Evidence for an increase in the intensity of inter‐seasonal influenza, Queensland, Australia, 2009‐2019

BACKGROUND: Inter‐seasonal influenza cases have been increasing in Australia. Studies of influenza seasonality typically focus on seasonal transmission in temperate regions, leaving our understanding of inter‐seasonal epidemiology limited. We aimed to improve understanding of influenza epidemiology during inter‐seasonal periods across climate zones, and explored influenza intensity and strain dominance patterns over time. METHODS: Queensland state‐wide laboratory‐confirmed influenza notifications and public laboratory influenza test data from 2009‐2019 were described by demographics, time period, region and strain type. We compared influenza intensity over time using the WHO Average Curve method to provide thresholds for seasonal and inter‐seasonal periods. RESULTS: Among the 243 830 influenza notifications and 490 772 laboratory tests reported in Queensland between 2009 and 2019, 15% of notifications and 40% of tests occurred during inter‐seasonal periods, with 6.3% of inter‐seasonal tests positive. Inter‐seasonal notifications and tests substantially increased over time and increases in weekly proportions positive and intensity classifications suggested gradual increases in virus activity. Tropical inter‐seasonal activity was higher with periods of marked increase. Influenza A was dominant, although influenza B represented up to 72% and 42% of notifications during some seasonal and inter‐seasonal periods, respectively. CONCLUSIONS: Using notification and testing data, we have demonstrated a gradual increase in inter‐seasonal influenza over time. Our findings suggest this increase results from an interplay between testing, activity and intensity, and strain circulation. Seasonal intensity and strain circulation appeared to modify subsequent period intensity. Routine year‐round surveillance data would provide a better understanding of influenza epidemiology during this infrequently studied inter‐seasonal time period.