As the Southern Hemisphere’s influenza season draws to a close, Australia’s experience is offering an early indication of what could lie ahead for countries in the Northern Hemisphere this winter.
The A(H3N2) strain was dominant in Australia during a prolonged flu wave, with 171,400 confirmed cases recorded by the end of September, according to the Australian Centre for Disease Control.
This year’s flu season began later than in previous years. Even after Australia entered spring, influenza infections continued at a steady rate.
Flu season returns to a pre-pandemic pattern
According to Kirsty Short, a virologist at the University of Queensland, one likely explanation is that influenza seasonality is gradually returning to the pattern seen before the COVID-19 pandemic.
“After COVID, we observed a shift in the seasonality of influenza, as the peak of influenza was occurring earlier, while the current situation is more consistent with what we saw before the pandemic, with the peak of influenza occurring somewhat later,” Short said.
She also noted that the unpredictable behavior of influenza during the post-pandemic period may have contributed to people getting vaccinated earlier.
“Immunity is certainly not lost completely, but it declines during the season. Therefore, it is really difficult to find the balance between the optimal timing for vaccination to protect oneself and ensuring the longevity of immunity,” she said.
The genetic group “K” of A(H3N2) influenza A viruses was also dominant in the Southern Hemisphere, having been associated with increased transmission in 2025.
Despite the pronounced peak and increased hospitalizations toward the end of the season, however, Australia recorded fewer infections this year than last year.
Vaccination offers significant protection
Australian data also highlight the role of vaccination in protecting against influenza and severe illness.
Preliminary estimates showed substantial protection against influenza and flu-related hospitalization, while the viruses circulating in the country remained largely antigenically compatible with the vaccine.
According to the Australian Centre for Disease Control, the data showed a 58% reduction in the risk of visiting a general practitioner because of influenza and a 35% reduction in the risk of hospitalization due to influenza.
Flu season begins in the Northern Hemisphere
October marks the gradual start of the influenza vaccination season in Greece and other Northern Hemisphere countries as the Southern Hemisphere’s flu season comes to an end.
Japan and South Korea are already recording increases in influenza activity, while the World Health Organization has reported increased activity in several parts of Asia.
In Europe, however, influenza virus activity remains at low levels for the time being, according to the European Centre for Disease Prevention and Control (ECDC).
Greece is preparing to procure 3.6 million doses of influenza vaccine to cover the population.
According to a Health Ministry circular issued last week and signed by Deputy Health Minister Eirini Agapidaki, vaccination is preferably recommended from mid-October through the end of November, as approximately two weeks are needed for immunity to develop.
People who do not get vaccinated during that period can still receive the vaccine throughout the seasonal influenza outbreak, experts say.
Who is prioritized for vaccination
Greek health authorities are prioritizing several groups for influenza vaccination, including:
- People aged 60 and older
- Infants and children aged 6 months to 5 years
- Adults and children with chronic conditions, including respiratory and heart diseases, diabetes, kidney disease and immunosuppression
- Pregnant women
- People with severe obesity
- Caregivers
- People living with infants younger than 6 months
- People living with individuals who have underlying health conditions
- Health care workers and employees in health care settings
Australia’s prolonged season and the continued circulation of A(H3N2) provide an early reference point for the Northern Hemisphere, although the timing and intensity of the influenza season can vary from one region to another.
