Flu season arrives late in Australia as cases surge 27% in spring

Australia's flu season has arrived late this year, and it's arriving with force. In the two weeks leading up to September 6, influenza cases jumped 27 percent compared to the previous fortnight. The numbers keep climbing, defying the country's historical pattern where flu typically peaks in August and fades by late October.

What makes this year unusual is how quiet things were until mid-August. Laboratory surveillance systems, community data, GP records, and hospital reports all painted the same picture through most of 2026: influenza activity was running below what Australians had seen in previous years. Then something shifted. Cases began rising in mid-August and have continued upward since. The culprit is primarily the H3N2 subtype of influenza A, though some H1N1 and influenza B cases have also surfaced. Within that H3N2 category, the "super K" strain—the same variant that caused a late surge in 2025—is doing most of the work. This strain spreads more readily than usual, though it doesn't appear to cause worse disease when people do catch it.

The timing is not uniform across the country. New South Wales and Western Australia both saw influenza activity pick up as early as late June, a pattern that didn't emerge in other states and territories. This regional variation hints at how unpredictable influenza has become since the pandemic. Before COVID-19, the rhythm was predictable: cases would start rising in June or July, peak in August, and decline through October. The pandemic disrupted that entirely. Influenza essentially vanished in 2020 and 2021, then roared back in 2022 as international borders reopened and public health measures relaxed. The 2022 and 2023 seasons arrived unusually early, peaking in late June. Last year threw another curveball—an initial peak in mid-July followed by persistent activity stretching all the way to December, again driven by the super K strain now circulating this year.

Why influenza behaves differently from one year to the next remains largely mysterious. The virus itself changes constantly, with each strain carrying different properties—some spread more easily, others cause more severe illness. Most seasonal influenza represents only minor variations on previously circulating strains, which means most people retain at least partial immunity from prior exposure or vaccination. But human behavior matters too. School holidays reduce transmission among children. Temperature and humidity likely play a role, though the exact mechanism is unclear. The interplay between the virus itself, the people it infects, and the environment they share creates an equation that scientists still cannot reliably predict.

Vaccination offers some protection, though this year's numbers are sobering. Preliminary data suggests the flu vaccine reduces the risk of hospitalization from severe influenza by about 35 percent—lower than historical averages. Vaccination rates have also slipped. Among people 65 and older, about 60 percent have received a flu shot this year, down from around 70 percent in 2022. In preschool children aged two to five, 28 percent have been vaccinated, holding steady with 2022 figures but still leaving most unprotected.

Influenza is not the only virus causing flu-like symptoms. Rhinovirus can produce similar illness, though it rarely becomes serious. Respiratory syncytial virus and COVID-19 can both cause significant disease, though RSV patterns have remained consistent with previous years and COVID activity continues to decline. For people deciding whether to get vaccinated now, the unusual timing of this season argues in favor of doing so. While vaccine protection does wane over three to six months, people who were vaccinated earlier in the year still retain some defense. A second dose in the same season is not generally recommended, except for young children who have never been vaccinated before and people with specific medical conditions such as those undergoing bone marrow transplants. The vaccine is free for high-risk groups including older adults, young children, and people with chronic illnesses. As the season continues its late surge, the window to act remains open.