Influenza causes seasonal epidemics in temperate regions of Australia, with peak activity occurring in winter-spring seasons (July-September). Here we analyse influenza viruses that have been circulating in unusually high numbers Australia-wide during the most recent inter-seasonal period (Dec 2018- April 2019)
We analysed influenza positive samples sent to the WHO Collaborating Centre for Influenza (WHO CC) and notifications of laboratory-confirmed influenza from the Australian National Notifiable Diseases Surveillance System (NNDSS).
During the influenza inter-seasonal period (December 2018-April 2019), the WHO CC received 1890 samples compared to the combined inter-seasonal periods of the previous three years, of 1669 samples. This data was reflected in the NNDSS reporting of laboratory-confirmed cases of influenza, with 35,257 (to 15 April 2019) cases reported in the current inter-seasonal period, compared to 49,275 for the previous three years combined. The current inter-seasonal period has been dominated by the A(H1N1)pdm virus, with 63% of viruses analysed here belonging to this subtype, followed by A(H3N2) (31%), and influenza B (5%). Antigenically, the A(H1N1)pdm viruses were similar to the virus included in the 2019 southern hemisphere vaccine though there was considerable genetic heterogeneity. The A(H3N2) viruses also fell into several distinct genetic clades, with the predominant subclade being 3C2a1b with a 131K substitution in the HA, similar to A(H3) viruses circulating in Japan and China during this period.
Summer influenza activity in Australia has been at record levels in 2018-19, with both A(H3N2) and A(H1N1)pdm09 viruses co-circulating. The causes of this dramatic increase in influenza activity are unclear at this stage but may be related to a late epidemic of H1N1pdm09 in Northern Australia in 2018 combined with a late and extended influenza 2018-9 season in Asia and elsewhere, that may have resulted in A(H3N2)-infected tourists returning to Australia.