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Euro Surveill. 2019 Aug;24(33). doi: 10.2807/1560-7917.ES.2019.24.33.1900421.

Intense interseasonal influenza outbreaks, Australia, 2018/19.

Author information

Department of Microbiology and Immunology, University of Melbourne, Melbourne, Australia.
WHO Collaborating Centre for Reference and Research, Melbourne, Australia.
Department of Microbiology, Biomedicine Discovery Institute Monash University, Clayton, Australia.
National University of Singapore (NUS) Graduate School for Integrative Sciences and Engineering, Singapore.
Bioinformatics Institute, Agency for Science, Technology and Research (A*STAR), Singapore.
Communicable Diseases Branch, Health Protection New South Wales, St. Leonards, Australia.
Rapid Surveillance, Centre for Epidemiology, New South Wales Ministry of Health, St. Leonards, Australia.
Centre for Disease Control, Northern Territory Department of Health, Darwin, Northern Territory, Australia.
Serology/Molecular Biology Territory Pathology, Royal Darwin Hospital, Northern Territory Government Health, Darwin, Australia.
Microbiology and Infectious Disease Directorate, SA Pathology, Adelaide, Australia.
Department of Paediatrics and Adolescent Health, Faculty of Health and Medicine, University of Sydney, Sydney, Australia.
National Centre for Immunisation Research and Surveillance (NCIRS), Westmead, Australia.
Department of Biological Sciences, National University of Singapore, Singapore.


BackgroundInterseasonal influenza outbreaks are not unusual in countries with temperate climates and well-defined influenza seasons. Usually, these are small and diminish before the main influenza season begins. However, the 2018/19 summer-autumn interseasonal influenza period in Australia saw unprecedented large and widespread influenza outbreaks.AimOur objective was to determine the extent of the intense 2018/19 interseasonal influenza outbreaks in Australia epidemiologically and examine the genetic, antigenic and structural properties of the viruses responsible for these outbreaks.MethodsThis observational study combined the epidemiological and virological surveillance data obtained from the Australian Government Department of Health, the New South Wales Ministry of Health, sentinel outpatient surveillance, public health laboratories and data generated by the World Health Organization Collaborating Centre for Reference and Research on Influenza in Melbourne and the Singapore Agency for Science, Technology and Research.ResultsThere was a record number of laboratory-confirmed influenza cases during the interseasonal period November 2018 to May 2019 (n= 85,286; 5 times the previous 3-year average) and also more institutional outbreaks, hospitalisations and deaths, than what is normally seen.ConclusionsThe unusually large interseasonal influenza outbreaks in 2018/19 followed a mild 2018 influenza season and resulted in a very early start to the 2019 influenza season across Australia. The reasons for this unusual event have yet to be fully elucidated but are likely to be a complex mix of climatic, virological and host immunity-related factors. These outbreaks reinforce the need for year-round surveillance of influenza, even in temperate climates with strong seasonality patterns.


Australia; human; influenza; seasonality

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