Bird flu, aka the H5N1 virus, has been around since 1996. Since then it has led to intermittent cases of avian flu in humans. It has also steadily infected more species – from raccoons and foxes to seals and sealions to, most recently, dairy cattle in the US. Between January 1st, 2003, and December 21st, 2023, 882 cases of human infection with the H5N1 virus were reported from 23 countries, of which 52 per cent were fatal.
Surely this makes it a likely candidate as the source of the next global pandemic?
An editorial in this month’s Lancet Infectious Diseases certainly thinks so and has a clear warning: “The increasing host range of the virus, [its] potential spread among mammals and between a mammal and a human, its wide geographical spread, and the unprecedented scale of the outbreaks in birds, raise concerns about the pandemic potential (of H5N1),” it says.
And a recently published international survey of leading scientists found that some 57 per cent of disease experts now think that a strain of flu virus will be the cause of the next global outbreak to follow Covid-19. The belief that influenza is the world’s greatest pandemic threat is based on long-term research showing it is constantly evolving and mutating, said study author Jon Salmanton-García of University of Cologne.
The World Health Organisation (WHO) has also weighed in. Last month it raised fears about the alarming spread of the H5N1 strain of influenza that is causing millions of cases of avian flu across the globe. The H5N1 variant has become “a global zoonotic animal pandemic”, UN health agency’s chief scientist Jeremy Farrar told reporters in Geneva. “The great concern, of course, is that in . . . infecting ducks and chickens and then increasingly mammals, that virus now evolves and develops the ability to infect humans and then critically the ability to go from human to human.”
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His colleague, Dr Michael Ryan, executive director of the WHO health emergencies programme, is a little more measured in his response. ”Nobody is suggesting that H5N1 is the new next pandemic. I don’t believe anybody can predict that. But it’s certainly concerning when a virus like this begins to infect multiple mammalian species, which means the virus is adapting to [animals] that are more like us than birds, and therefore there’s a higher level of alert.”
What aspect of the virus in particular has scientists on tenterhooks?
The virus likely spilt from birds into cows as far back as late 2023, and spread undetected among cattle for months before the first cases were reported this March. It has been detected in unpasteurised cows’ milk. An analysis of a recent case of a cattle worker infected in Texas outlined how the worker was in close contact with H5N1-infected dairy cattle, and developed eye inflammation and broken blood vessels as a result. Significantly, the virus identified in the worker’s specimen had a genetic change that is associated with viral adaptation to mammalian hosts.
But there is no sign yet of an adaptation that would allow this virus to better bind to human-specific receptors in the upper airway, which would give it the ability to transmit easily between people. However, the more a virus spreads the chances of it mutating to infect humans increases.
Aren’t there vaccines against H5N1 already available? And is it time to ramp up production of these vaccines?
Yes, there are two H5N1 vaccine formulations already available that could be used in a pandemic. There are two types of injections that are well matched with the currently circulating strain in dairy cattle.
But there is no need to begin manufacturing these shots now, the WHO’s Dr Ryan says. Scaling up to produce pandemic vaccines would mean a trade-off where manufacturers are forced to stop producing annual shots for our seasonal strains of flu.