COVID-19 may have increased people's susceptibility to other respiratory viruses in different ways, Newcastle respiratory specialist Peter Wark says.
Professor Wark said restrictions on movement during the pandemic caused "a dramatic reduction in influenza and RSV" and people's immunity to those viruses lapsed.
But he said there was also discussion that "having COVID may well have increased susceptibility to the other respiratory viruses and their consequences".
Some studies suggest COVID can damage the immune system.
"It's a valid possibility, particularly given this year's flu season is looking just as bad as what it did last year," he said.
"There's a lot of seasonal influenza and RSV [respiratory syncytial virus]. RSV kind of goes unnoticed."
A University of Sydney-led study found last year that the pandemic led to "the emergence of new RSV strains" after COVID control measures were eased.
The study also cited an earlier influenza season as an effect of the pandemic.
Professor Wark, a respiratory specialist, said there had been "a lot of hospital admissions" due to influenza and RSV this winter flu season.
"They cause very similar illnesses and will impact on similar populations, the very young and very old."
The Hunter New England district recorded 456 influenza cases (up 19 per cent in a month) in the week to July 8, the latest NSW Health data shows.
There were 168 RSV cases (down 23 per cent in a month) and 398 COVID cases (down 62 per cent in a month).
The figures were published in the NSW Respiratory Surveillance Report on Thursday. They refer only to notification data obtained from positive PCR laboratory tests for infections.
"Influenza continues to dominate respiratory virus activity in NSW," the report said.
"Emergency department presentations and subsequent admissions for influenza-like illness remain high, particularly in children and young people aged less than 17."
A female year 9 student, who lived on the Central Coast, died from influenza B at the weekend.
The Newcastle Herald reported last week that 16 children had been admitted to intensive care since May with "life-threatening complications" from influenza at three NSW hospitals. Four affected children were treated at John Hunter Hospital.
NSW Health data shows Hunter New England recorded 80 to 190 influenza and pneumonia-related deaths each year over the past 20 years. The median age of influenza-associated deaths in Australia last year was 82.
Professor Wark, who is also a researcher at Hunter Medical Research Institute, said influenza "can make very young children sick and they can become quite sick, quite quickly".
"RSV is the same, particularly for very young children under the age of one. So if they're very sick, then yes, take them to hospital."
The NSW government has urged people to get the flu vaccine.
Professor Wark said RSV vaccines were also on the way, having undergone phase 3 clinical trials.
"There will be vaccinations that combine flu and RSV. The RSV vaccines are looking very effective," he said.
"In the future I think we will need to be more proactive in getting people vaccinated against RSV. "
Asked why respiratory viruses were worse in winter, he said "if you go to the tropics, influenza bubbles away all year round".
"If you're in a temperate climate, where you have fluctuations in temperature, it really is heightened in winter and it goes away more or less in summer.
"That's complex and relates to a number of factors. Being inside is very likely one of those factors.
"But the cold weather probably also influences the ability of the virus to be transmitted, survive and evade the immune response."
He said there was "quite a complex interplay going on there that we don't understand".
"Whether dressing warmly and wearing a scarf or beanie protects you or not, I can't be sure, but if that's what your mother's telling you to do, you probably should listen."
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