Report on influenza viruses received and tested by the Melbourne WHO Collaborating Centre for Reference and Research on Influenza during 2025.

As part of its role in the World Health Organization (WHO) Global Influenza Surveillance and Response System (GISRS), the WHO Collaborating Centre for Reference and Research on Influenza in Melbourne (the Centre) received 13,817 human influenza-positive samples during 2025. Viruses were analysed for their antigenic, genetic, and antiviral susceptibility properties. Selected viruses were propagated in qualified cells or embryonated hens' eggs for potential use in seasonal influenza virus vaccines. Of the 13,817 samples received or processed, influenza A(H1N1)pdm09 viruses predominated, accounting for 46.1% of samples, compared to 21.2% for A(H3N2) viruses and 19.5% for influenza B viruses; one influenza C virus was received. Among viruses analysed at the Centre, the majority of A(H1N1)pdm09 (> 99%) and influenza B (98%) viruses were antigenically similar to their respective WHO recommended vaccine strains for the Southern Hemisphere in 2025. In contrast, only 43% of A(H3N2) viruses were antigenically similar to their respective WHO recommended vaccine strains. Of 3,307 samples tested for susceptibility to the neuraminidase inhibitors oseltamivir and zanamivir, 37 A(H1N1)pdm09 viruses showed highly reduced inhibition by oseltamivir and no influenza viruses tested showed highly reduced inhibition by zanamivir. Of 5,080 samples with sequencing of the polymerase acidic (PA) gene, no genetic markers associated with highly reduced susceptibility to baloxavir marboxil were identified.