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

Abstract

Background: Few 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. Methods: A 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 Anti-SARS-CoV-2 electrochemiluminescence immunoassay on the Cobas e601 analyser (Roche Diagnostics). Findings: A total of 3156 individuals were interviewed. Seroprevalence of total antibodies against SARS-CoV-2 was 40.4% (95%CI 35.6-45.3). 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, 62.2% had more than three symptoms, 11.1% had one or two symptoms, and 26.0% were asymptomatic. The infection fatality rate was 0.17%, higher for males and advanced age groups. The ratio of estimated infections to reported cases was 22.2. Interpretation: To 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.

Seroepidemiology of SARS-CoV-2 infections in an urban Nicaraguan population

“…This suggests the actual number of SARS-CoV-2 infections in Nicaragua was higher than official reports, which might be explained by a high rate of asymptomatic and mild cases that did not seek medical attention, or by limited testing of patients who received medical attention. Our seroprevalence results are similar to those of another study from Brazil, 7 and those reported from SARS-CoV-2 hot spots in high income countries, such as Massachusetts and Northern Italy, although the Massachusetts study was not carried out in a population-based cohort. 2,8 In these studies, the seroprevalence of SARS-CoV-2 antibodies was higher than expected based on the reported incidence of symptomatic SARS-CoV-2 infections in the area.…”

Population-based prevalence surveys during the COVID-19 pandemic: a systematic review

“…The same proportion of studies validated their methods internally to report accuracy (16,21,23,24,30,32,(34)(35)(36)(37)(38)(39)45,50,52) or used sensitivity and specificity given by manufacturers or other external studies (18)(19)(20)22,36,(40)(41)(42)(44)(45)(46)(47)(48)(49)(50)(51). We also noticed that it is quite unclear if the field teams followed standardized protocols for the data collection and testing.…”

“…Valid methods consistently stated the identification of the condition and the manufacturer indicated this accuracy. The majority of the sampling methods was conducted appropriately regarding randomness, and the participants were well described and stratified, thus mitigating possible selection bias (18,19,24,26,28,30,33,37,39,43,44,46,(48)(49)(50)(51). A strong trend was observed in relation to the sampling procedures used in Brazil.…”

Inference of COVID-19 epidemiological distributions from Brazilian hospital data

“…Ninety-five per cent credible intervals are given in brackets for mean and variance. The parameters p 1 , p 2 and p 3 for the preferred PDFs gamma and generalized lognormal (GLN) are given in the form gamma(x|p 1 , p 2 ) = gamma(α, β) and GLN(x|p 1 , p 2 , p 3 ) = GLN(μ, σ, s), with the formulae of the PDFs given in appendix B, Across Brazil, the epidemic has strong geographical heterogeneity, with some states such as Amazonas and Maranhão reported to be at advanced stages [40,41]. To describe the observed differences at subnational level accurately, using a mathematical model, it is essential to account for variation in model parameters by state.…”

“…Across Brazil, the epidemic has strong geographical heterogeneity, with some states such as Amazonas and Maranhão reported to be at advanced stages [ 40 , 41 ]. To describe the observed differences at subnational level accurately, using a mathematical model, it is essential to account for variation in model parameters by state.…”