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

## TL;DR

This study tracks how SARS-CoV-2 spread among young women in Finland from 2020 to 2022, showing the impact of vaccination and regional differences in infection rates.

## Contribution

The study provides detailed seroprevalence data among young Finnish women, highlighting regional disparities and the shift from vaccination-induced to infection-induced immunity.

## Key findings

- SARS-CoV-2 spike-WT seropositivity in young women reached 96.3% by mid-2022 due to high vaccination rates.
- Nucleocapsid IgG seropositivity increased more in the Helsinki region compared to other areas, indicating higher infection rates.
- Low infection-related seroprevalence in 2020-2021 suggests effective infection control before widespread vaccination.

## Abstract

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 (<5%). Spike-WT seropositivity increased rapidly concomitant with mass vaccinations in 2021 and reached 96.3% in the 2nd quartile of 2022. Antibodies to nucleocapsid IgG remained relatively infrequent throughput 2020–2021, increasing rapidly in the 1st and 2nd quartiles of 2022 (to 19.7% and 56.6% respectively). The nucleocapsid IgG seropositivity increased more profoundly in participants residing in the Helsinki metropolitan region (4.5%, 8.4% and 43.9% in 2020, 2021 and 2022 respectively) compared to those residing in communities outside the capital region (4.5%, 4.3% and 34.7%).

Low SARS-CoV-2 infection-related seroprevalence during 2020–2021 suggest a comparatively successful infection control. Antibodies to the SARS-CoV-2 WT spike protein became extremely common among young women by the end of 2021, in line with the high uptake of SARS-CoV-2 vaccination. Finally, the rapid increase of seroprevalences to the SARS-CoV-2 nucleocapsid protein during the first and second quartile of 2022, imply a high incidence of infections with SARS-CoV-2 variants able to escape vaccine-induced protection.