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

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.