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Ulrike Baum

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The Finnish influenza surveillance system monitors the influenza prevalence, disease burden, vaccine uptake and effectiveness. Since the start of the COVID-19 pandemic the publication of the yearly surveillance report has been paused. Therefore, this paper summarizes the past five seasons based on surveillance data from 2019 to 2024 and describes the epidemiology of influenza during the COVID-19 pandemic in Finland. Clinical surveillance showed that the 2019/20 epidemic ended concomitantly with the introduction of COVID-19 containment measures in March 2020, in 2020/21 influenza was absent, and the 2021/22 epidemic peaked exceptionally late. Influenza activity has roughly returned to pre-pandemic levels since 2022/23, when there were 13,728 and 2,172 laboratory-confirmed influenza A and influenza B cases, 1,485 and 179 of which were hospitalized due to influenza. Virological surveillance confirmed that circulation of the influenza B/Yamagata lineage had ceased. One to two million influenza vaccinations were given each season. While the percentage of vaccinated children younger than seven years was constant (ranging from 31% to 37%), the percentage of vaccinated people aged 65 years and above increased from 48% in 2019/20 to 63% in 2021/22 and has remained moderately high. 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 2022/23 and slightly lower in 2023/24. The COVID-19 pandemic had two hopefully lasting effects on the epidemiology of influenza: elimination of the influenza B/Yamagata lineage and improved influenza vaccination coverage among the elderly population in Finland.