Flu predicted to roar back during Australia’s winter after two-year reprieve

Australia's coming flu season is likely to be worse than in previous pandemic years, with experts warning against complacency around influenza vaccines.

There was a historically low level of influenza transmission nationally last year, largely a result of the closure of international borders and other public health measures related to Covid-19.

Data from the National Notifiable Disease Surveillance System showed there were 598 laboratory-confirmed cases of influenza in all of 2021, according to the Doherty Institute's Prof Ian Barr.

In comparison, the same number of cases has already been recorded this year, as of April 6 data.

"There is normally low-level activity through these summer and autumn months, and the real uptick doesn't start normally until May," Barr, who is deputy director of the World Health Organization's collaborating centre for reference and research on influenza, said.

"Covid is front and centre of people's minds and influenza isn't really," he said.

"The virus is going to be back this year, after a two-year layoff, and vaccination is still the best way to protect yourself."

Barr was optimistic that this year's flu season would would not be as severe as 2019, which had 313,033 laboratory-confirmed cases and 953 deaths (in comparison, there were 21,266 cases in 2020).

But Nigel Stocks, a professor of general practice at the University of Adelaide, said this year brings "unknown territory".

"We face an unusual and novel situation where a lot of the community have not been exposed to influenza for at least a couple of years," he said.

Promisingly, however, the recent northern hemisphere winter saw shorter influenza outbreaks compared to pre-pandemic years.

"It's not quite clear why, but it was welcome because obviously Covid was co-circulating at the same time," Stocks said.

Co-infection with both Covid and the flu - dubbed "flurona" - is possible but not highly common, estimated to affect less than 1% of people with Covid in the general community.

One UK study of more than 212,000 people, published last month in the Lancet journal, found an influenza co-infection rate of 3% among individuals admitted to hospital with Covid. Those co-infected had four times greater odds of being put on mechanical ventilation, and twice the odds of dying in hospital.

"In every other season, people have had the chance of getting two infections at once," Stocks said. "In the past it was influenza and respiratory syncytial virus or rhinovirus."

People can be simultaneously infected with both influenza A and B strains, Stocks said, but he emphasised this was uncommon.

Young children receiving the influenza vaccine for the first time need two jabs for adequate immunisation, rather than just the one dose necessary for all other groups, Stocks said.

Free influenza vaccinations are now available for groups at highest risk, including people who are over 65, pregnant or with chronic health conditions, as well as children aged six months to five years.

The drop in international movement during the pandemic significantly reduced the diversity of influenza strains circulating, according to new research co-authored by Barr.

The study, published in peer-reviewed journal Nature Communications, also noted that one lineage of influenza B virus, known as B/Yamagata, still appeared to have been wiped out. It has not been conclusively detected since April 2020.

Barr said that if B/Yamagata had been eliminated for good, "it should make it easier for us to make the [flu] vaccine because there'll only be three components in the standard vaccine rather than four".

"For influenza B viruses, the big advantage is there's no animal reservoir, so it's unlikely that B/Yamagata viruses will be hiding out in a chicken or a pig or anything like that," Barr said.

"They just don't replicate in anything but us to any measurable extent.

"It's taken us over 20 years to get rid of this virus, not that we really managed to do it ourselves; the pandemic seems to have done it. It would be nice if it did stay away, but we can't be sure."