Report of the National Influenza Surveillance Scheme, 2022-2023

This report describes influenza surveillance activities in Australia in 2022 to 2023. Data were extracted from several sources constituting the National Influenza Surveillance Scheme. Following the relaxation of public health and social measures which had been applied in Australia in response to the coronavirus disease 2019 (COVID-19) pandemic in 2020 and 2021, the influenza seasons in 2022 and 2023 were characterised by high activity overall, with 1.4 and 1.8 times the number of laboratory-confirmed influenza notifications than those of the 2015-2019 pre-pandemic five-year mean, respectively. There were differences in the timing of the 2022 and 2023 influenza seasons compared to most pre-pandemic years, with earlier season starts and earlier peaks (approximately six to eight weeks earlier than in pre-pandemic years). There was also considerable variation in the timing of peak influenza activity between the states and territories, particularly in 2023. Notification rates were highest in the 5-9 years age group, with rates in this age group about two and a half times as high in 2022 and three times as high in 2023 as the overall notification rate in the corresponding years. Influenza A(unsubtyped) comprised most notifications in 2022 and 2023. Of the remaining samples, influenza A(H3N2) was dominant in 2022; however, in 2023, there was substantial circulation of influenza B, which predominantly affected children and younger adults, and of A(H1N1). Sentinel surveillance supports the assessment that both the 2022 and 2023 influenza seasons were characterised by early, high activity. High notification rates were also reflected in increased sentinel hospital admissions with confirmed influenza in 2023, particularly among children. The burden placed on sentinel hospitals due to people admitted with influenza was moderate in 2022 but high in 2023. Severity, measured through the proportion of patients admitted to sentinel hospital intensive care units, was moderate in both years and consistent with historical estimates. Influenza was recorded as the underlying cause of death in 0.16% of all deaths that occurred in 2022, and 0.24% of all deaths in 2023. Overall, the 2022 influenza season had high activity with low-to-moderate severity, while 2023 had high activity with moderate severity.