As COVID-19 spread to hundreds of millions of people around the world, another potentially lethal disease — influenza — hardly reared its head at all.
And seasonal flu rates globally have been so low for the past 18 months, it looks as though one flu virus has been stamped out altogether.
Two studies — one published in Nature Reviews Microbiology in September and the other currently under review — show one of four flu viruses that infect humans each year hasn’t been detected anywhere in the world since April 2020.
So does that mean it’s gone for good? It’s still too early to say.
There is a chance this particular virus — the Yamagata virus — might be lurking in a pocket of the world somewhere, according to Ian Barr, deputy director of the World Health Organization’s Collaborating Centre for Reference and Research on Influenza at the Doherty Institute, and co-author of one of the studies.
“It may re-emerge, but we haven’t had a single detection of that virus in 18 months.
“That’s unusual, so it could be gone. We hope it’s gone.”
Which is good news. But … while Australia’s historically low flu count over the pandemic has undoubtedly saved lives, it also means the population may be a sitting duck when autumn and winter roll around next year.
But more on that later. First: how did the Yamagata virus disappear?
A fraction of the flu
The extent to which the flu has been crushed by the COVID-19 pandemic is extraordinary.
On average, around 100,000 flu cases are detected in Australia each year, and a few hundred people die from it, although those numbers do bounce around year to year.
In 2019, one of our worst flu seasons in recent times, more than 300,000 people became sick and more than 900 died from flu.
This year so far in Australia, there have been just 550 reported flu cases — and not a single flu death.
“We did have early cases in 2020, going up ’til about March, April,” Professor Barr said.
“But after that, the cases dropped off, and haven’t really reappeared since.”
Some parts of the world, such as countries in the Middle East, East Asia and West Africa, still endured flu outbreaks during the pandemic.
The vanishing virus
The Yamagata virus belongs to the influenza B group of flu viruses. With very rare exceptions, this group only infects humans.
Because they don’t jump between humans and other animals, they won’t cause a pandemic, unlike influenza A viruses, such as the one that causes swine flu.
But influenza B viruses are still behind roughly a quarter of total flu infections each year.
Something else that sets them apart from influenza A viruses is they mutate slowly.
Viruses that mutate quickly are more likely to evade the protection we get from vaccines.
The Yamagata virus, Professor Barr says, “has slowed down in terms of evolutionary change over the last 10 years”.
This, plus the fact that it doesn’t hide out in animals, had some scientists thinking the Yamagata virus might eventually be wiped out by vaccination alone.
Then COVID-19 hit. Social distancing, better hygiene and masks all contributed to driving flu levels down, and finished off the Yamagata virus faster than expected.
But what had the biggest effect, Professor Barr said, was closed borders.
Flu viruses generally don’t survive summer, so each country’s flu season is kicked off and sustained by a constant influx of new viruses brought in from travellers abroad during autumn and winter.
When international arrivals are put in quarantine for two weeks, that chain is broken.
“The quarantine stations have been a bit of a godsend for us, especially the one at Howard Springs,” Professor Barr said.
“The majority of [flu] viruses we’ve had this year in Australia have come through the quarantine station.
“So the viruses are coming in, but they’re just not getting out — or if they are getting out, they’re only getting out in very small numbers.”
How does this affect the flu jab?
The Yamagata virus is one of four we’re protected against by the yearly quadrivalent flu vaccine in Australia.
Despite not being seen for 18 months, the Yamagata virus might yet make a comeback, so will be included in next year’s flu vaccine.
“We don’t have sampling in every corner of every village in every country of the world, so there is probably a good chance that the virus is still out there,” Professor Barr said.
“Whether it gets back into the mainstream or not remains to be seen.
“I think we need to be cautious. This virus, only a couple of years ago, was one of the dominant viruses in circulation. It can circulate widely, and it can cause illness and hospitalisations and death.”
But if it’s still undetected next year, then 2023’s flu vaccine may drop the Yamagata component, and become a trivalent or three-part vaccine.
And that would be a good thing in some ways, Professor Barr added, because a vaccine made with three components is cheaper to make, and could help get more flu vaccines into developing countries.
And what about the 2022 flu season?
As Australia begins to reopen borders and NSW ditches hotel quarantine next month for passengers fully vaccinated against COVID-19, new flu viruses will trickle in and spread.
“But we’re in a situation now, which we’ve never been before, where flu viruses haven’t circulated, essentially, in two winters in Australia,” Professor Barr said.
Because we’ve not been exposed to the flu for a couple of years, the population doesn’t have much in the way of flu immunity at the moment.
The longer borders are closed, the more that herd immunity wanes.
On top of this, flu vaccine uptake has been particularly low this year.
Only around a third of the total population is vaccinated against the flu, Professor Barr says.
“While older people have held up well — 75 to 80 per cent of people over 65 are vaccinated — it’s the kids and adults who are way down on their vaccinations compared to previous years.
“And that makes us more susceptible.”
Getting as many people as possible vaccinated against the flu next year will be the best way to keep a lid on flu numbers over winter as viruses inevitably arrive on our shores.
“2022 could be a very interesting year,” Professor Barr said.
“I’m sure influenza will be back once borders are open. And then it’s just a numbers game, whether we have a moderate season or we have a big season.”