Inter‐seasonality of influenza in Australia

BACKGROUND: It appears inter‐seasonal influenza notifications have been increasing in summer months in Australia. This study aims to determine changes in inter‐seasonal influenza activity in Australia over time. METHODS: Routine influenza surveillance data and hospitalisations data were analysed to study the epidemiology of inter‐seasonal influenza and to examine the impact of inter‐seasonal influenza on morbidity in Australia at a national level. To adjust for changes in testing over time, we calculated a ratio of summer‐to‐winter notifications for each year in the study. A P‐value of <0.05 was used for statistical significance. RESULTS: Nationally, 18 933 notifications were reported during summer months from December to February 2005‐2016. There have been increasing summer notifications over time, which corresponded to similarly increased notifications in winter. A significant upward trend was observed for rate of notification during summer period over these years, P < 0.01. However, the ratio of summer‐to‐winter notifications demonstrated that while notifications have increased, the ratio has not increased markedly over the years and did not show a significant trend. No seasonal trend in rates of hospitalisation for influenza and pneumonia, respiratory and circulatory diagnosis was observed over the studied years. CONCLUSION: This study provides a clearer understanding of the epidemiology and burden of inter‐seasonal influenza and trends over time in Australia. The ratio of summer‐to‐winter notifications remains relatively constant and is supported by reasonably constant hospitalisation rates over the years.