Trajectories of COVID-19: a longitudinal analysis of many nations and subnational regions

The COVID-19 pandemic is the first to be rapidly and sequentially measured by nation-wide PCR community testing for the presence of the viral RNA at a global scale. We take advantage of the novel “natural experiment” where diverse nations and major subnational regions implemented various policies including social distancing and vaccination at different times with different levels of stringency and adherence. Initially, case numbers expanding exponentially with doubling times of ∼1-2 weeks. In the nations where lockdowns were not implemented or ineffectual, case numbers increased exponentially but then stabilized around 10 2 -to-10 3 new infections (per km 2 built-up area per day). Dynamics under strict lockdowns were perturbed and infections decayed to low levels. They rebounded following the lifting of the policies but converged on an equilibrium setpoint. Here we deploy a mathematical model which captures this behavior, incorporates a direct measure of lockdown efficacies, and allows derivation of a maximal estimate for the basic reproductive number R o (mean 1.6-1.8). We were able to test this approach by comparing the approximated “herd immunity” to the vaccination coverage observed that corresponded to rapid declines in community infections during 2021. The estimates reported here agree with the observed phenomena. Moreover, the decay rates d (0.4-0.5) and rebound rates r 0 (0.2- 0.3) were similar throughout the pandemic and among all the nations and regions studied. Finally, a longitudinal analysis comparing multiple national and regional results provides insights on the underlying epidemiology of SARS-CoV-2 and lockdown and vaccine efficacy, as well as evidence for the existence of an endemic steady state of COVID-19.