Household Transmission of SARS-COV-2: Insights from a Population-based Serological Survey

Bi, Q.; Lessler, J.; Eckerle, I.; Lauer, S. A.; Kaiser, L.; Vuilleumier, N.; Cummings, D. A.; Flahault, A.; Petrovic, D.; Guessous, I.; Stringhini, S.; Azman, A. S.; SEROCoV-POP,

Importance: Knowing the transmissibility of asymptomatic infections and risk of infection from household and community exposures is critical to SARS-CoV-2 control. Limited previous evidence is based primarily on virologic testing, which disproportionately misses mild and asymptomatic infections. Serologic measures are more likely to capture all previously infected individuals. Objective: Estimate the risk of SARS-CoV-2 infection from household and community exposures, and identify key risk factors for transmission and infection. Design: Household serosurvey and transmission model. Setting: Population-based serosurvey in Geneva, Switzerland Participants: 4,524 household members five years and older from 2,267 households enrolled April-June 2020. Exposures: SARS-CoV-2 infected (seropositive) household members and background risk of community transmission. Main outcomes and measures: Past SARS-CoV-2 infection confirmed through anti-SARS-CoV-2 IgG antibodies by ELISA. Chain-binomial models based on the number of infections within households were used to estimate extra-household infection risk by demographics and reported extra-household contacts, and infection risk from exposure to an infected household member by demographics and infector's symptoms. Infections attributable to exposure to different types of infectious individuals were estimated. Results: The chance of being infected by a single SARS-CoV-2 infected household member was 17.2% (95%CrI 13.6-21.5%) compared to a cumulative extra-household infection risk of 5.1% (95%CrI 4.5-5.8%). Infection risk from an infected household member increased with age, from 7.5% (95%CrI 1.3-20.3%) among 5-9 years to 30.2% (95%CrI 14.3-48.2%) among those [≥]65 years. Working-age adults (20-49 years) had the highest extra-household infection risk. Seropositive household members not reporting symptoms had 74.8% lower odds (95%CrI 43.8-90.3%) of infecting another household member compared to those reporting symptoms, accounting for 19.6% (95%CrI 12.9-24.5%) of all household infections. Conclusions and Relevance: The risk of infection from exposure to a single infected household member was four-times that of extra-household exposures over the first wave of the pandemic. Young children had a lower risk from infection from household members. Asymptomatic infections are far less likely to transmit than symptomatic ones but do cause infections. While the small households in Geneva limit the contribution of household spread, household transmission likely plays a greater role in other settings.