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How 2018's Minor Flu Caused This Year's 'Out Of Order' Virus

A "remarkably early" flu season has struck down tens of thousands across the country, despite a record number of Australians getting the flu jab. So what's going on?

Across Australia, states are reporting flu cases in higher numbers than recent years. In the latest influenza surveillance reports, NSW has 3000 confirmed cases, Victoria 18,000, and Western Australia nearly 9000.

Despite Australians getting the flu jab in record numbers -- those hit with it are in reality even higher across all states -- as not every person suffering with the flu will present to hospital.

WA's flu notifications in 2019 are nearly 6.5 times higher than at the same point in 2018, and 27 flu deaths have been recorded in the state's aged care homes this year -- compared to zero in 2018, and just two in 2017.

Photo: WA Department of Health

"It's a very early season. Compared to last year, it’s a lot busier. We haven't hit our peak yet," Professor Robert Booy, senior professorial fellow at the National Centre for Immunisation Research and Surveillance, told 10 daily.

Experts can't point to one single cause behind the stunning spike in flu cases, but the finger has been pointed at the fact that Australia escaped with a relatively easy flu season in 2018 -- and now we're paying for it.

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"Last year was a very quiet one for influenza. One of the theories is that we didn't get the usual amount of activity last year that would have given the population a bit of protection boost into the next year," David Smith, a microbiologist and clinical professor at the University of Western Australia, told 10 daily.

"We’ve gone into this season with a population more susceptible than usual."

Photo: Getty

Smith, who is also a director of the National Influenza Centre, said there are two strains of the flu doing the rounds this year, with one of them -- an influenza A(H3N2) -- of particular concern.

"The strains causing the high activity so far are both quite different to the strains circulated previously, so any past protection we had is not great against these particular viruses," he said.

"[H3N2] mutates more quickly than other strains. So it does tend to be more difficult to make sure the strain you pick for the vaccine, six months before the flu season hits, will be the one that actually circulates."

Photo: NSW Department of Health

Booy said this year's flu was "totally out of order" and bucked recent trends. Such high flu numbers are rarely seen at this point of the year, and with the peak of cases traditionally coming in July, there are fears this season has more sickness to dole out to Aussies.

"The flu virus changes each year, and with more susceptible people this year, that might be leading to this very unusual surge," Booy said.

"In 1942 we had a surge in April, but there haven't been many early surges like this."

READ MORE: Flu Deaths Reach 35 In 'Unprecedented' Season In South Australia

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The NCIRS fellow said he was pleased to see Australians getting vaccinated, but said more could still be done to further stamp out the flu.

"We're seeing an unprecedented high uptake of vaccinations this year. The public is increasingly aware and doing something about it.

But the fact there's so much disease shows too many people are still not vaccinated," Booy said, outlining that more than half of Australians did not get a flu vaccine each year.

"We’re now approaching 50 percent of the population, which means we get some herd immunity in the community."

Young children are one of those most at risk. Photo: Getty

Children aged under five, those over 65, pregnant woman, people with chronic medical conditions and Indigenous people are particularly urged to get a flu jab each year.

As flu can set people up for further bacterial infections, young people may consider getting measles and meningococcal vaccinations as well, while older people should consider a pneomococcal injection, Booy said.

"The season has started early, we’re not sure how long it will go on for. If you haven't been vaccinated, you should get it, as it could be quite some time of virus circulation still to come," Smith said.

Contact the author jobutler@networkten.com.au