Changes in SARS-CoV-2 seroprevalence and population immunity in Finland, 2020–2022

1. Studying the prevalence of SARS-CoV-2 specific antibodies (seroprevalence) allows assessing the impact of epidemic containment measures and vaccinations, as well as estimation of the number of infections regardless of viral testing. We assessed antibody-mediated immunity to SARS-CoV-2 induced by infections and vaccinations from April 2020 to December 2022 in Finland by measuring serum IgG to SARS-CoV-2 nucleoprotein (N-IgG) and spike glycoprotein from randomly selected 18-85-year-old subjects (n=9794). N-IgG seroprevalence remained at <7% until the last quartile (Q) of 2021. After the emergence of the omicron variant, N-IgG seroprevalence increased rapidly and was 31% in Q1/2022 and 54% in Q4/2022. Seroprevalence was highest in youngest age groups from Q2/2022 onwards. We estimated that 51% of the Finnish 18-85-year-old population had antibody-mediated hybrid immunity induced by a combination of vaccinations and infections by the end of 2022. In conclusion, major shifts in the COVID-19 pandemic and resulting population immunity could be observed by serological testing.