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. 2025 Sep;30(36):2500658.
doi: 10.2807/1560-7917.ES.2025.30.36.2500658.

An interseasonal outbreak of influenza A(H1N1)pdm09 related to a music festival, Denmark, August 2025

Amanda Bolt Botnen  1 , Hanne-Dorthe Emborg  2 , Casper Thorup  1 , Jesper Krog  1 , Sofia Myrup Otero  3 , Stine Nielsen  2 , Bolette Søborg  2 , Ramona Trebbien  1
Affiliations

An interseasonal outbreak of influenza A(H1N1)pdm09 related to a music festival, Denmark, August 2025

Amanda Bolt Botnen et al. Euro Surveill. 2025 Sep.

Abstract

In week 33, 2025, the integrated respiratory virus surveillance at Statens Serum Institut, Denmark, detected an atypical sixfold increase in influenza A(H1N1)pdm09 cases. Rapid sequencing of viruses collected in week 33 identified a highly related cluster of 17 cases. Telephone interviews with patients belonging to this cluster revealed that they all had attended a large open-air music festival. This is the first time an influenza outbreak has been detected during the summer in Denmark related to a large gathering.

Keywords: influenza; integrated surveillance; outbreak, festival, gathering.

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Conflict of interest statement

Conflict of interest: None declared.

Figures

Both line graphs show a peak around week 23 and a sharp rise from week 32.
Figure 1
National summary of detected influenza cases and hospital admissions for influenza, Denmark, from week 21, 2025
Column graph showing an increase to >50 cases in weeks 33 and 34 from a baseline of <20 cases in the weeks before. The vast majority of cases are influenza H1N1(pdm09).
Figure 2
Subtype distribution of influenza A and B, received at Statens Serum Institut, Denmark, since week 21, 2025 (n = 225)
Two phylogenetic trees showing the outbreak sequences clustering on a single unique branch.
Figure 3
Phylogenetic tree of influenza H1 and N1 segments in the national influenza surveillance, Denmark, 2024/25 season (n = 29 outbreak sequences)
See this image and copyright information in PMC

References

    1. Statens Serum Institut (SSI). Overvågning af influenza, covid-19, RS-virus og andre luftvejssygdomme. [Surveillance of influenza, COVID-19, RSV and other respiratory diseases]. Copenhagen: SSI. [Accessed: 25 Aug 2025]. Danish. Available from: https://www.ssi.dk/sygdomme-beredskab-og-forskning/overvaagning-af-luftv...
    1. Gaillat J, Dennetière G, Raffin-Bru E, Valette M, Blanc MC. Summer influenza outbreak in a home for the elderly: application of preventive measures. J Hosp Infect. 2008;70(3):272-7. 10.1016/j.jhin.2008.07.009 - DOI - PubMed
    1. Barr IG, Deng YM, Grau ML, Han AX, Gilmour R, Irwin M, et al. Intense interseasonal influenza outbreaks, Australia, 2018/19. Euro Surveill. 2019;24(33):1900421. 10.2807/1560-7917.ES.2019.24.33.1900421 - DOI - PMC - PubMed
    1. Smukfest. Om festivalen. [Smukfest. About the festival]. Skanderborg: Skanderborg Festivalklub. [Accessed: 26 Aug 2025]. Danish. Available from: https://www.smukfest.dk/festivalen
    1. Teicher A. Super-spreaders: a historical review. Lancet Infect Dis. 2023;23(10):e409-17. 10.1016/S1473-3099(23)00183-4 - DOI - PubMed

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