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Journal Article Ppublish

PubMed: 40528677 DOI: 10.1111/irv.70131

Influenza Epidemiology in Finland During and After the COVID-19 Pandemic: Surveillance Data Analysis (2019-2024).

Influenza Other Respir Viruses 19(6):e70131

Authors: Ulrike Baum, Niina Ikonen, Oskari Luomala, Eero Poukka, Tuija Leino, Hanna Nohynek

Affiliations

  • Department of Public Health, Finnish Institute for Health and Welfare, Helsinki, Finland.
  • Department of Public Health, Faculty of Medicine, University of Helsinki, Helsinki, Finland.

Abstract

BACKGROUND

The Finnish influenza surveillance system combines traditional virological surveillance and analyses of electronic health records. This paper describes the influenza epidemiology in Finland (population: 5.5 million) during and after the COVID-19 pandemic based on national surveillance data from 2019 to 2024.

METHODS

Influenza incidence was evaluated based on three register-based outcomes: laboratory-confirmed infections, primary health care visits, and hospitalizations. Virus-type distributions were analyzed from respiratory specimens. In register-based analyses, vaccination coverage and vaccine effectiveness were assessed for the two cohorts universally included in the Finnish vaccination program: children aged ≤ 6 years and adults aged ≥ 65 years.

RESULTS

The 2019/2020 influenza epidemic ended with the introduction of COVID-19 containment measures. In 2020/2021, influenza was largely absent. The 2021/2022 epidemic peaked exceptionally late. Influenza activity returned to prepandemic levels in 2022/2023. None of the 717 sentinel specimens tested positive for B/Yamagata. Although the percentage of vaccinated young children was constant (31% [100,387/323,614] to 37% [126,984/346,344]), the percentage of vaccinated elderly people increased from 48% (577,404/1,211,732) in 2019/2020 to 63% (787,771/1,255,644) in 2021/2022. The vaccine effectiveness against hospitalization due to laboratory-confirmed influenza in young children and elderly people was 68% (95% confidence interval: 38%; 83%) and 42% (34%; 50%) in 2022/2023, respectively, and slightly lower in 2023/2024.

CONCLUSIONS

The COVID-19 pandemic had two potentially lasting effects on influenza: elimination of the B/Yamagata lineage and improved vaccination coverage in the elderly population in Finland. To strengthen the Finnish influenza surveillance system, participation in sentinel surveillance must be improved.

MeSH

Epidemiology, Laboratories, Hospitalization, Vaccines, Vaccination, Aged, Covid-19, Influenza, Human, Influenza Vaccines, Pandemics, Data Analysis, Vaccination Coverage, Epidemics, Child

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