Severity of the 2019 influenza season in Australia- a comparison between 2017 and 2019 H3N2 influenza seasons

Background: Annual seasonal influenza infection is unpredictable, and varies from season to season.  Australia experienced a severe season in 2017, followed by a mild season in 2018.  In 2019, it featured unusually high inter-seasonal activity followed by a severe H3N2 season, proclaimed by the media as “the worst ever”. Following a severe season in 2017, enhanced vaccines were introduced in 2018 for people aged >65 years. Aim: To compare the seasonal severity of the 2017 and 2019 influenza season. Methods: Data were collated from the National Notifiable Diseases Surveillance System (NNDSS) report. Epidemiological trends of seasonal influenza infection were compared between the two years 2017 and 2019. Reported hospitalisations, ICU admissions and deaths were obtained from published sources and compared. Results: A(H3N2) dominated in both years. There was an early start to influenza circulation in 2019 with an unusually high number of cases seen during summer. There was no significant difference in the number of influenza hospitalisations or deaths reported in 2019 and 2017, but the proportion of hospitalisations that were admitted to ICU was significantly lower in 2019 (6.4% vs 8.9%, p Conclusion: Both 2017 and 2019 were H3N2 seasons with large numbers of infections. Higher numbers of notifications to NNDSS were observed in 2019, probably due to an unusually high level of summer activity and increased testing. Clinical severity was estimated as similar to or less in 2019 compared to 2017. Estimated VE was higher in 2019 compared to 2017, and suggests a benefit of enhanced vaccines.