The mysterious future of the flu
Flu could come back as if 2020 didn’t happen or something stranger could happen. Keith Lynch explains.
Typically, about 20,000 people catch the flu and between 400 and 500 die of it every year in New Zealand. But last winter, the flu season never eventuated, with only a handful of cases found.
It appears the Covid-19 response, particularly tighter border restrictions, kept flu out in 2020.
‘‘I would say this is a 1 in 100 year event we captured,’’ says ESR virologist and flu expert Sue Huang.
Flu transmission has also fallen significantly in the northern hemisphere, even where Covid-19 spread widely. In the United States, for example, mask wearing and social distancing could not eliminate Covid-19, but these measures do seem to have vanquished the flu.
This is the good news. But there is bad news: the flu is still out there, and we’re not sure how it’s going to react to these weird last 12 months.
Let’s go back to basics. How does flu work?
There are four strains of influenza or flu: Type A, B, C and D. Types A and B typically make millions of people sick annually.
The H1N1 strain of Type A flu caused the 1918 flu pandemic, the 1977 Russian flu pandemic and the 2009 swine flu pandemic.
H3N2 is another Type A flu strain. It caused the Hong Kong flu, which killed more than 1 million people worldwide in the late 1960s.
Seasonal flu has tended to be a variant (or descendant) of one of these two strains. The 2018-2019 season is an illuminating example of how flu works. That year, the virus killed nearly 80,000 people in the US.
At the start of that season, the main strain was an H1N1 version of the virus. But another type of flu was lurking, and by February it (an H3N2 strain) had taken over.
It appears, however, that Type B flu – which only spreads among humans – is on the rise. According to new research, Type B flu was behind a virulent strain in the US in 2019/2020.
Flu is Darwinism in action – the strains are constantly fighting to survive and the fittest (the one that spreads the most easily) wins out.
Why seasons matter with flu
Flu season occurs during winter in the northern and southern hemispheres. This allows us to look at what’s happening in the US and the UK, for instance, and react accordingly and vice versa.
‘‘Here in the northern hemisphere we look at the strains circulating down where you are and produce the vaccines based on that. It allows us to stay one step ahead of the flu,’’ says Stephen Kissler, an epidemiologist at Harvard.
Over winter people are vaccinated or tend to catch the flu. (The virus is much more common when it’s cold and less humid.) So you see some immunity build up to a particular flu strain.
But immunity doesn’t last long. Over summer, flu isn’t spreading and the immunity in the population to last winter’s strain tends to wane. And when winter arrives again, bringing a new flu strain, there are vulnerable people who are susceptible.
‘‘It’s almost like a forest growing, getting overgrown with trees, and then a wildfire hits,’’ Kissler says.
What now for flu?
Normally, flu is relatively rampant and just as we’ve seen with Covid-19, that means multiple strains pop up and compete for global dominance.
‘‘There’s usually really only one strain that’s spreading at any given time that’s replaced the next year by a slightly different one,’’ Kissler says.
‘‘You might expect there to be multiple variants spreading and causing flu in
different places. But that’s not what happens.’’
That suggests the flu is under significant evolutionary pressure. It needs to find a winning strategy and stick with it to thrive.
Right now, flu is out there but there’s a lot less of it – so it may well be that there are fewer opportunities for the virus to mutate and for new strains to emerge.
If this is what happens, the flu strains that circulate in the future (when Covid-19 restrictions are lifted) could be similar to those we saw in 2019, Kissler says.
But there’s another possibility.
The flu may have the chance to develop a whole heap of winning strategies (or new strains) all at once.
In normal times when people are travelling freely, not wearing masks or distancing, flu competition is fierce. All the strains are battling for dominance and eventually one wins.
But right now, competition isn’t fierce. Think of the flu season like the rugby season. Think of right now almost as an extended pre-season for flu. It has time to try new things, to change up strategies, and reimagine itself.
‘‘We might see a lot more diversity with flu,’’ Kissler says. ‘‘It might have more opportunities to come up with a new variant different to the one we saw circulating in 2019.’’
New strains could emerge that are potentially stranger and more infectious. And it could be our vaccines – which are based on observing what happened during the last flu season – are less effective.
‘‘I think it’s more likely to be that evolution of flu is slowing,’’ Kissler says, ‘‘but there are good reasons to believe that may not be true.’’
Huang says: ‘‘The question is which strain is most fit to survive? And what is the factor that causes one to become the dominant strain is something we’re very keen to understand.’’
‘‘There’s usually really only one strain that’s spreading at any given time that’s replaced the next year by a slightly different one.’’ Stephen Kissler
Harvard epidemiologist