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

  1. Antônio Augusto Moura da Silva
  2. Lídio Gonçalves Lima-Neto
  3. Conceição de Maria Pedrozo e Silva Azevedo
  4. Léa Márcia Melo da Costa
  5. Maylla Luana Barbosa Martins Bragança
  6. Allan Kardec Duailibe Barros Filho
  7. Bernardo Bastos Wittlin
  8. Bruno Feres Souza
  9. Bruno Luciano Carneiro Alves de Oliveira
  10. Carolina Abreu de Carvalho
  11. Erika Barbara Abreu Fonseca Thomaz
  12. Eudes Alves Simões-Neto
  13. Jamesson Ferreira Leite Júnior
  14. Lécia Maria Sousa Santos Cosme
  15. Marcos Adriano Garcia Campos
  16. Rejane Christine de Sousa Queiroz
  17. Sérgio Souza Costa
  18. Vitória Abreu de Carvalho
  19. Vanda Maria Ferreira Simões
  20. Maria Teresa Seabra Soares de Brito Alves
  21. Alcione Miranda dos Santos
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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 Maranhão, 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.

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