We usually have to wait until winter approaches before we see an increase in cases of influenza, or the flu. But we have already seen a lot of flu this year, with 25,000 cases reported from January to March - and that's only a fraction of actual case numbers.
Authors
- Meru Sheel
Professor of Infectious Diseases and Global Health, University of Sydney
- Allen Cheng
Professor of Infectious Diseases, Monash University
Most people with the flu recover without treatment. But it can cause serious complications in older people, young children, pregnant women and those with underlying chronic diseases such as asthma or heart disease. Influenza kills around 3,500 Australians a year and lands 18,000 in hospital.
Vaccination is the best way to protect against flu and is recommended for everyone over the age of six months.
Flu vaccines are free for certain risk groups . Others can access them (usually for a fee) at pharmacies, GP clinics and local council clinics in some states and territories. Some employers also offer vaccinations for staff.
Influenza has been unpredictable since the COVID pandemic, with off-season circulation. There are also concerns protection might lag at the end of winter. So when is the best time to get vaccinated?
What are the symptoms and how does it spread?
Flu symptoms include a cough, sore throat, fever, body aches and fatigue.
Flu spreads from person to person via small respiratory droplets when you talk, cough and sneeze. It may also spread by touching a surface or object where infected droplets have landed, but this is less common.
So if you have flu-like symptoms , it's important to stay at home.
Flu spreads more in winter months due to increased contact between people and time spent indoors. Some studies also suggest influenza viruses transmit better in the cold .
What strains are around this year? And what is super-K?
Typically, human flu cases are caused by four virus strains, A(H3N2), A(H1N1), B/Victoria and B/Yamagata.
So far this year in Australia , almost 98% of cases have been influenza A(H3N2) and the remaining have been influenza B/Victoria.
In late 2025 , a new variant of the H3N2 strain known as subclade K or "super-K" emerged in the northern hemisphere winter.
Super-K contributed to the unusual increase in flu numbers over summer in Australia. But there is no evidence to suggest it's more severe than other H3N2 strains.
What does this year's vaccine protect against?
Each year , the flu vaccine is designed to protect against a mix of different virus strains, depending on what strains are currently circulating.
This year's flu vaccine contains two new strains for the influenza A(H1N1)pdm09 and A(H3N2) subtype virus components and an existing B strain:
- an A/Missouri/11/2025 (H1N1)pdm09-like virus
- an A/Singapore/GP20238/2024 (H3N2)-like virus
- a B/Austria/1359417/2021 (B/Victoria lineage)-like virus.
The Singapore strain in the vaccine is closely related to the super-K strains that have been circulating, so should provide better protection than the vaccine used last year in the northern and southern hemispheres.
The composition of the southern hemisphere vaccine for use in Australia is different to the northern hemisphere composition for the 2025-26 winter.
How effective will this year's vaccine be?
It's too early to know how well the vaccine will work against preventing infection and severe disease.
Preliminary Australian data suggests people who received the flu vaccine in 2025 were 53% less likely to be hospitalised with influenza or visit a GP for flu symptoms compared with unvaccinated people.
In the UK, during the 2025-26 winter, influenza vaccines were 72-75% effective at protecting against flu cases needing medical attention in children and adolescents, including infections caused by super-K.
Herd immunity from influenza vaccines can also help prevent transmission to others who are unable to get vaccinated.
A three-year study in the United States, which ended in 2020 found influenza vaccines were 21% effective at preventing infection among others household members.
Another study in the UK and Ireland from 2010 to 2017 found vaccinating school-aged children reduced the amount of respiratory illnesses GPs saw across all age groups.
When does the flu season peak?
While we see flu cases throughout the year, the "flu season" in temperate Australia typically lasts from May to October, peaking in June to July.
Theoretically, the best time to get the flu vaccine is about two weeks before flu cases start to rise.
However, this is difficult to predict and the rise can start anywhere between April and July in temperate Australia, and even earlier in tropical northern Australia.
How long does the flu vaccine last?
There is some evidence the protection provided by influenza vaccines falls over six months. Immunity to flu is optimal for three to four months after you are vaccinated.
However, some studies suggest this may be an artifact of the methods by which vaccine effectiveness is measured.
Other factors may also be important. Waning protection may be more prominent in older people and may also depend on the degree to which mutations in circulating influenza strains accumulate during the season.
So when is the best time to get vaccinated?
When working out when is the best time to get your flu vaccine, you might be balancing a theoretical benefit by waiting, against a chance of actually getting the flu before you get vaccinated.
Our advice is to get the vaccine when it's available and convenient, sometime around April or May .
But if you're travelling overseas, particularly for those travelling to the northern hemisphere in winter, aim to get your flu vaccine around two to four weeks before you leave.
If you're pregnant , an ideal time to get the influenza vaccine is with RSV (respiratory syncytial virus) and pertussis vaccines from 28 weeks of pregnancy. This protects both mother and baby.
Finally, young children getting their first flu vaccines should have two doses four weeks apart. Getting their first dose in earlier in the season will leave plenty of time for the second dose before the influenza season starts.
Meru Sheel receives funding from Australian Government Department of Foreign Affairs and Trade and the National Health and Medical Research Council (NHMRC). She is a member of the World Health Organization's Immunization and Vaccines related Implementation Research Advisory Committee (IVIR-AC).
Allen Cheng receives funding from the National Health and Medical Research Council and the Australian Department of Health, Ageing and Disability Care. He was previously a member of the Australian Technical Advisory Group on Immunisation and the Advisory Committee on Vaccines.
