Seroprevalence of anti-SARS-CoV-2 antibodies after the second pandemic peak

After the first pandemic wave in Europe, seroprevalence surveys revealed that roughly one in ten individuals had been infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).¹ Our Geneva-based seroprevalence study revealed that infections were less common in young children (<9 years) than in older children and adults, but at the time of the study individuals were confined and schools were closed.² Since autumn, 2020, Europe has experienced a rapid increase in reported infections, with SARS-CoV-2 incidence in some countries largely surpassing that of the first wave. Due to changes in test availability, policy, and care-seeking behaviours, it is unclear how to compare current case reports with the first wave and how these relate to undetected infection rates.

To estimate SARS-CoV-2 seroprevalence in the general population and determine whether age disparities have persisted through the second wave, we repeated a representative serosurvey of the Geneva population using a stratified random sample (based on age, sex, and education level) of individuals aged 18–64 years from our previous study² and an independent random sample of individuals aged 0–18 years and 65 years and older who were identified from resident registers of the Swiss Federal Office of Statistics. We tested participants for anti-SARS-CoV-2 total immunoglobulins targeting the spike protein (Elecsys anti-SARS-CoV-2 S; Roche Diagnostics, Rotkreuz, Switzerland) following manufacturer's recommendations (≥0·8 U/mL considered seropositive). We used a previously published Bayesian model accounting for household clustering, test performance, and age distribution in the Geneva population.²

Between Nov 23, and Dec 23, 2020, we recruited 4000 participants aged 0–96 years (53·4% women; 25·4% <18 years), of whom 820 were seropositive, yielding a seroprevalence of 21·1% (95% credible interval [CrI] 19·2–23·1). We found similar seroprevalence among men and women, but large differences across age groups (appendix p 2). Compared with adults aged 25–34 years, children aged 6 years and older and adolescents had similar seroprevalence, whereas children aged 0–5 years were 43% less likely to be seropositive, and adults aged 65–74 years and those aged 75 years and older were 42% and 64% less likely to be seropositive, respectively (appendix p 2). We estimated that each virologically confirmed SARS-CoV-2 infection represented 2·7 infections (95% CrI 2·3–3·1; appendix pp 3–5) in the community, substantially lower than in the first wave (11·6),² probably due to changed testing practices.

Despite seroprevalence doubling in Geneva since the end of the first wave, most of the population remains unexposed, including more than 90% of adults aged 75 years and older, who have very high mortality risk.3, 4 Although children aged 6 years and older have a similar infection risk as adults, younger children have a lower infection risk. These results should inform policy-makers worldwide, reinforcing the need for continuous measures to contain SARS-CoV-2 spread, despite growing pandemic fatigue in the population,⁵ and to avoid potentially catastrophic COVID-19-related hospitalisations and deaths in the critical months ahead.