Seroepidemiological assessment of the spread of SARS-CoV-2 among 25 and 28 year-old adult women in Finland between March 2020-June 2022

Serological surveys of the prevalence of SARS-CoV-2 are instrumental to understanding the course of the COVID-19 epidemic. We evaluate the seroprevalence of SARS-CoV-2 among young adult Finnish females residing in 25 communities all over Finland from 2020 until 2022. Between 1st March 2020 and 30th June 2022, 3589 blood samples were collected from 3583 women born in 1992-95 when aged 25 or 28 years old attending the follow-up of an ongoing population-based trial of cervical screening strategies. The crude and population standardized SARS-CoV-2 seroprevalence was measured using nucleocapsid (induced by infection) and spike wild-type (WT) protein (induced both by infection and by vaccination) antigens over time and stratified by place of residence (inside or outside the Helsinki metropolitan region). During 2020 (before vaccinations), spike-WT and nucleocapsid IgG antibodies followed each other closely, at very low levels (