Intense interseasonal influenza outbreaks, Australia, 2018/19

Area of Science:

• Public Health
• Epidemiology
• Virology

Background:

• Interseasonal influenza outbreaks are common but typically small in temperate climates.
• Australia's 2018/19 summer-autumn period saw unusually large and widespread influenza outbreaks.
• These events preceded an early start to the 2019 main influenza season.

Purpose of the Study:

• To epidemiologically assess the scale of the 2018/19 Australian interseasonal influenza outbreaks.
• To investigate the genetic, antigenic, and structural characteristics of the responsible influenza viruses.

Main Methods:

• Utilized observational study design.
• Combined epidemiological and virological surveillance data from multiple Australian health authorities and research institutions.
• Included data from the World Health Organization Collaborating Centre for Reference and Research on Influenza.

Main Results:

• A record 85,286 laboratory-confirmed influenza cases occurred between November 2018 and May 2019, five times the prior 3-year average.
• Increased institutional outbreaks, hospitalizations, and deaths were observed during this interseasonal period.
• The outbreaks followed a mild 2018 influenza season.

Conclusions:

• The 2018/19 interseasonal outbreaks were exceptionally large and contributed to an early 2019 influenza season in Australia.
• The exact causes remain under investigation, likely involving a combination of climatic, virological, and host immunity factors.
• Reinforces the necessity of continuous, year-round influenza surveillance, irrespective of established seasonal patterns.