Anti SARS-CoV-2 seroprevalence in Zanzibar in 2021 before the Omicron wave

•Population-based data on COVID-19 seroprevalence in Zanzibar were analyzed.•57% of this representative cohort were positive for antibodies against SARS-CoV-2.•The highest seroprevalence was observed in adolescents.•No regional seroprevalence hotspots were detected in Unguja or Pemba.•The results form the basis for a follow-up survey on susceptibility to the Omicron variants. For Tanzania, including Zanzibar, the development of the COVID-19 pandemic has remained unclear since the reporting of cases was suspended during 2020/21. Our study was the first to analyze data on COVID-19 seroprevalence in the Zanzibari population before the Omicron variant wave began in late 2021. During August through October 2021, representative cross-sectional data were collected from randomly selected households in 120 wards of the two main islands, Unguja and Pemba. Participants voluntarily provided blood samples to test their sera for antibodies against SARS-CoV-2 using a semiquantitative enzyme-linked immunosorbent assay (ELISA). 58.9% of the 2051 sera analysed were positive, without significant differences between Unguja and Pemba or between rural and urban areas. The results were in agreement with observations from other sub-Saharan African countries. The antibody levels observed were most likely due to previous infections with SARS-CoV-2, since vaccination was generally not available before the survey. Therefore, this study offers the first insights into how many Zanzibari had COVID-19 before the Omicron variant emerged. Furthermore, it provides an appropriate basis for a follow-up survey addressing how this seroprevalence has influenced susceptibility to the Omicron variants, given the use of harmonized methodologies. [Display omitted]