Seroprevalence of SARS-CoV-2 in Tanzania: results from a cross-sectional study documenting the Omicron wave in five regions of Tanzania

BACKGROUND: Between August and October 2021, before the Omicron wave and before COVID-19 vaccines became widely available in Tanzania, two studies estimated that half of the population in Zanzibar and Mwanza in Tanzania had SARS-CoV-2 antibodies. From May 2020 to Dec 2021, Tanzania adopted a very different COVID-19 mitigation strategy with no lockdowns or quarantines and limited community-level COVID-19 testing. We conducted population-based SARS-CoV-2 seroprevalence surveys in five regions of Tanzania between November 2021 and July 2022 to estimate community-level exposure to SARS-CoV-2and vaccine coverage. METHODS: Regionally representative cross-sectional serosurveys were progressively conducted via multistage cluster sampling in five regions in Tanzania. Twenty enumeration areas were randomly selected from each region. Seropositivity was determined using the WANTAI SARS-CoV-2 Ab ELISA kit. RESULTS: We found that nine out of every ten people living in five geographically dispersed regions in Tanzania had SARS-CoV-2 antibodies despite only 8% vaccination coverage in our sample. CONCLUSION: Given the high seroprevalence of SARS-CoV-2 in these regions, targeting COVID-19 vaccination efforts to those at greater risk of severe disease may help maximize the public health impact.