Population-based seroprevalence of SARS-CoV-2 is more than halfway through the herd immunity threshold in the State of Maranhao, Brazil

ABSTRACT

BackgroundFew population-based studies on the prevalence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have been performed to date, and most of them have used lateral flow immunoassays with finger-prick, which may yield false-negative results and thus underestimate the true infection rate. MethodsA population-based household survey was performed in the State of Maranhao, Brazil, from 27 July 2020 to 8 August 2020 to estimate the seroprevalence of SARS-CoV-2 using a serum testing electrochemiluminescence immunoassay. A three-stage cluster sampling stratified by four state regions was used. The estimates took clustering, stratification, and non-response into account. Qualitative detection of IgM and IgG antibodies was performed in a fully-automated Elecsys(R) Anti-SARS-CoV-2 electrochemiluminescence immunoassay on the Cobas(R) e601 analyser (Roche Diagnostics). FindingsA total of 3156 individuals were interviewed. Seroprevalence of total antibodies against SARS-CoV-2 was 40{middle dot}4% (95%CI 35{middle dot}6-45{middle dot}3). Population adherence to non-pharmaceutical interventions was higher at the beginning of the pandemic than in the last month. SARS-CoV-2 infection rates were significantly lower among mask wearers and among those who maintained social and physical distancing in the last month compared to their counterparts. Among the infected, 62{middle dot}2% had more than three symptoms, 11{middle dot}1% had one or two symptoms, and 26{middle dot}0% were asymptomatic. The infection fatality rate was 0{middle dot}17%, higher for males and advanced age groups. The ratio of estimated infections to reported cases was 22{middle dot}2. InterpretationTo the best of our knowledge, the seroprevalence of SARS-CoV-2 estimated in this population-based survey was the highest and the closest to the herd immunity threshold reported to date. Our results suggest that the herd immunity threshold is not as low as 20%, but at least higher than or equal to around 40%. The infection fatality rate was one of the lowest reported so far, and the proportion of asymptomatic cases was low.