Seroprevalence of anti-SARS-CoV-2 IgG antibodies in Geneva, Switzerland (SEROCoV-POP): a population-based study

Background Assessing the burden of COVID-19 on the basis of medically attended case numbers is suboptimal given its reliance on testing strategy, changing case definitions, and disease presentation. Population-based serosurveys measuring anti-severe acute respiratory syndrome coronavirus 2 (anti-SARS-CoV-2) antibodies provide one method for estimating infection rates and monitoring the progression of the epidemic. Here, we estimate weekly seroprevalence of anti-SARS-CoV-2 antibodies in the population of Geneva, Switzerland, during the epidemic. Methods The SEROCoV-POP study is a population-based study of former participants of the Bus Sante study and their household members. We planned a series of 12 consecutive weekly serosurveys among randomly selected participants from a previous population-representative survey, and their household members aged 5 years and older. We tested each participant for anti-SARS-CoV-2-IgG antibodies using a commercially available ELISA. We estimated seroprevalence using a Bayesian logistic regression model taking into account test performance and adjusting for the age and sex of Geneva's population. Here we present results from the first 5 weeks of the study. Findings Between April 6 and May 9, 2020, we enrolled 2766 participants from 1339 households, with a demographic distribution similar to that of the canton of Geneva. In the first week, we estimated a seroprevalence of 4 center dot 8% (95% CI 2 center dot 4-8 center dot 0, n=341). The estimate increased to 8 center dot 5% (5 center dot 9-11 center dot 4, n=469) in the second week, to 10 center dot 9% (7 center dot 9-14 center dot 4, n=577) in the third week, 6 center dot 6% (4 center dot 3-9 center dot 4, n=604) in the fourth week, and 10 center dot 8% (8 center dot 2-13 center dot 9, n=775) in the fifth week. Individuals aged 5-9 years (relative risk [RR] 0 center dot 32 [95% CI 0 center dot 11-0 center dot 63]) and those older than 65 years (RR 0 center dot 50 [0 center dot 28-0 center dot 78]) had a significantly lower risk of being seropositive than those aged 20-49 years. After accounting for the time to seroconversion, we estimated that for every reported confirmed case, there were 11 center dot 6 infections in the community. Interpretation These results suggest that most of the population of Geneva remained uninfected during this wave of the pandemic, despite the high prevalence of COVID-19 in the region (5000 reported clinical cases over <2 center dot 5 months in the population of half a million people). Assuming that the presence of IgG antibodies is associated with immunity, these results highlight that the epidemic is far from coming to an end by means of fewer susceptible people in the population. Further, a significantly lower seroprevalence was observed for children aged 5-9 years and adults older than 65 years, compared with those aged 10-64 years. These results will inform countries considering the easing of restrictions aimed at curbing transmission. Copyright (c) 2020 Elsevier Ltd. All rights reserved.