Australia had a couple of virtually flu-free years, but the disease has bounced back — and then some — over the past few weeks.
In the year to last Sunday, almost 40,000 influenza cases were reported to the National Notifiable Diseases Surveillance System, with 26,000 of those cases in the fortnight of May 9-22.
That's compared to just 598 reported flu cases in the whole of 2021, thanks largely to closed borders and COVID restrictions.
Now, as Australians head into winter with little natural immunity, and face the potential risk of a co-infection with COVID-19, experts say getting the flu vaccine is more important than ever.
"We've got around 600,000 children under the age of two who've never seen flu, so that's quite a sizeable population of young kids who are entirely susceptible," Sheena Sullivan, an infectious diseases epidemiologist at the Doherty Institute, told ABC News.
"And then for our older populations … we don't know exactly how frequently people get infected, but normally, there would be a higher level of protection in the community than there is now."
Sydney paediatrician Nick Wood said hospitals were already seeing more flu cases in young children, who — alongside older adults, pregnant people and adults with chronic health conditions — face a higher risk of complications should they be infected.
"In the paediatric space, there are definitely more hospitalisations in under 5s than we saw with COVID-19," said Dr Wood, who is also an associate director at the National Centre for Immunisation Research and Surveillance (NCIRS).
Flu shot uptake slow
Some 6 million Australians have already had their flu shot this year, but that's less than a quarter of the eligible population, said Ian Barr, deputy director of the World Health Organization's Collaborating Centre for Reference and Research on Influenza at the Doherty Institute.
"It's low. Last year, we had about 35 per cent of eligible Australians vaccinated. So we're even below that at the moment."
That said, older Australians generally have much higher seasonal flu vaccination rates than their younger counterparts. (In 2019, only 15 per cent of 20-to-50-year-olds received a flu shot.)
In Australia, everyone over the age of 6 months is recommended to have a seasonal flu vaccine. Vaccination protects you against serious disease and also helps to protect people around you by reducing your risk of passing it on.
Dr Wood suspected "vaccine fatigue" may be contributing to lagging uptake rates in some age groups.
The rollout of COVID-19 vaccines, especially boosters in the last six months, may also have led some people to postpone their flu jab, Dr Sullivan said.
"There was a bit of confusion around whether you can get the influenza vaccine at the same time, and so that meant that vaccine uptake was a bit lower."
(You can now get your COVID-19 booster and flu jab at the same time.)
Flu vaccine an 'excellent match' so far
While vaccination is the best way to protect yourself against influenza, flu vaccines give better protection in some years than others.
That's because the viruses that cause the flu can mutate quickly and slip past our immune defences — as well as any protection bestowed by vaccines.
To stay a step ahead, the World Health Organization (WHO) keeps tabs on what viruses are circulating and where, and uses this to predict, in September each year, what will be in our next seasonal flu vaccine.
So how did they go this year?
Things are looking pretty good at the moment, according to Professor Barr.
"How that actually pans out in terms of vaccine effectiveness is too early to tell yet, but in terms of looking at the viruses which are circulating, and the match in the vaccine, they're an excellent match at this stage."
Of the four flu viruses covered by the vaccine, two are influenza A viruses and two are influenza B.
Influenza B viruses generally only infect people, while influenza A viruses — such as H1N1, or swine flu — can bounce between us and other animals.
This year, influenza A viruses have been behind the lion's share of laboratory-confirmed cases, Professor Barr said: "It's about 75 per cent H3N2 and 25 per cent H1N1."
While there's very little B virus getting around — only comprising 0.2 per cent of laboratory-confirmed cases — the vaccine seems to be less effective against it, University of Queensland virologist Ian Mackay said.
"That's one to keep an eye on, because if it was to scale up or we were to get more cases of it from [travellers] introducing it into Australia … it could cause more disease."
More potent vaccines for older Australians
How well the flu vaccine works also depends on your age and overall health.
The NCIRS estimates the flu jab can prevent illness in about 50-60 per cent of young children and healthy adults under the age of 65, though this figure varies year by year.
But older people and those with compromised immune systems may not respond as well to vaccination, meaning they may be less protected.
That's why Australians over 65 are recommended one of two enhanced flu jabs, specifically designed to increase their immune system's response to the vaccine.
"They have an adjuvant [in the vaccine] or higher dose vaccine, basically because the immune system just needs more of an immune stimulant," Dr Wood said.
"For younger kids, their immune system is a bit more robust … it's the same thing we were seeing with COVID vaccines."
How bad is this year's flu?
While influenza can be a mild disease — leading to a few days off work with fever, aches and pains and a sore throat — it can also cause very serious illness, and should not be confused with the common cold.
"I think people get a bit complacent about the flu," Dr Mackay said.
"But we saw in 2017 and in 2019 very big flu years that had a lot of people in hospital. We saw deaths, ICU visits, and lots of time off work."
According to this year's annual FluTracking data, respiratory illness levels are "moderate and sharply increasing, particularly in children aged 17 and under".
Officially reported numbers suggest influenza hospitalises around 5,100 people and kills 100 every year in Australia, but these figures are widely believed to under-represent the true burden of the disease.
One mathematical modelling study in 2008 suggests the flu is likely responsible for more than 3,000 deaths and 13,500 hospitalisations each year, and that's just in people over the age of 50.
It's never too late to get vaccinated
The NCIRS recommends eligible Australians get their flu shot around April or May, when the flu season is starting to ramp up.
Like some other vaccines, a flu shot's effectiveness wanes over time, but an April/May vaccine should still provide good protection when the flu season peaks in August or September.
If you're yet to get your flu shot this year, it's not too late, but try to get it as soon as you can so you get some protection, according to the NCIRS.
As long as they're available to give, flu shots will be offered throughout the whole flu season.
And if you've already had the flu? You should still get your shot.
It will help reduce your risk of getting sick from other flu virus strains doing the rounds.
Other ways to reduce your risk of catching the flu — and other diseases — should be pretty familiar by now: social distancing, wearing a face mask when around other people, washing your hands and staying at home.
