Flu is hitting the country earlier, and H3N1 and H1N1 subtypes have become dominant with very little influenza B.
Just 32% of Australians were vaccinated against the flu last year, amid a record 365,500 confirmed cases of the disease, according to the latest 2024 data.
It was a “strange” season for a few reasons, Professor Ian Barr told audiences at the Immunisation Coalition Annual Scientific Meeting in Victoria this week.
The season started in April and peaked in early June before tailing off in October, but even last month the cases were “quite high”, said the deputy director of the WHO Collaborating Centre for Reference and Research on Influenza.
“January is usually a quiet time for influenza,” Professor Barr said. “The NNDSS has recorded some nearly 14,000 cases of influenza, so I think Australians are travelling more than ever and bringing home some little souvenirs from the northern hemisphere.”
For comparison, there were 9700 cases in January 2024 and 4000 in January 2023.
Different types and subtypes of influenza have also emerged. Influenza B/Yamagata hadn’t been seen since March 2020, he said, and most states had a combination of influenza A(H3N2) and influenza A(H1N1)pdm09.
The H3N2 subtype was dominant earlier in the season, and it was taken over by H1N1 later in the season, he said. There was very little influenza B overall.
Vaccines for 2025 will now include influenza A/Croatia/10136RV/2023-like virus instead of influenza A(H3N2). Professor Barr there was also a push to remove the Yamagata component of the quadrivalent vaccine, and that will “hopefully” be gone by 2026.
Another thing Professor Barr said he was looking forward to in 2026 was Australia scheduling the self-administered nasal flu vaccine, approved as FluMist in the US, for ages 2-49. “It’s obviously very simple to use and I think there is definite benefit in the younger children, as has been shown in the UK.”
Five hundred people died last year – up from 376 the year before – and seven of those were children under nine, according to NNDSS data. Laboratory-confirmed cases underestimated the true community rate, and Professor Barr said he thought the real rate of infections could be 10 times as high.
Professor Barr also said he was “chagrined” to see that only around 32% of Australians were vaccinated against the disease, “something we should not be very proud of”. This equated to half a million doses fewer than the year before, and substantially less than 2022 when 43% of the population was vaccinated.
The good news was that there was very little antiviral resistance to the licensed drugs oseltamivir and zanamivir, or baloxavir marboxil. Professor Barr said that while baloxavir marboxil was “a very good antiviral drug”, it had only been registered here in Australia and not yet distributed. “Maybe that will change in the future,” he added.
Most data on vaccine effectiveness was only interim, but so far has been better than usual for community infections and hospitalised cases, he said.
When asked what might have driven three successive years of exceptionally high flu cases, Professor Barr pointed to the high rates of H3 and possibly a legacy from low levels of herd infection after the massive drop in flu cases during the covid pandemic.
“It is unusual to have these big seasons, one after another. It’s usually every second or every third year,” he said.
“People always tell me, ‘Well, it’s just more testing and more testing’. But I don’t think there’s radically more testing in the last couple of years … and in fact, many of the public hospitals are doing less testing in 2024 and going forward than they have in the past.”
“My prediction for 2025 … [is] a low-to-moderate year with H3 virus predominating and an increased circulation of B.”
