Social restrictions set due to COVID‐19 interrupted the circulation of influenza in March 2020 in Finland and globally. 1 , 2 , 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 , 9 , 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.
We included a total of 151,833 influenza detections from July 2010 to June 2022. Of these 110,742 (72.9%) were influenza A and 41,091 (27.1%) influenza B. During the study period, the highest recorded monthly peak incidence of influenza A was in January 2017 among patients aged 70 or more, whereas the highest recorded peak of influenza B was in January 2018 in both children and elderly (Figure 1). Influenza causes yearly epidemics in Finland, whereas influenza B epidemic do not occur every year. The reported peak incidence in 2021–2022 season was in May 2022 which was unusually late. Of the previous 26 influenza seasons, the epidemic peak has once been in November, twice in December, four times in January, 14 times in February, and five times in March. The epidemic peak in 2022 was caused by influenza A, whereas influenza B was practically undetected (Figure 1). The influenza epidemic peak occurred simultaneously in all age groups, and the highest peak was detected in children (117.3 per 100,000).
FIGURE 1.
Monthly incidences of laboratory confirmed influenza A and B findings per 100,000 stratified by age (0–14, 15–69, and 70 or more years) in Finland from July 2010 to June 2022
The ending of social restrictions led to normal timing of influenza season beginning in Finland. However, the restrictions implemented due to Omicron in January 2022 interrupted the epidemic. Relaxation of restrictions in March 2022 lead to record late peak of influenza A in May 2022, whereas the influenza B was undetected in spring.
Influenza A returned to viral circulation for winter season 2021–2022 in Europe. 12 Similar late influenza season was reported from Denmark, where the incidence peaked after the restrictions were ended. 13 Studies have reported increased influenza vaccination coverage during the pandemic. 14 Influenza vaccination coverage in Finland in season 2021–2022 was reported to be similar to previous five seasons in children but records high in people aged 65 or more (+20% increase). The vaccinations were given in November, so the time from the vaccination to epidemic peak was longer than previously, which might have influenced the influenza detection rates. However, the current epidemic dynamic in Finland was most likely due to social restrictions rather than vaccination coverage or effectiveness.
Our main limitation is the lack of testing numbers as it would have enabled to conduct test‐negativity analyses. Furthermore, we lack information on hospitalization rates and individual vaccination data, as these data are not available prior 2023.
The ending and reimplementation of social restriction during influenza season 2021–2022 affected the epidemic as the start of the epidemic in December 2021 was normal, but it was interrupted by the social restrictions in January 2022. The ending of social restriction in March 2022 lead to record late influenza A season in Finland. Our results highlight the importance of continuous epidemiological surveillance of respiratory pathogens.
AUTHOR CONTRIBUTIONS
Ilari Kuitunen: Conceptualization; formal analysis; investigation; methodology; validation; visualization. Marjo Renko: Conceptualization; funding acquisition; investigation; methodology; project administration; resources; software; supervision. Terhi Tapiainen: Conceptualization; data curation; funding acquisition; project administration; resources; software; supervision.
CONFLICT OF INTEREST
None to report.
PEER REVIEW
The peer review history for this article is available at https://publons.com/publon/10.1111/irv.13036.
ACKNOWLEDGMENT
None.
Funding information
None.
DATA AVAILABILITY STATEMENT
Data are available per request from the corresponding author or alternatively can be downloaded straight from the open access repository of the Finnish Insitute of Health and Welfare (database in Finnish). https://sampo.thl.fi/pivot/prod/fi/ttr/shp/fact_shp%E2%80%9Ccd_value_code%3D%E2%80%9Dtext.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
Data are available per request from the corresponding author or alternatively can be downloaded straight from the open access repository of the Finnish Insitute of Health and Welfare (database in Finnish). https://sampo.thl.fi/pivot/prod/fi/ttr/shp/fact_shp%E2%80%9Ccd_value_code%3D%E2%80%9Dtext.