Population-based seroprevalence of SARS-CoV-2 and the herd immunity threshold in Maranhão

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.

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.…”

“…We identified high seroprevalence among younger age groups, particularly in children between 5 and 14 years of age (42%). Age-based differences in seropositivity were observed in Brazil and Italy, 2,7 however, they were more pronounced in our population. High seropositivity in children might be due to continued operation of public schools throughout the epidemic.…”

Prevalência e fatores associados à hesitação vacinal contra a covid-19 no Maranhão, Brasil

“…Para o cálculo do tamanho da amostra, adotou-se a prevalência de infectados por SARS-Cov-2 estimada no primeiro inquérito realizado no estado do Maranhão 16 . Em cada estrato, o cálculo do tamanho da amostral foi estimado a partir da seguinte equação: A seleção dos domicílios em cada um dos 150 setores ou agrupamentos foi obtida por amostra sistemática.…”

“…Os sintomas também foram investigados por meio da recordação, segundo o mês em que estiveram presentes. Porém, para averiguar a frequência dos sintomas, foram usados os mais associados ao diagnóstico da covid-19, anosmia/hiposmia ou ageusia/disgeusia 16 . Sobre a intenção de tomar a vacina, a pergunta foi feita quando a vacinação ainda não estava ocorrendo, com poucos detalhes divulgados sobre como seria executada.…”

“…Foram apresentadas as estimativas para as variáveis em cada bloco.RESULTADOSA prevalência de hesitação ao uso da vacina contra o vírus SARS-CoV-2 foi de 17,5% (IC95% 16,1-19,1). Essa prevalência foi estatisticamente maior (p = 0,0036) no município de Imperatriz (24,0%; IC95% 18,1-31,1) e naqueles que formam a Grande Ilha de São Luís (20,7%; IC95% 17,6-24,3) do que entre as cidades com menos de 20.000 habitantes (14,0%;IC95% 11,[7][8][9][10][11][12][13][14][15][16]6). A hesitação vacinal também foi estatisticamente maior entre pessoas do Prevalência da hesitação vacinal ao uso da vacina contra o novo coronavírus, caso ela estivesse já disponível para a população, segundo comportamentos e condições de saúde dos entrevistados no inquérito sorológico no Maranhão, Brasil, 2020.…”