Surveillance Open Access
Like 0



Influenza 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.


To 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.


Infection 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.


Test 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.


The 2021/22 influenza season had a different age distribution compared with pre-COVID-19 pandemic seasons.


Article metrics loading...

Loading full text...

Full text loading...



  1. World Health Organization (WHO). Comparison of number of influenza detections by subtype. Geneva: WHO. [Accessed: 4 May 2023]. Available from: https://app.powerbi.com/view?r=eyJrIjoiZTIxMzAwMzYtZWE4NC00YTU2LWE3MTUtMTI0OGY1ZjQyMWViIiwidCI6ImY2MTBjMGI3LWJkMjQtNGIzOS04MTBiLTNkYzI4MGFmYjU5MCIsImMiOjh9
  2. Emborg HD, Carnahan A, Bragstad K, Trebbien R, Brytting M, Hungnes O, et al. Abrupt termination of the 2019/20 influenza season following preventive measures against COVID-19 in Denmark, Norway and Sweden. Euro Surveill. 2021;26(22):2001160.  https://doi.org/10.2807/1560-7917.ES.2021.26.22.2001160  PMID: 34085632 
  3. Emborg HD, Vestergaard LS, Botnen AB, Nielsen J, Krause TG, Trebbien R. A late sharp increase in influenza detections and low interim vaccine effectiveness against the circulating A(H3N2) strain, Denmark, 2021/22 influenza season up to 25 March 2022. Euro Surveill. 2022;27(15):2200278.  https://doi.org/10.2807/1560-7917.ES.2022.27.15.2200278  PMID: 35426361 
  4. Nielsen RT, Dalby T, Emborg HD, Larsen AR, Petersen A, Torpdahl M, et al. COVID-19 preventive measures coincided with a marked decline in other infectious diseases in Denmark, spring 2020. Epidemiol Infect. 2022;150:e138.  https://doi.org/10.1017/S0950268822001145  PMID: 35899864 
  5. Fricke LM, Glöckner S, Dreier M, Lange B. Impact of non-pharmaceutical interventions targeted at COVID-19 pandemic on influenza burden - a systematic review. J Infect. 2021;82(1):1-35.  https://doi.org/10.1016/j.jinf.2020.11.039  PMID: 33278399 
  6. Munkstrup C, Lomholt FK, Emborg HD, Møller KL, Krog JS, Trebbien R, et al. Early and intense epidemic of respiratory syncytial virus (RSV) in Denmark, August to December 2022. Euro Surveill. 2023;28(1):2200937.  https://doi.org/10.2807/1560-7917.ES.2023.28.1.2200937  PMID: 36695451 
  7. van Summeren J, Meijer A, Aspelund G, Casalegno JS, Erna G, Hoang U, et al. Low levels of respiratory syncytial virus activity in Europe during the 2020/21 season: what can we expect in the coming summer and autumn/winter? Euro Surveill. 2021;26(29):2100639.  https://doi.org/10.2807/1560-7917.ES.2021.26.29.2100639  PMID: 34296672 
  8. Bermúdez Barrezueta L, Matías Del Pozo V, López-Casillas P, Brezmes Raposo M, Gutiérrez Zamorano M, Pino Vázquez MA. Variation in the seasonality of the respiratory syncytial virus during the COVID-19 pandemic. Infection. 2022;50(4):1001-5.  https://doi.org/10.1007/s15010-022-01794-y  PMID: 35316529 
  9. Cohen R, Ashman M, Taha MK, Varon E, Angoulvant F, Levy C, et al. Pediatric Infectious Disease Group (GPIP) position paper on the immune debt of the COVID-19 pandemic in childhood, how can we fill the immunity gap? Infect Dis Now. 2021;51(5):418-23.  https://doi.org/10.1016/j.idnow.2021.05.004  PMID: 33991720 
  10. Ali ST, Lau YC, Shan S, Ryu S, Du Z, Wang L, et al. Prediction of upcoming global infection burden of influenza seasons after relaxation of public health and social measures during the COVID-19 pandemic: a modelling study. Lancet Glob Health. 2022;10(11):e1612-22.  https://doi.org/10.1016/S2214-109X(22)00358-8  PMID: 36240828 
  11. Turbelin C, Souty C, Pelat C, Hanslik T, Sarazin M, Blanchon T, et al. Age distribution of influenza like illness cases during post-pandemic A(H3N2): comparison with the twelve previous seasons, in France. PLoS One. 2013;8(6):e65919.  https://doi.org/10.1371/journal.pone.0065919  PMID: 23755294 
  12. Park JE, Ryu Y. Transmissibility and severity of influenza virus by subtype. Infect Genet Evol. 2018;65:288-92.  https://doi.org/10.1016/j.meegid.2018.08.007  PMID: 30103034 
  13. Segaloff H, Melidou A, Adlhoch C, Pereyaslov D, Robesyn E, Penttinen P, et al. Co-circulation of influenza A(H1N1)pdm09 and influenza A(H3N2) viruses, World Health Organization (WHO) European Region, October 2018 to February 2019. Euro Surveill. 2019;24(9):1-8.  https://doi.org/10.2807/1560-7917.ES.2019.24.9.1900125  PMID: 30862331 
  14. Caini S, Spreeuwenberg P, Kusznierz GF, Rudi JM, Owen R, Pennington K, et al. Distribution of influenza virus types by age using case-based global surveillance data from twenty-nine countries, 1999-2014. BMC Infect Dis. 2018;18(1):269.  https://doi.org/10.1186/s12879-018-3181-y  PMID: 29884140 
  15. Rosu ME, Lexmond P, Bestebroer TM, Hauser BM, Smith DJ, Herfst S, et al. Substitutions near the HA receptor binding site explain the origin and major antigenic change of the B/Victoria and B/Yamagata lineages. Proc Natl Acad Sci USA. 2022;119(42):e2211616119.  https://doi.org/10.1073/pnas.2211616119  PMID: 36215486 
  16. Benedetti G, Krause TG, Schneider UV, Lisby JG, Voldstedlund M, Bang D, et al. Spotlight influenza: Influenza surveillance before and after the introduction of point-of-care testing in Denmark, season 2014/15 to 2018/19. Euro Surveill. 2021;26(37):2000724.  https://doi.org/10.2807/1560-7917.ES.2021.26.37.2000724  PMID: 34533117 
  17. World Health Organization (WHO) and European Centre for Disease Prevention and Control (ECDC). Operational considerations for influenza surveillance in the WHO European Region during COVID-19: interim guidance. Stockholm: ECDC; 16 Oct 2020. Available from: https://www.ecdc.europa.eu/en/publications-data/operational-considerations-influenza-surveillance-european-region-during-covid-19
  18. Voldstedlund M, Haarh M, Mølbak K, MiBa Board of Representatives. The Danish Microbiology Database (MiBa) 2010 to 2013. Euro Surveill. 2014;19(1):20667.  https://doi.org/10.2807/1560-7917.ES2014.19.1.20667  PMID: 24434175 
  19. Schmidt M, Pedersen L, Sørensen HT. The Danish Civil Registration System as a tool in epidemiology. Eur J Epidemiol. 2014;29(8):541-9.  https://doi.org/10.1007/s10654-014-9930-3  PMID: 24965263 
  20. Lynge E, Sandegaard JL, Rebolj M. The Danish national patient register. Scand J Public Health. 2011;39(7) Suppl;30-3.  https://doi.org/10.1177/1403494811401482  PMID: 21775347 
  21. Statistics Denmark. Borgare. [People]. Copenhagen: Statistics Denmark. [Accessed: 1 Jan 2024]. Danish. Available from: https://www.statistikbanken.dk/statbank5a/default.asp?w=2195
  22. Lomholt FK, Emborg HD, Nørgaard S, Nielsen J, Munkstrup C, Møller KL, et al. Out-of-season epidemic of respiratory syncytial virus in Denmark in the summer/autumn of 2021, with more cases and admissions than seen in previous winter seasons and a shift in affected age groups towards older children aged 2-5 years. EMJ Microbiol Infect Dis. 2023;4(1):51-3.  https://doi.org/10.33590/emjmicrobiolinfectdis/10309487 
  23. Munch PK, Espenhain L, Hansen CH, Müller L, Krause TG, Ethelberg S. Societal activities associated with SARS-CoV-2 infection: a case-control study in Denmark, November 2020. Epidemiol Infect. 2021;150:e9.  https://doi.org/10.1017/S0950268821002478  PMID: 34784997 
  24. Melidou A, Ködmön C, Nahapetyan K, Kraus A, Alm E, Adlhoch C, et al. Influenza returns with a season dominated by clade 3C.2a1b.2a.2 A(H3N2) viruses, WHO European Region, 2021/22. Euro Surveill. 2022;27(15):2200255.  https://doi.org/10.2807/1560-7917.ES.2022.27.15.2200255  PMID: 35426364 
  25. Steel J, Staeheli P, Mubareka S, García-Sastre A, Palese P, Lowen AC. Transmission of pandemic H1N1 influenza virus and impact of prior exposure to seasonal strains or interferon treatment. J Virol. 2010;84(1):21-6.  https://doi.org/10.1128/JVI.01732-09  PMID: 19828604 
  26. Ang LW, Cui L, Mak TM, Ng Y, Leo YS, Lee VJM, et al. Differential age-specific distribution of influenza virus types and subtypes in tropical Singapore, 2011 to 2017. J Med Virol. 2019;91(8):1415-22.  https://doi.org/10.1002/jmv.25473  PMID: 30927452 
  27. Kelvin AA, Zambon M. Influenza imprinting in childhood and the influence on vaccine response later in life. Euro Surveill. 2019;24(48):1900720.  https://doi.org/10.2807/1560-7917.ES.2019.24.48.1900720  PMID: 31796156 
  28. Emborg HD, Vestergaard LS, Botnen AB, Nielsen J, Krause TG, Trebbien R. A late sharp increase in influenza detections and low interim vaccine effectiveness against the circulating A(H3N2) strain, Denmark, 2021/22 influenza season up to 25 March 2022. Euro Surveill. 2022;27(15):2200278.  https://doi.org/10.2807/1560-7917.ES.2022.27.15.2200278  PMID: 35426361 
  29. Statens Serum Institut (SSI). Influenzasæsonen - opgørelse over sygdomsforekomst 2021/22. [Influenza season – statement of disease incidence 2021/22]. Copenhagen: SSI; 2021. [Accessed: 1 Jan 2024]. Danish. Available from: https://www.ssi.dk/sygdomme-beredskab-og-forskning/sygdomsovervaagning/i/influenzasaesonen---opgoerelse-over-sygdomsforekomst-2021-22
  30. Kissling E, Rose A, Emborg HD, Gherasim A, Pebody R, Pozo F, et al. Interim 2018/19 influenza vaccine effectiveness: six European studies, October 2018 to January 2019. Euro Surveill. 2019;24(8):1900121.  https://doi.org/10.2807/1560-7917.ES.2019.24.1900121  PMID: 30808440 
  31. Rose A, Kissling E, Emborg HD, Larrauri A, McMenamin J, Pozo F, et al. Interim 2019/20 influenza vaccine effectiveness: six European studies, September 2019 to January 2020. Euro Surveill. 2020;25(10):2000153.  https://doi.org/10.2807/1560-7917.ES.2020.25.10.2000153  PMID: 32183932 

Data & Media loading...

Supplementary data

Submit comment
Comment moderation successfully completed
This is a required field
Please enter a valid email address
Approval was a Success
Invalid data
An Error Occurred
Approval was partially successful, following selected items could not be processed due to error