Population-based seroprevalence of SARS-CoV-2 and the herd immunity threshold in Maranhão
Antônio Augusto Moura da Sílva, Lídio Gonçãlves Lima Neto, Conceição de Maria Pedrozo e Silva de Azevedo, Léa Márcia Melo da Costa, Maylla Luanna Barbosa Martins Bragança, Allan Kardec Duailibe Barros Filho, Bernardo Bastos Wittlin, Bruno Feres de Souza, Bruno Luciano Carneiro Alves de Oliveira, Carolina Abreu de Carvalho, Érika Bárbara Abreu Fonseca Thomaz, Eudes Alves Simões Neto, Jamesson Ferreira Leite Júnior, Lécia Maria Sousa Santos Cosme, Marcos Adriano Garcia Campos, Rejane Christine de Sousa Queiroz, Sérgio Souza Costa, Vitória Abreu de Carvalho, Vanda Maria Ferreira Simões, Maria Teresa Seabra Soares de Britto e Alves, Alcione Miranda dos Santos
Abstract
OBJECTIVE: To estimate the seroprevalence of SARS-CoV-2 in the state of Maranhão, Brazil. METHODS: A population-based household survey was performed, from July 27, 2020 to August 8, 2020. The estimates considered clustering, stratification and non-response. Qualitative detection of IgM and IgG antibodies was performed in a fully-automated Elecsys® Anti-SARS-CoV-2 electrochemiluminescence immunoassay on the Cobas® e601 analyzer (Roche Diagnostics). RESULTS: In total, 3,156 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, 26.0% were asymptomatic. The infection fatality rate (IFR) was 0.14%, higher for men and older adults. The IFR based on excess deaths was 0.28%. The ratio of estimated infections to reported cases was 22.2. CONCLUSIONS: To the best of our knowledge, the seroprevalence of SARS-CoV-2 estimated in this population-based survey is one of the highest reported. The local herd immunity threshold may have been reached or might be reached soon.