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Abstract Objectives For Tanzania including Zanzibar, the development of the COVID-19 pandemic has remained unclear, since reporting cases was suspended during 2020/21. The present study provides first data on the COVID-19 seroprevalence among Zanzibari before the omicron variant wave starting in late 2021. Design 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-CoV2 in a semiquantitative enzyme-linked immunosorbent assay (ELISA). Results 58.9% of the 2051 sera analysed were positive without significant differences between Unguja and Pemba or between rural and urban areas, similar to observations from other sub-Saharan Africa countries. Conclusions The antibody levels observed are most likely to previous infections with SARS-CoV2, since vaccination was basically not available before the survey. Therefore, this study provides first insight, how many Zanzibari have had COVID-19 before the Omicron variant. Further, it provides the appropriate basis for a follow-up survey addressing how this seroprevalence influenced the susceptibility to the Omicron variants, given harmonised methodologies are used.