
0.3
0.4
0.0
0.5
1.0
1.5
2.0
0.0
0.1
0.2
0.3
0.4
0
2
4
6
0.00
0.25
0.50
0.75
0
1
2
3
4
5
0.0
0.3
0.6
0.9
1.2
0.0
0.5
1.0
1.5
0
1
2
3
4
5
0.0
0.5
1.0
0.0
0.1
0.2
0.3
0.4
0
1
2
3
0.00
0.05
0.10
0.15
0.0
0.1
0.2
0.3
0.0
0.1
0.2
0.3
0.4
0.5
0
10
20
0.0
0.5
1.0
1.5
2.0
0.0
0.1
0.2
0
5
10
15
20
0.0
0.2
0.4
0.6
0.0
0.5
1.0
1.5
0
1
2
3
4
0.0
0.2
0.4
0.6
0.8
0
2
4
6
0
2
4
6
0.0
0.2
0.4
0.6
0.0
0.1
0.2
0.3
0.4 Daily cases per 100,000
Figure S1: Time series of reported cases for all countries of Africa. Cases per 100,000 individ-
uals between March 1 and August 13 are shown.
Togo Tunisia Uganda Zambia Zimbabwe
Somalia South Africa South Sudan Sudan Swaziland Tanzania The Gambia
Namibia Niger Nigeria Republic of Congo Rwanda Senegal Sierra Leone
Libya Madagascar Malawi Mali Mauritania Morocco Mozambique
Ethiopia Gabon Ghana Guinea Kenya Lesotho Liberia
Central African Republic Chad Cote d'Ivoire Democratic Republic of the Congo Djibouti Egypt Eritrea
Algeria Angola Benin Botswana Burkina Faso Burundi Cameroon
Mar Apr May Jun Jul AugMar Apr May Jun Jul AugMar Apr May Jun Jul AugMar Apr May Jun Jul AugMar Apr May Jun Jul Aug
Mar Apr May Jun Jul AugMar Apr May Jun Jul Aug
0
25
50
75
100
0
25
50
75
100
0
25
50
75
100
0
25
50
75
100
0
25
50
75
100
0
25
50
75
100
0
25
50
75
100 Stringency index
Figure S2: Time series of stringency index for all countries of Africa. This stringency index
ranges from 0 (no government stringency policies) to 100 (very strict stringency policies).
32
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The copyright holder for thisthis version posted November 16, 2020. ; https://doi.org/10.1101/2020.11.13.20231241doi: medRxiv preprint
Togo Tunisia Uganda Zambia Zimbabwe
Somalia South Africa South Sudan Sudan Swaziland Tanzania The Gambia
Namibia Niger Nigeria Republic of Congo Rwanda Senegal Sierra Leone
Libya Madagascar Malawi Mali Mauritania Morocco Mozambique
Ethiopia Gabon Ghana Guinea Kenya Lesotho Liberia
Central African Republic Chad Cote d'Ivoire Democratic Republic of the Congo Djibouti Egypt Eritrea
Algeria Angola Benin Botswana Burkina Faso Burundi Cameroon
Mar Apr May Jun Jul AugMar Apr May Jun Jul AugMar Apr May Jun Jul AugMar Apr May Jun Jul AugMar Apr May Jun Jul Aug
Mar Apr May Jun Jul AugMar Apr May Jun Jul Aug
0
1
2
3
0
1
2
3
0
1
2
3
0
1
2
3
0
1
2
3
0
1
2
3
0
1
2
3 Testing regime
Figure S3: Time series of testing policy for all countries of Africa. This categorical variable
(H2 in the OxCGRT codebook) ranges from 0 to 3, reο¬ecting: 0-no testing policy, 1-testing those
meeting certain criteria, 2-testing of anyone showing COVID-19 symptoms, and 3-open public
testing.
Togo Tunisia Uganda Zambia Zimbabwe
Somalia South Africa South Sudan Sudan Swaziland Tanzania The Gambia
Namibia Niger Nigeria Republic of Congo Rwanda Senegal Sierra Leone
Libya Madagascar Malawi Mali Mauritania Morocco Mozambique
Ethiopia Gabon Ghana Guinea Kenya Lesotho Liberia
Central African Republic Chad Cote d'Ivoire Democratic Republic of the Congo Djibouti Egypt Eritrea
Algeria Angola Benin Botswana Burkina Faso Burundi Cameroon
Mar Apr May Jun Jul AugMar Apr May Jun Jul AugMar Apr May Jun Jul AugMar Apr May Jun Jul AugMar Apr May Jun Jul Aug
Mar Apr May Jun Jul AugMar Apr May Jun Jul Aug
0
10
20
30
0
10
20
30
0
10
20
30
0
10
20
30
0
10
20
30
0
10
20
30
0
10
20
30
Temperature (Co)
Figure S4: Time series of population-weighted mean temperature for all countries of Africa.
33
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Togo Tunisia Uganda Zambia Zimbabwe
Somalia South Africa South Sudan Sudan Swaziland Tanzania The Gambia
Namibia Niger Nigeria Republic of Congo Rwanda Senegal Sierra Leone
Libya Madagascar Malawi Mali Mauritania Morocco Mozambique
Ethiopia Gabon Ghana Guinea Kenya Lesotho Liberia
Central African Republic Chad Cote d'Ivoire Democratic Republic of the Congo Djibouti Egypt Eritrea
Algeria Angola Benin Botswana Burkina Faso Burundi Cameroon
Mar Apr May Jun Jul AugMar Apr May Jun Jul AugMar Apr May Jun Jul AugMar Apr May Jun Jul AugMar Apr May Jun Jul Aug
Mar Apr May Jun Jul AugMar Apr May Jun Jul Aug
0
20
40
60
0
20
40
60
0
20
40
60
0
20
40
60
0
20
40
60
0
20
40
60
0
20
40
60 Rain (mm)
Figure S5: Time series of population-weighted mean rain for all countries of Africa.
Togo Tunisia Uganda Zambia Zimbabwe
Somalia South Africa South Sudan Sudan Swaziland Tanzania The Gambia
Namibia Niger Nigeria Republic of Congo Rwanda Senegal Sierra Leone
Libya Madagascar Malawi Mali Mauritania Morocco Mozambique
Ethiopia Gabon Ghana Guinea Kenya Lesotho Liberia
Central African Republic Chad Cote d'Ivoire Democratic Republic of the Congo Djibouti Egypt Eritrea
Algeria Angola Benin Botswana Burkina Faso Burundi Cameroon
Mar Apr May Jun Jul AugMar Apr May Jun Jul AugMar Apr May Jun Jul AugMar Apr May Jun Jul AugMar Apr May Jun Jul Aug
Mar Apr May Jun Jul AugMar Apr May Jun Jul Aug
5
10
15
20
5
10
15
20
5
10
15
20
5
10
15
20
5
10
15
20
5
10
15
20
5
10
15
20 Specific Humidity (g/kg)
Figure S6: Time series of population-weighted mean speciο¬c humidity for all countries of
Africa.
34
. CC-BY-NC-ND 4.0 International licenseIt is made available under a
is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.(which was not certified by peer review)preprint
The copyright holder for thisthis version posted November 16, 2020. ; https://doi.org/10.1101/2020.11.13.20231241doi: medRxiv preprint
A B
C D E
Figure S7: Population weighting of mean daily temperatures. (A) 10km mean temperature
raster for April 1, 2020. (B) Gridded population density. (C) Unweighted mean temperature by
country for the same day, taken as a simple average over each raster pixel within a countryβs
borders. (D) Mean temperature by country, weighted by population. (E) Differential of mean
temperature by country between weighted and unweighted calculations.
35
. CC-BY-NC-ND 4.0 International licenseIt is made available under a
is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.(which was not certified by peer review)preprint
The copyright holder for thisthis version posted November 16, 2020. ; https://doi.org/10.1101/2020.11.13.20231241doi: medRxiv preprint
A B
C D E
Figure S8: Population weighting of mean daily rainfall. (A) 10km mean rainfall raster for April
1, 2020. (B) Gridded population density. (C) Unweighted mean rainfall by country for the same
day, taken as a simple average over each raster pixel within a countryβs borders. (D) Mean rainfall
by country, weighted by population. (E) Differential of mean rainfall by country between weighted
and unweighted calculations.
36
. CC-BY-NC-ND 4.0 International licenseIt is made available under a
is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.(which was not certified by peer review)preprint
The copyright holder for thisthis version posted November 16, 2020. ; https://doi.org/10.1101/2020.11.13.20231241doi: medRxiv preprint
A B
C D E
Figure S9: Population weighting of mean daily humidity. (A) 10km mean humidity raster for
April 1, 2020. (B) Gridded population density. (C) Unweighted mean speciο¬c humidity by country
for the same day, taken as a simple average over each raster pixel within a countryβs borders. (D)
Mean humidity by country, weighted by population. (E) Differential of mean speciο¬c humidity by
country between weighted and unweighted calculations.
β
β
β
β
β
β
β
β
β
β
β
β
β β
β1500
β1250
β1000
β750
2 3 4 5 6 7 8
D
Improvement in AIC
β βGeometric PoissonA
0.00
0.05
0.10
0.15
0.20
1 2 3 4 5 6 7
d
wdB
Geometric Poisson
Figure S10: Distributed lags. (A) Improvement in AIC (relative to the ο¬rst-order model, D =
1) for increasing lags and different weights parameterizations.The largest improvement (βAIC =
-1560) is achieved by a model with t = 7 and with geometric weights. However we preferred the
Poisson weights due to the weights distribution that is biologically consistent with the infection
dynamics. AIC increases again at D = 8. For the ο¬nal parsimonious model, we chose D=7 because
of the largest improvement in AIC. (B) Estimated model weightsΛwd. The Poisson weights show an
increase in infectiousness up to day 4 followed by a decrease while geometric weights degenerate
to a uniform distribution.
37
. CC-BY-NC-ND 4.0 International licenseIt is made available under a
is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.(which was not certified by peer review)preprint
The copyright holder for thisthis version posted November 16, 2020. ; https://doi.org/10.1101/2020.11.13.20231241doi: medRxiv preprint
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Tunisia Uganda Zambia Zimbabwe
South Africa South Sudan Sudan Swaziland Tanzania The Gambia Togo
Niger Nigeria Republic of Congo Rwanda Senegal Sierra Leone Somalia
Libya Malawi Mali Mauritania Morocco Mozambique Namibia
Ethiopia Gabon Ghana Guinea Kenya Lesotho Liberia
Central African Republic Chad Cote d'Ivoire Democratic Republic of the Congo Djibouti Egypt Eritrea
Algeria Angola Benin Botswana Burkina Faso Burundi Cameroon
Apr May Jun Jul Aug Apr May Jun Jul Aug Apr May Jun Jul Aug Apr May Jun Jul Aug
Apr May Jun Jul Aug Apr May Jun Jul Aug Apr May Jun Jul Aug
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Reported cases
Between Within Endemic
Figure S11: Model contributions by country. Observed case counts are shown with black traces.
Between and within country components are shown for the period of analysis.
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β
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ββ
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βββ
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β
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β
β
βββ
β
β
β
β
β
β
β
βββββ
βββ
ββ
β
Tunisia Uganda Zambia Zimbabwe
South Africa South Sudan Sudan Swaziland Tanzania The Gambia Togo
Niger Nigeria Republic of Congo Rwanda Senegal Sierra Leone Somalia
Libya Malawi Mali Mauritania Morocco Mozambique Namibia
Ethiopia Gabon Ghana Guinea Kenya Lesotho Liberia
Central African Republic Chad Cote d'Ivoire Democratic Republic of the Congo Djibouti Egypt Eritrea
Algeria Angola Benin Botswana Burkina Faso Burundi Cameroon
Apr May Jun Jul Aug Apr May Jun Jul Aug Apr May Jun Jul Aug Apr May Jun Jul Aug
Apr May Jun Jul Aug Apr May Jun Jul Aug Apr May Jun Jul Aug
0
500
1000
1500
2000
0
25
50
75
0
50
100
0
100
200
300
400
500
0
100
200
300
0
25
50
75
0
20
40
60
80
0
2000
4000
6000
8000
0
50
100
0
100
200
300
0
50
100
150
0
100
200
300
400
0
30
60
90
0
250
500
750
0
1000
2000
3000
0
1000
2000
3000
4000
0
200
400
600
0
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200
0
100
200
0
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500
750
0
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300
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500
0
500
1000
0
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500
750
1000
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150
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1000
1500
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1000
2000
3000
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150
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400
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500
750
1000
1250
0
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500
750
1000
1250
0
50
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150
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250
0
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150
0
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400
600
0
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400
0
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2000
3000
0
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400
0
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0
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2000
3000
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0
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2000
0
500
1000
1500
2000
0
100
200
0
20000
40000
60000
0
50
100
150
Reported Cases
Reported cases β β Predicted mean retrospective forecast 50% β 95% CI retrospective forecast
Figure S12: Week-ahead predictions by country. Reported cases used to ο¬t the model are ο¬lled
black circles. Unobserved cases from the period for which predictions are made are open black
circles. The 50% and 95% conο¬dence intervals are shown in dark/light shading for both the retro-
spective model ο¬tting and the week-ahead forecast.
38
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0 500 1000 1500 2000 2500 km
N
Pβvalue
0.10
Not analyzed
Calibration Test
Figure S13: Calibration test results by country. P-value obtained from a calibration test per-
formed on the hold-out data (last week of cases) for each country using the logarithmic score.
Small p-values correspond to poorly calibrated predictions. A global test for the whole continent
conο¬rms that our model is well calibrated (p= 0.96).
39
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The copyright holder for thisthis version posted November 16, 2020. ; https://doi.org/10.1101/2020.11.13.20231241doi: medRxiv preprint
0 500 1000 1500 2000 2500 km
N
Population
597000 β 2241750
2241750 β 4887000
4887000 β 11428375
11428375 β 15378000
15378000 β 20658500
20658500 β 31209500
31209500 β 52767250
52767250 β 206140000
Population in 2020
0 500 1000 1500 2000 2500 km
N
HDI
High
Low
Medium
Human Development Index
0 500 1000 1500 2000 2500 km
N
Access to Coast
Coastal
Landlocked
Access to Coast
0 500 1000 1500 2000 2500 km
NUN Region
North Africa
SubβSaharan Africa
Regions
A B
C D
Figure S14: Time-constant covariates utilized in the model. (A) Population. (B) Human Devel-
opment Index. (C) Access to coast. (D) North African and Sub-Saharan Africa regions.
40
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The copyright holder for thisthis version posted November 16, 2020. ; https://doi.org/10.1101/2020.11.13.20231241doi: medRxiv preprint
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
β
40
50
60
70
80
15 20 25 30
Median Age (years)
Human Development Index (%)
Figure S15: Correlation of human development index of a country with median age. The
Pearsonβs correlation coefο¬cient is high (R=0.71, p<0.0001). To avoid multicolinearity with HDI,
we removed median age from the model ο¬t.
41
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β
β
β
β β β β β β0.00
0.25
0.50
0.75
1.00
1 2 3 4 5 6 7 8 9
Adjacency order
Nonβnormalized weights
A
Alger
iaAngolaBeninBotswanaBurkina FasoBurundiCameroonCentral African RepublicChadCote d'IvoireDem. Rep. of the Congo DjiboutiEgyptEritreaEthiopiaGabonGhanaGuineaKenyaLesothoLiberiaLibyaMalawiMaliMauritaniaMoroccoMozambiqueNamibiaNigerNigeriaRepublic of CongoRwandaSenegalSierra LeoneSomaliaSouth AfricaSouth SudanSudanSwazilandTanzaniaThe GambiaTogoTunisiaUgandaZambiaZimbabwe
Zimbabwe
Zambia
Uganda
Tunisia
Togo
The Gambia
Tanzania
Swaziland
Sudan
South Sudan
South Africa
Somalia
Sierra Leone
Senegal
Rwanda
Republic of Congo
Nigeria
Niger
Namibia
Mozambique
Morocco
Mauritania
Mali
Malawi
Libya
Liberia
Lesotho
Kenya
Guinea
Ghana
Gabon
Ethiopia
Eritrea
Egypt
Djibouti
Dem. Rep. of the Congo
Cote d'Ivoire
Chad
Central African Republic
Cameroon
Burundi
Burkina Faso
Botswana
Benin
Angola
Algeria
0.1
0.2
0.3
wji
B
Figure
S16: Spatial connectivity of countries. (A) Spatial connectivity weights as a function
of country border distance. (B) A matrix of countries showing the connectivity weights. The
connectivity weights decrease as the number of borders crossed to reach a certain country increase.42
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Table S1: Model selection for AR7: Sequential model building from intercept-only model (model
1) to the ο¬nal model (model 5). Statistically signiο¬cant explanatory factors are inbold.
Model
1 Model 2 Model 3 Model 4 Model 5 ο¬xed effects Model 5 (Final) with random effects
V
ariable RR 95 % CI RR 95 % CI RR 95 % CI RR 95 % CI RR 95 % CI RR 95 % CI
Endemic
Intercept 33.707 21.473
- 52.91 11.83 4.12 - 33.972 16.33 7.416 - 35.959 23.371 13.275 - 41.144 27.949 19.407 - 40.252 11.071 7.15 - 17.142
W
ithin-country
Intercept 0.988 0.939 - 1.039 0.93 0.783 - 1.104 0.513 0.374 - 0.705 0.521 0.38 - 0.714 0.767 0.552 - 1.064 0.958 0.05 - 1.669
log(population) 0.981 0.944 - 1.018 0.977 0.939 - 1.017 0.975 0.936 - 1.015 0.99 0.949 - 1.032 1.036 0.956 - 1.123
HDI 1.054 0.973 - 1.141 0.991 0.913 - 1.075 0.995 0.914 - 1.083 0.957 0.819 - 1.118
Landlocked 0.874 0.766 - 0.998 0.862 0.754 - 0.985 0.783 0.685 - 0.895 0.674 0.531 - 0.857
Stringency 1.91 1.379 - 2.645 1.94 1.405 - 2.679 1.674 1.195 - 2.345 1.872 1.170 - 3.00
Testing 1.058 0.994 - 1.127 1.066 1.002 - 1.134 0.925 0.863 - 0.991 0.817 0.729 - 0.918
Rain 1.033 0.977 - 1.093 1.034 0.978 - 1.092 1.019 0.963 - 1.078 1.045 0.981 - 1.112
Temperature 1.082 1.015 - 1.153 1.07 1.005 - 1.138 1.059 0.995 - 1.127 1.106 1.014 - 1.206
Humidity 0.99 0.925 - 1.06 0.984 0.92 - 1.052 0.982 0.918 - 1.051 0.856 0.780 - 0.940
Between-country
Intercept 0.027 0.024
- 0.03 0.424 0.267 - 0.673 0.435 0.274 - 0.692 0.34 0.207 - 0.561 0.025 0.014 - 0.046 0.045 0.04 - 0.481
log(population) 1.776 1.611 - 1.958 1.776 1.612 - 1.957 1.837 1.65- 2.035 1.656 1.500- 1.828 1.428 0.923 - 2.21
HDI 1.746 1.486 - 2.052 1.71 1.45 - 2.017 1.239 0.584 - 2.638
Landlocked 1.117 0.902 - 1.384 1.35 1.082 - 1.685 0.844 0.306 - 2.323
Stringency 2.622 1.544 - 4.45 1.630 0.560 - 4.749
Testing 2.899 2.482 - 3.385 2.322 1.676 - 3.218
AIC
48824.32 48690.89 48663 48618.73 48437.99 -
Ο 0.64 0.43 - 0.85 0.79 0.55 - 1.04 0.86 0.62 - 1.10 0.64 0.38 - 0.89 0.74 0.45 - 1.02 2.19 1.53 - 2.84
Ο 2.11 2.025 - 2.195 2.062 1.978 - 2.145 2.044 1.961 - 2.127 2.024 1.942 - 2.107 1.947 1.867 - 2.027 1.703 1.631 - 1.775
Logarithmic Score 4.772 4.817 4.809 4.765 4.745 4.713
43
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Table S2: Codebook for the factors included in the OxCGRT stringency index. Full codebook
here, and methodology for stringency index calculation given here. *Testing policy H2 does not
contribute to the stringency index. It is included as a separate variable in the model.
Item
Description Targeted Levels
C1
Record closing of schools and universities Y/N
0-no measures
1-recommend closing
2-require closing (only some levels)
3-require closing all levels
C2
Record closings of workplaces Y/N
0-no measures
1-recommend closing (or work from home)
2-require closing or work from home (for some sectors or categories of workers)
3-require closing or work-from-home for all but essential workplaces
C3
Record cancelling public events Y/N
0-no measures
1-recommend cancelling
2-require cancelling
C4
Record limits on private gatherings Y/N
0-no restrictions
1-restrictions on gatherings >1000 people
2-restrictions on gatherings of 101-1000 people
3-restrictions on gatherings of 11-100 people
4-restrictions on gatherings ofβ€10 people
C5
Record closing of public transit Y/N
0-no measures
1-recommend closing or signiο¬cantly reduce service
2-require closing
C6
Record orders to shelter in place Y/N
0-no measures
1-recommend not leaving house
2-require not leaving house with exeptions for daily exercise, grocery shopping, and βessentialβ trips
3-require not leaving house with minimal exceptions
C7
Record restrictions on internal movement between cities/regions Y/N
0-no measures
1-recommend not to travel between regions/cities
2-internal movement restrictions in place
C8
Record restrictions on international travel of foreigners Y/N
0-no restrictions
1-screening arrivals
2-quarantine arrivals from some or all regions
3-ban arrivals from some regions
4-ban on all regions or total border closure
H1
Record presence of public information campaigns Y/N
0-no COVID-19 public information campaign
1-public ofο¬cials urging caution about COVID-19
2-coordinated public information campaign
*H2
Record government policy on who has access to testing
0-no testing policy
1-testing those meeting certain criteria
2-testing of anyone showing COVID-19 symptoms
3-open public testing
44
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Table S3: Sources for data included in the model. Results presented in this work can be repro-
duced with code and archived versions of this data that have been stored in a github repository.
Current versions of this data can be found at the links included.
Data Source
Country-level COVID-19 in-
cidence
Johns Hopkins University the Center for Systems Science and Engi-
neering (JHU-CSSE) Coronavirus Resource Center β JHU-CSSE
Website β Data Repository.
Country Population, 2020 United Nations (UN) Population Division of the Department of Eco-
nomic and Social Affairs β UN Website
Geography - Shapeο¬les ESRI: Version November 21, 2018. This shapeο¬le has been mod-
iο¬ed from the original ESRI version to represent the current UN
country designations. This modiο¬ed shapeο¬le has been included in
the data repository.
Geography - Sub-Saharan
Africa
United Nations Statistics Division (UNSD) β UNSD Website
Geography - Landlocked UNSD β UNSD Website
Government Containment
Policies
Oxford COVID-19 Government Response Tracker (OxCGRT) β
OxCGRT Websiteβ Data Repository
Human Development Index United Nations Development Programme (UNDP) Human Devel-
opment Index for 2018 β UNDP Website & Data
Weather data UK Met Ofο¬ce β UK Met Ofο¬ce Websitesee methods section for
procedure to extract population-weighted weather variables
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