Respiratory illnesses are rising as cold and flu season and people's immunity adjusts to the effects of COVID, University of Newcastle Professor Nathan Bartlett says.
The scenario has prompted the NSW government to urge people to get vaccinated for flu and COVID.
"The incidents of cold/flu-like symptoms, as monitored by Flutracking, are definitely indicating an uptick at this point," said Professor Bartlett, a virologist.
"There are increasing numbers of flu cases being detected in hospital."
He said it was unclear what the magnitude of the flu season would be.
"But it certainly looks earlier. And that's prompting the advice to go and get your flu vaccine sooner rather than later," he said.
The latest NSW Respiratory Surveillance Report, released on April 6, states that "influenza activity is currently at low levels but continues to increase".
NSW Premier Chris Minns said on Tuesday that people at greatest risk of severe illness from flu or COVID-19 should book vaccinations as soon as possible.
"Influenza is highly contagious and can be deadly for some people," Mr Minns said.
Mr Minns urged priority groups to "take advantage of the free flu vaccine as soon as possible".
Flu vaccines are free for children aged six months to under five, people 65 and over, Aboriginal residents, pregnant women and those with serious health conditions.
NSW Health Minister Ryan Park encouraged people to ask their pharmacist or GP if they are eligible for a free flu vaccine.
"There is plenty of stock available throughout the state, so now is the time to book your shot," Mr Park said.
Chief Health Officer Kerry Chant recommended that people over 65 and those at higher risk of severe illness get a flu shot and COVID-19 booster at the same time.
COVID vaccines can be administered on the same day as an influenza vaccine.
Royal Australian College of General Practitioners president Nicole Higgins said Australia had already recorded more than 21,000 cases of flu this year.
"People have been travelling more and we've stopped measures like wearing masks and washing our hands, which allows flu to spread," she said.
Bupa Health Insurance chief medical officer Tony MacDermott said modelling from the northern hemisphere winter indicates the flu season will arrive earlier than expected in Australia and be worse than last year, with more hospitalisations likely.
Australian Medical Association vice president Danielle McMullen said those most at risk from the flu are people under five and over 65, as well as those with serious health conditions.
Professor Bartlett said "a very early flu season" happened last year.
"Flu came back with a vengeance. It peaked in May. Flu season normally peaks in August/September," he said.
"That was our first indication that the respiratory viruses were circulating again, because we were all interacting after the lockdowns.
"Things are a bit out of whack for want of a more scientific term."
He said cold and flu season will take time to reset after the disruption of COVID.
"Population immunity won't immediately go back to where it was pre-COVID," he said.
"It's going to take multiple years."
Immunity to other viruses like flu, RSV and other common cold viruses was being reset with COVID "circulating at relatively high levels".
"That will be altering or impacting immunity to these other infections too," he said.
"These viruses can cause co-infections. A response to one virus can impact your ability to respond to a different virus.
"We're still trying to come back to a level of immunity against these other viruses that we lost due to 2.5 years of lockdowns."
He said the immunity landscape was "a complicated situation".
"COVID still hasn't reached what we'd call an endemic virus. It's still circulating at higher than acceptable levels.
"We're a long way from having a predictable level of immunity and disease. We're nowhere near that stage yet with COVID."
The Hunter-New England district had 1006 reported cases of COVID in the week ending April 1, with one death and 24 people in hospital and two in ICU.
Meanwhile, The Guardian reported on Saturday that new mRNA vaccines could be used for a range of conditions by 2030.
The story raised the prospect of potential vaccines for cancer, cardiovascular and autoimmune diseases, along with multiple respiratory illnesses and other conditions.
Professor Bartlett said pharmaceutical companies are "very optimistic about their potential to expand to other diseases".
"Of course, we need to be a little careful here and temper the exuberance of the companies spruiking the massive potential from the very thing they want to sell and make money from," he said.
"But the science certainly suggests that mRNA or RNA-based therapeutics have huge potential."
The NSW government announced in February a $96 million RNA research and pilot manufacturing centre will be built at Macquarie University and operated by Myeloid Therapeutics.
"Clearly there's recognition of the incredible importance and potential of these kinds of therapeutics and there's investment to follow that," Professor Bartlett said.
"But there's still a lot of work to be done in order to develop new drugs and see how well they'll work for different diseases.
"It's nice to be optimistic, but now it's up to the scientists to work out how we can utilise this technology to provide better drugs for these conditions."
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