Social restrictions set due to COVID-19 interrupted the circulation of influenza in March 2020 in Finland and globally.^1-3 Finland had lockdown from March 2020 to May 2020 and since then, Finland has had regional restrictions and traveling restrictions based on the COVID-19 incidences. These actions prevented the influenza season 2020–2021 completely, and only few cases were detected in Finland and Northern Europe.^4, 5

As the COVID-19 vaccination coverage increased in Finland, the social restrictions were ended in September 2021. This was seen as unforeseen high epidemic peak of parainfluenza, and the respiratory syncytial virus season came unusually early.^6, 7 Many countries have reported similar findings of atypical epidemics after the ending of social restrictions.^8-10 Although the restrictions were ended in September 2021, the arrival of Omicron in December 2021 lead to restrictions set back to use in January 2022.¹¹ As the omicron wave started to decrease, the restrictions were again loosened in March 2022. The aim of our study was to analyze the influenza season 2021–2022 and how the ending and reimplementation of social restrictions affected the epidemic.

We conducted a nationwide retrospective surveillance study. We included all influenza A and B detections from the Finnish National Infectious Diseases Register. The register holds information on notifiable pathogen findings, and all Finnish laboratories are mandated by the law of contagious diseases to report the pathogen findings without delays to register. Therefore, the register has high validity, excellent coverage, and it provides timely information. Study period was from July 1996 to June 2022, but we focused for the period of July 2010 to June 2022 more precisely. The month of the epidemic peak was gathered for each year. We used age-stratification 0–14 years, 15–69 years, and 70 or more years. We calculated monthly incidence per 100,000 in each age group. Population size for each year was retrieved from the open-access population statistics provided by the Statistics Finland. Influenza epidemic season 2021–2022 was compared with the mean of the 10 previous pre-pandemic seasons (2010–2020). Season 2020–2021 was not analyzed as it has been reported previously.⁴ Due to the register-based study design, we did not need ethical committee evaluation.

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