Inter-Seasonal Influenza is Characterized by Extended Virus Transmission and Persistence

Area of Science:

• Virology
• Epidemiology
• Public Health

Background:

• The drivers of seasonal influenza patterns remain unclear.
• Existing models often attribute off-season influenza to imported viruses.
• Understanding inter-seasonal transmission is crucial for public health interventions.

Purpose of the Study:

• To investigate the origins and evolution of influenza viruses during inter-seasonal periods in Australia.
• To identify factors contributing to influenza seasonality beyond simple importation.
• To explore complex transmission dynamics of influenza A (H1N1pdm, H3N2) and B viruses.

Main Methods:

• Conducted a large-scale phylogenetic analysis of hemagglutinin (HA) gene sequences.
• Analyzed 9912 (A/H1N1pdm), 3804 (A/H3N2), and 3941 (B) HA sequences globally (2009-2014).
• Examined 1475 Australian influenza virus samples, focusing on those collected outside the typical surveillance season.

Main Results:

• Identified complex inter-seasonal transmission patterns, including 'stragglers' and 'heralds' of seasons.
• Documented influenza B virus persistence between epidemic seasons.
• Observed a high frequency of inter-seasonal transmission events in Australia's tropical and subtropical regions.

Conclusions:

• Influenza seasonality is influenced by more than just viral importation.
• Local transmission dynamics and persistence contribute significantly to off-season influenza.
• Climate in tropical and subtropical regions may play a critical role in shaping influenza seasonality.