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.
Botnen et al. (Mon,) studied this question.