Risk factors for household transmission of SARS-Cov-2: a modelling study in the French national population-based EpiCov cohort

ABSTRACT Background Households are specific transmission settings, as they involve close and repeated contacts between individuals of different generations. Household surveys provide a unique opportunity to better understand SARS-CoV-2 transmission and the role of individual characteristics. Here, we assessed the risk of SARS-CoV-2 acquisition from household and community exposure according to age, family ties, and socioeconomic and living conditions using data from the nationwide population-based EpiCov cohort/ORCHESTRA collaboration in November-December 2020. Methods A history of SARS-CoV-2 infection was defined by a positive Euroimmun Anti-SARS-CoV-2 ELISA IgG result in November-December 2020. We applied stochastic chain binomial models fitted to the final distribution of infections in households to data from 17,983 individuals ≥5 years enrolled from 8,165 households. Models estimated the competing risks of being infected from community and household exposure. Results Young adults aged 18-24 years had the highest risk of extra-household infection (8.9%, [95% credible interval, Crl]: 7.5 – 10.4), whereas the oldest (>75) and the youngest (6-10) had the lowest risk, 2.6% (1.8 – 3.5) and 3.4% (1.9 – 5.2), respectively. Extra-household infection was also independently associated with socioeconomic conditions. Within households, the probability of person-to-person transmission increased with age: 10.6% (5.0 – 17.9) among 6-10-year-olds to 43.1% (32.6 – 53.2) among 65-74-year-olds. It was higher between partners 29.9% (25.6 - 34.3) and from mother to child 29.1% (21.4 – 37.3) than between individuals related by other family ties. Conclusion In 2020 in France, the main factors identified for extra-household infection were age and socioeconomic conditions. Intra-household infection mainly depended on age and family ties. Key Messages Young adults aged 18-24 years had the highest probability of extra-household SARS-Cov-2 acquisition over the year 2020: 8.9%, 95% credible interval (95%Crl) 7.5 – 10.4. The probability of extra-household infection increased with family income and population density in the municipality of residence and was higher in the French regions most affected by the waves of SARS-CoV-2. When estimating the probability of person-to-person transmission of SARS-CoV-2, the 65-74 year-olds had the highest susceptibility, i.e. the highest probability of SARS-CoV-2 acquisition when exposed to an infected household member (22.1%, 16.4 – 28.2) The probability of transmission was the highest between partners (29.9%, 25.6 – 34.3). The probability of transmission was higher from mother to child than from father to child: 29.1%, (21.4 – 37.3) and 14.0% (5.9 – 22.8), respectively. The probability of transmission from child to parent was higher from children <12 years than for older children: 11.8% (2.5 – 25.1) and 4.1% (0.9 – 9.0), respectively.