Population-Level Seropositivity Trend for SARS-Cov-2 in Rio Grande do Sul, Brazil: Results of 10 Repeated Surveys of the EPICOVID19-RS Study

Background: Rio Grande do Sul, the southernmost state in Brazil, had a slow start in the pandemic compared to other regions of the country, but the number of new cases rose sharply in December 2020, with a huge spike in March 2021, when the number of new cases was close to 800 per million people. In this article, we describe the pandemic in the State through the lens of 10 consecutive surveys conducted between April 2020 and April 2021. Methods: We used a sentinel city approach, covering all regions of the State. In each city we selected 50 urban census tracts with probability proportional to size. In each tract, 10 households were selected using a systematic approach. Within each selected household, all residents were listed, and one was selected randomly. In the first rounds of the study, we used the rapid point-of-care lateral- flow WONDFO SARS-CoV-2 Antibody Test (Wondfo Biotech Co., Guangzhou, China). In rounds nine and 10, we used an in-house direct ELISA test that identifies the presence of IgG to the viral spike (S) protein from dried blood spot samples (S-UFRJ). In terms of social distancing, individuals were asked three questions, from which we generated an exposure score through principal components analysis. Findings: Antibody prevalence in early April 2020 was 0.07 (95% CI 0.01-0.32), increasing to 10.0% (95% CI 9.1-11.0) in February 2021, and to 18.2% (95% CI 16.9-19.5) in April 2021. Self-reported whites showed the lowest seroprevalences (9.3%, 95% CI 8.3–10.4 in round 9 and 17.3%, 95% CI 15.9–18.8 in round 10), while indigenous individuals presented the highest levels (20.0%, 95% CI 8.5– 40.3 in round 9 and 44.4%, 95% CI 22.5–68.7 in round 10), followed by blacks (14.4%, 95% CI 11.3– 18.1 in round 9 and 21.6%, 95% CI 17.4–26.4 in round 10). We found an increase in seropositivity in relation to our exposure score; the most exposed 10% of the population presented a 40% higher seropositivity. Interpretation: The proportion of the population already infected by SARS-Cov-2 in the State is still far from any perspective of herd immunity, suggesting that herd immunity will only be achieved through vaccination. Funding: the Brazilian Ministry of Health, Banco do Estado do Rio Grande do Sul (BANRISUL), Instituto Cultural Floresta, UNIMED Porto Alegre, “Todos pela Saude” Group, Instituto Serrapilheira, Brazilian Collective Health Association (ABRASCO) and the JBS S.A. initiative “Fazer o Bem Faz Bem”. Declaration of Interest: None to declare. Ethical Approval: Ethical approval was obtained from the Brazilian’s National Ethics Committee (30415520.2.0000.5313), and we obtained written informed consent from all participants.