The earliest start to the flu season in 10 years and the rise of a strain known to cause more severe illness is putting pressure on WA's already stretched health resources.
Experts have warned it could be a bad flu season this year, with flu notifications rising sharply and continuing to flood in.
Around half of cases diagnosed so far are influenza A/H3N2 which is known to be more deadly and cause worse illness than other strains.
Influenza specialist group chair Alan Hampson said influenza A/H3N2 was responsible for more deaths and hospitalisation than influenza B which represents the other half of notifications and tends to cause milder disease.
"Two viruses we are seeing at the moment haven't been prominent in our population for the last three years, so the population's immunity is probably low against those," Professor Hampson said.
"We do know that when we get H3 outbreaks they are the ones that tend to be the more severe outbreaks and have greater effects, certainly on the older adult population."
Professor Hampson said H3N2 viruses were most commonly associated with "excess mortality, more deaths than you would normally expect in the population".
He said each year 2500 to 3500 people died from influenza, but most of those were not recorded as such because the flu symptoms precipitated heart attacks and strokes.
This year's flu vaccine is the same as last year and provides immunity against the H3N2 virus as well as H1N1 and influenza B.
Paul Armstrong, director Communicable Disease Control Directorate, said there had been a slight change in the H3N2 virus compared to previous years.
"So some of the virus that would be circulating would be slightly different from the one that's in the vaccine but the vaccine will still afford good immunity against it," he said.
While older people had good immunity against the H1N1 pandemic strain (because they were infected with a similar virus in the 1950s) they were vulnerable to the H3N2.
"So now that H1N1 pandemic strain has almost disappeared ... it is more likely that older people will get this H3N2 strain, they won't have that protection against as they did with that pandemic H1N1 strain."
Dr Armstrong said it was difficult to forecast what would happen this flu season.
"It started several weeks earlier this year than any time for the past 10 years and we are just on the upswing at the moment so we just don't know where that peak is going to be and we don't know how severe it is.
"Flu seasons are quite variable, they can last anywhere from two months to four or five months."Dr Armstrong said there had been an unusually high number of flu notifications over the warmer months of the year, which may have contributed to the early start to the flu season this year.
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