A1 Refereed original research article in a scientific journal
SARS-CoV-2 seroprevalence and seroconversion in Finnish households with school-aged children between June 2020 and April 2022
Authors: Ahti, Jaakko; Ollila, Helena; Toivonen, Laura; Salo-Tuominen, Krista; Ivaska, Lauri; Julkunen, Ilkka; Peltola, Ville
Publisher: Informa Healthcare
Publication year: 2026
Journal: Infectious Diseases
ISSN: 2374-4235
eISSN: 2374-4243
DOI: https://doi.org/10.1080/23744235.2026.2648678
Publication's open availability at the time of reporting: No Open Access
Publication channel's open availability : Partially Open Access publication channel
Web address : https://doi.org/10.1080/23744235.2026.2648678
Self-archived copy’s web address: https://research.utu.fi/converis/portal/detail/522859583
Self-archived copy's licence: CC BY NC ND
Self-archived copy's version: Final draft
Background
SARS-CoV-2 spreads efficiently in households with children. Here, we aimed to examine whether serological methods, in addition to PCR, can improve understanding of SARS-CoV-2 transmission within households.
Methods
Within the prospective birth-cohort study called the STEPS Study, 175 households with 376 children and 324 adults were prospectively followed for acute respiratory infections from June 2020 to April 2022. SARS-CoV-2 was tested by PCR and antigen tests. IgG antibodies against SARS-CoV-2 nucleoprotein (N) and spike (S) proteins were determined at recruitment and at two later timepoints to assess seroprevalence, and five weeks after a PCR-positive infection in the household to assess seroconversion. Secondary attack rates (SARs) based on seroconversion or PCR-positivity were calculated and compared among index and secondary cases.
Results
SARS-CoV-2 anti-N-IgG seroprevalence showed a minor increase from 0.5% at recruitment, starting in June 2020, to1.9% at the last serum collection. SARS-CoV-2 RNA was detected by PCR in altogether 334 (48%) participants. The SAR for the adult index cases was higher than the SAR for the child index cases (77% vs 48%; p = 0.0027). The SARs were similar for unvaccinated (0–1 vaccinations) and vaccinated (2–3 vaccinations) index cases (53% and 63%, respectively; p = 0.28), but the SAR for unvaccinated secondary cases was higher compared to the SAR for vaccinated secondary cases (83% vs 52%; p = 0.0067).
Conclusion
In this study, adults spread SARS-CoV-2 in households more efficiently than children. While the index case vaccination status showed no difference, unvaccinated household members were more prone to SARS-CoV-2 infection than vaccinated individuals.
Key points
A cohort of households with children was prospectively studied for SARS-CoV-2 transmission through repeated PCR, and serology was obtained 5 weeks post-infection. Adults transmitted the virus more effectively than children, and unvaccinated participants were more susceptible than vaccinated participants.
Keywords: children, COVID-19, household transmission, SARS-CoV-2, secondary attack rate, seroconversion, serology
Funding information in the publication:
This work was supported by Research Funds from Specified Government Transfers, Hospital District of Southwest Finland, Jane and Aatos Erkko Foundation and by grants to J.A. from the Finnish Cultural Foundation, the Research Foundation of the pulmonary diseases, the Finnish Medical Foundation, Tampere Tuberculosis Foundation, the Juho Vainio Foundation, and the Foundation for Paediatric Research.
