No 25 - 2023

Influenza and other respiratory viruses in the 2022/23 season

The 2022/23 influenza season was long and recorded three waves. The season started in December 2022 and ended in April 2023 and was dominated by influenza virus B/Victoria and influenza virus A(H1N1) with some circulation of influenza virus A(H3N2).

This week’s EPI-NEWS summarises the most important details from the current influenza season, whereas a more comprehensive description of the influenza occurrence is available in the 2022/23 seasonal influenza report.

Data from the Danish Microbiology Database (MiBa), which records all laboratory-confirmed cases, showed that the entire season was characterised by co-circulation of influenza A and influenza B. For influenza A, the A(H1N1) subtype was dominant, but with co-circulation of A(H3N2). Only one of the two influenza B types (B/Victoria) circulated. The highest occurrence of influenza A infections was detected among 0-1-year-olds, who are more frequently affected by A(H1N1), followed by the oldest citizens aged 85 years or more, who are typically affected by A(H3N2). Influenza B/Victoria was primarily detected among persons below 50 years of age, especially in the age groups 0-1 years and 7-14 years. In the 2022/23 season, the number of laboratory-confirmed cases was 27% higher than in the 2021/22 season, whereas the number of hospital admissions was only 8% higher. The number of influenza-related deaths was similar for the two seasons. This may be explained by the fact that the influenza subtypes in circulation primarily affected the younger part of the population.

In the sentinel surveillance system, general practitioners (GPs) report influenza-like illnesses among their patients. The sentinel surveillance system revealed a three-wave pattern in patients who contacted the GPs with influenza-like symptoms. In contrast, the Influmeter monitoring system, which involves voluntary weekly symptom reporting by citizens, only captured the first and the last influenza wave, which was anticipated as Influmeter participants are adult citizens who were less affected by the B/Victoria wave in February.

In addition to reporting the number of patients with influenza-like symptoms, the sentinel surveillance system also comprises testing of swabs from a subset of patients. The swabs are tested for a range of respiratory viruses that may also cause influenza-like symptoms. In the beginning of the season, rhinovirus was the predominant pathogen detected among the patients. The RSV season started late in the summer of 2022, which is atypical compared with the preceding seasons before COVID-19 but in line with the onset of the 2021 season, which already started in late summer. The 2022/23 RSV season was prolonged and caused a larger number of admissions and deaths than seen in previous seasons.
see EPI-NEWS 35/2021. Influenza gained footing in December 2022 and continued through to April 2023.

Statens Serum Institut has developed an interactive dashboard for monitoring of influenza and RSV. The two dashboards follow influenza and RSV activity year-round, and alsos presents data from previous seasons.

(The SSI Influenza Team: H.-D. Emborg, C.L. Munkstrup, L.K. Knudsen, C. Kjelsø, L.S. Vestergaard, F.K. Lomholt, J. Nielsen, H. Bang, Infectious Disease Epidemiology and Prevention, R. Trebbien, C.K. Hjulsager, A.B. Botnen, J.S. Krog, Department for Virus and Microbiological Special Diagnostics)