Report on influenza viruses received and tested by the Melbourne WHO Collaborating Centre for Reference and Research on Influenza during 2023.

As part of its role in the World Health Organization's (WHO) Global Influenza Surveillance and Response System (GISRS), the WHO Collaborating Centre for Reference and Research on Influenza in Melbourne received a record 15,014 human influenza-positive samples during 2023. 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. During 2023, influenza A(H1N1)pdm09 and influenza B/Victoria viruses predominated, accounting for 37% and 28% respectively of all viruses received, compared to 12% for influenza A(H3N2). The majority of A(H1N1)pdm09, A(H3N2) and influenza B viruses analysed at the Centre were found to be antigenically and genetically similar to the respective WHO recommended vaccine strains for the southern hemisphere in 2023. Of 5,531 samples tested for susceptibility to the neuraminidase inhibitors oseltamivir and zanamivir, seven A(H1N1)pdm09 viruses showed highly reduced inhibition against oseltamivir.