No, you’re not imagining it. Flu season is late and cases are up

The weather is warming up and the hay fever season has begun, but more people are experiencing cases of influenza than expected for spring.

In the two weeks to September 6, flu cases were up 27% on the previous two weeks. And case numbers are rising.

How is this year’s flu season different?

This year, surveillance systems based in laboratories, data from members of the community, GPs and hospitals all suggest influenza activity has been lower than in previous years for most of the year.

But influenza case numbers have increased since mid-August.

So far, the majority of influenza cases have been due to the H3N2 subtype of influenza A, although some H1N1 and influenza B have also been reported.

The H3N2 subtype has mostly been due to the “super K” strain that was associated with a late surge of cases in 2025. This strain seemed to be more transmissible than usual, although it didn’t cause more severe disease.

There are also some differences in activity in different parts of Australia. New South Wales and Western Australia both had an increase in influenza activity from late June that was not seen in other other states and territories.

What is the usual pattern of influenza in Australia?

Since the COVID pandemic, influenza epidemiology has been more unpredictable than usual. Before the pandemic, influenza cases usually started to rise in June-July and peaked in August, falling to lower levels by late October.

During the pandemic, influenza activity was essentially absent for 2020 and 2021, but resumed with the re-opening of international borders and relaxation of public health measures in 2022.

The influenza seasons in 2022 and 2023 were unusually early, peaking in late June.

Last year was also unusual, with an initial peak in mid-July and then persisting activity until late December. This was attributed to the “super K” strain of H3N2 influenza that has also been circulating this year.

Influenza activity does vary between different parts of Australia. Tropical regions tend to get two peaks of influenza each year: one early in the year and a second around the same time as the other states.

Why does the influenza season vary from year to year?

The drivers of influenza transmission are still unknown, which makes long-term predictions difficult.

In general, infectious diseases are an interaction between the “agent” (characteristics of the influenza virus) the “host” (characteristics of the people who are susceptible to influenza) and the environment that brings them together.

The influenza virus changes from year to year, and each strain may have different properties, such as being able spread more readily, or to cause more severe disease.

Apart from pandemics due to completely new strains, seasonal influenza is usually a small variation on the previously circulating strain. So most people have some degree of at least partial immunity to influenza, which may also be enhanced by vaccination.

People’s behaviour can also impact influenza transmission. An obvious example is school holidays reducing transmission in school-aged children.

The role of the environment is less clear with influenza, but it is likely some combinations of temperature and humidity may increase the spread.

What else can cause flu-like illnesses?

Influenza is not the only virus that causes flu-like symptoms. Other viruses such as rhinovirus can cause flu-like symptoms, but less commonly cause serious illness.

However, respiratory syncytial virus (RSV) – and since 2020 SARS-CoV-2, the cause of COVID – can also cause significant illness. The pattern of RSV has been similar to previous years, and COVID activity continues to decline.

What about influenza vaccines?

Influenza vaccines are partially protective against influenza, and the degree of protection can vary year to year.

Preliminary data this year suggests the influenza vaccine reduces the risk of hospitalisation with severe influenza by about 35%. This is on the lower side when compared to estimates in previous years.

The proportion of people who get a vaccine varies by age group. In older people (65 and over), around 60% of people have received an influenza vaccine this year, down from around 70% in 2022.

In pre-school children (aged 2–5 years), 28% have received a flu vaccine, similar to the figures reported in 2022.

What does the unusual influenza season mean for me?

The unusual pattern of influenza means it’s still worth getting an influenza vaccine now, if you haven’t had one earlier this year.

Although there is some evidence that the protection from influenza vaccine does wane over three to six months, people vaccinated early in the season still have some protection.

A second vaccine in the same season is generally not recommended. (The exceptions to this are young children who have not previously received an influenza vaccine, and people with specific conditions such as those receiving bone marrow transplants.)

The influenza vaccine is free for high-risk groups including older people, young children and people with chronic medical illnesses. Talk to your pharmacist, GP, nurse or Aboriginal health practitioner. Influenza vaccines may also be available through workplace immunisation programs.

Read more: Can I get a free flu shot? And will it cover ‘super K’? Your influenza vaccine questions answered