The epidemiology of influenza during the COVID-19 pandemic in Finland --
A summary and interpretation of surveillance data from 2019 to 2024
Abstract
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.