COVID-19 Seroprevalence in Baixada Santista Metropolitan Area, Sao Paulo, Brazil.

Background: COVID-19 reached Brazil in February 2020 after spreading throughout Asia and Europe. The disease first struck in Brazil major cities, affecting high-income groups, subsequently spreading to the socially-vulnerable outlying areas. The objective of this preliminary study was to the estimate the seroprevalence of COVID-19 in the Baixada Santista Metropolitan Area - RMBS, to inform policies for prevention and preparation of primary care, hospital services, hospital beds, intensive and critical care in a bid to reduce lethality. Methods: This study is the first of a four-phase cross-sectional survey to include all cities within the Baixada Santista metropolitan area. The probabilistic population-based sample was based on a 7% prevalence with a 2% delta, a 5% level of significance and 90% power, and considered the estimated population, stratified by age, gender and city. The sample was randomized in each city and street selection was based on data drawn from the 2010 Brazilian census. The serology test was chosen from the available test kits approved by the National Health Agency (ANS) of the National Health Surveillance Agency (ANVISA), for the detection of SARS-CoV-2 antibodies. Information on socioeconomic characteristics, health service use, symptoms and adherence to social distancing measures was collected. All variables were expressed as absolute and relative frequencies and confidence intervals. Results: The final sample comprised 2,342 respondents with a mean age of 37.78 (19.98) years and equal gender distribution. Respondents predominantly lived in an abode with 4 or more rooms, 71.9% had an income of [≤]3 minimum wages and no formal employment. Over half of the sample reported being reliant on the National Health Service (SUS) for health care, while 26.5% held private health insurance. Social distancing was practiced by 61.4% of respondents who answered this question. Seroprevalence measured was 1.4% for a 95% CI (0.93-1.93) based on mathematical estimates, with a ratio of 15 actual infections for every case notified, and lethality of 0.48%. Conclusion: Results revealed a flattening of the epidemiological curve in the region and high underreporting of cases.