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. 2024 Jan;29(4):2300263.
doi: 10.2807/1560-7917.ES.2024.29.4.2300263.

Age-dependent influenza infection patterns and subtype circulation in Denmark, in seasons 2015/16 to 2021/22

Hanne-Dorthe Emborg  1 , Amanda Bolt Botnen  2 , Jens Nielsen  1 , Lasse S Vestergaard  1 , Frederikke Kristensen Lomholt  1 , Charlotte Munkstrup  1 , Karina Lauenborg Møller  3 , Charlotte Kjelsø  1 , Steen Hulthin Rasmussen  3 , Ramona Trebbien  2
Affiliations

Age-dependent influenza infection patterns and subtype circulation in Denmark, in seasons 2015/16 to 2021/22

Hanne-Dorthe Emborg et al. Euro Surveill. 2024 Jan.

Abstract

BackgroundInfluenza was almost absent for 2 years following the implementation of strict public health measures to prevent the spread of SARS-CoV-2. The consequence of this on infections in different age groups is not yet known.AimTo describe the age groups infected with the influenza virus in 2021/22, the first post-pandemic influenza season in Denmark, compared with the previous six seasons, and subtypes circulating therein.MethodsInfection and hospitalisation incidences per season and age group were estimated from data in Danish registries. Influenza virus subtypes and lineages were available from samples sent to the National Influenza Centre at Statens Serum Institut.ResultsTest incidence followed a similar pattern in all seasons, being highest in 0-1-year-olds and individuals over 75 years, and lowest in 7-14-year-olds and young people 15 years to late twenties. When the influenza A virus subtypes A(H3N2) and A(H1N1)pdm09 co-circulated in seasons 2015/16 and 2017/18 to 2019/20, the proportion of A(H1N1)pdm09 was higher in 0-1-year-olds and lower in the over 85-year-olds compared with the overall proportion of A(H1N1)pdm09 in these seasons. The proportion of A(H3N2) was higher in the over 85 years age group compared with the overall proportion of A(H3N2). The 2016/17 and 2021/22 seasons were dominated by A(H3N2) but differed in age-specific trends, with the over 85 years age group initiating the 2016/17 season, while the 2021/22 season was initiated by the 15-25-year-olds, followed by 7-14-year-olds.ConclusionThe 2021/22 influenza season had a different age distribution compared with pre-COVID-19 pandemic seasons.

Keywords: Influenza surveillance; age distribution; hospitalisation incidence; infection incidence; influenza subtypes and lineages.

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Conflict of interest statement

Conflict of interest: None declared.

Figures

Figure 1
Figure 1
Influenza (A) test incidence by age in years, (B) infection incidence by age in years and (C) weekly test incidence per age group during the 2021/22 season, Denmark, influenza seasons 2015/16–2021/22
Figure 2
Figure 2
Number of individuals with influenza A(H3N2), A(H1N1)pdm09, B/Victoria and B/Yamagata infections by age group during (A) the 2016/17 and 2021/22 influenza seasons, (B) the 2015/16 and 2017/18 influenza seasons and (C) the 2018/19 and 2019/20 influenza seasons, Denmark
Figure 3
Figure 3
Combined influenza A and B virus infection incidence per 10,000 individuals per week and age group during (A) the 2016/17 influenza season and (B) the 2021/22 influenza season, Denmark
Figure 4
Figure 4
Combined influenza A and B virus infection incidence per 10,000 per week and age group during (A) the 2015/16 influenza season (B) the 2017/18 influenza season, (C) the 2018/19 influenza season and (D) the 2019/20 influenza season, Denmark
Figure 5
Figure 5
Combined influenza A virus and influenza B virus hospitalisation incidence per 10,000 individuals per week and age group during (A) the 2015/16 influenza season, (B) the 2016/17 influenza season, (C) the 2017/18 influenza season, (D) the 2018/19 influenza season, (E) the 2019/20 influenza season and (F) the 2021/22 influenza season, Denmark
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References

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