Before the Covid, when virologists were asked which was the pathogen with the most ballots to cause the next pandemic, many replied that avian flu, A(H5N1). Faced with the same question today, the answer repeats itself. And for more reasons than we did then, although it should be made clear that most viruses suspected of causing a pandemic – which always occurs in humans – do not. And that an unknown person was to blame for the latter until 2020: SARS-CoV-2.
A(H5N1) has been on the radar of health authorities around the world since the mid-1990s, when it was first detected. But in recent months, alarm bells have been ringing that had never been heard before: while the virus had frequently jumped from birds to mammals, it was not transmitted significantly between them, the latest research suggests.
Could it mutate and evolve the ability to infect humans? The answer from the experts is “Yes, it could”. And the more contact people have with the virus, the more likely it is that it will happen, although today it is not easy: it would require the chaining of a series of unfortunate events considered unlikely. At least in the short term.
The World Health Organization has described the risk to humans as “low” precisely because there are no infections, but it has also reminded that we must be prepared “for any change in the status quo”.
There are still unanswered questions, but this is what is known about A(H5N1):
What is A(H5N1)?
It is an RNA virus (like the Covid virus) discovered in southern China in 1996 and mainly reproduces in aquatic birds, which often carry it without pathogenicity. It became endemic in Asia, but in Europe and North America it was previously detected only in autumn and winter, with these birds migrating to virtually disappear in summer, as is the case with human flu. Since 2021, however, it has been broadcast year-round and has recently reached South America, where it was very rare.
Can it be transmitted between people?
Virus-to-human transitions are not common, but they are not exceptional either. A dozen have been recorded since 2021, and fewer than 900 since their discovery. In humans, it is very serious, with mortality rates exceeding 50%. What has not been documented is transmission from one person to another. The virus doesn’t have the ability to do that today, but the big fear is that it will evolve based on mutations.
Why is it considered dangerous?
The virus has worried veterinarians for decades: in non-endemic places, the method to stop its spread in poultry is to slaughter it on farms where it is detected. The United States alone estimates that 58 million birds died from avian influenza last year.
Virologists have always viewed it as a possible but unlikely threat to humans. There are several reasons that lead many experts to consider it a danger: Since 2021, it has become more prevalent among non-aquatic birds such as scavengers. This has led to more contact with mammals (which are the diet of these species). And most importantly, possible infections between mammals are detected for the first time: at the end of last year it was discovered in a Spanish mink farm, and recently everything indicates that the same thing happened between sea lions on the beaches of Peru. On the one hand, this makes it more likely that the same thing will happen between people. Second, it brings the virus closer to humans with intermediate hosts, who tend to have more contact with them than birds, making them more likely to make the leap.
What is the probability of a mutation making it transmissible between humans?
What is known is that the more the virus circulates, and among more species, the more likely it will develop a mutation that makes it transmissible between humans. “I don’t see it as something immediate that could happen tomorrow. But it’s like a lottery, for which we buy more and more tickets,” says Albert Perlas, a researcher at the Pioneer Campus in Munich, who has been studying bird flu for years. One of the big fears is that a farmed mammal, like a pig or a mink, could harbor both the A(H5N1) and human influenza viruses at the same time and they would recombine with the worst of both: the virulence of the first and the transmission capacity of the second.
Why isn’t it considered an imminent threat?
Perlas explains that there are several “barriers” that protect us: “The receptors that interact with the avian virus are not easy to find in humans and prevent its replication. In addition, these viruses use avian proteins to replicate, proteins not found in mammals. Finally, it has recently been found that even when these barriers are overcome, the virus replicates separately in the different lobes of the lungs, isolating the variants that may happen to be better adapted to the new species and limiting their transmission. That might explain that they can infect humans, but not that they can be transmitted between them.”
What can be done to avoid a pandemic?
Elisa Pérez-Ramírez, veterinary virologist at CSIC’s Animal Health Research Center (CISA), assures that work is already underway on veterinary surveillance, both in the game reservoir and on the farms. “Surveillance of bird flu in Spain is very good, but it needs to be extended to cover the whole year and not just autumn and winter as has been the case up to now. In addition, there is a need to increase passive surveillance: the study of dead animals, both birds and mammals, to which much attention has not been paid so far”.
Perlas adds that it is important to prevent large farms from concentrating in the same areas, which makes it easier for the virus to spread: “It’s something that isn’t seen as much in Spain, but in other countries like the Netherlands . ” In these centers, the experts add, preventive measures should be strengthened among workers. One of the measures they propose is that they get vaccinated against seasonal flu in order to eliminate mutations due to a combination with the avian virus as much as possible to avoid.
Are there vaccines against A(H5N1)?
Yes, they exist for birds and experimentally for humans. In birds, they are widely used in Asia, but are banned in Europe. Aitor Nogales González, a CISA researcher, says there is a big debate on the subject in the EU. It is viewed with suspicion, among other things, because it prevents the export of meat from vaccinated animals. “The vaccine protects animals from serious diseases, but it doesn’t stop the virus from circulating and mutating. It would probably have to be done at very specific times and in very specific areas, along with very rigorous pre-vaccination and post-vaccination serological testing to find out if the virus is still being transmitted,” he stresses. In humans, Nogales said, these are emergency-release vaccines that could be used in case of infection.
How will the virus develop?
If global health experts have learned anything during the Covid pandemic, it’s to severely limit their predictions. “Until now, it’s broken with the mold,” says Pérez-Ramírez. “It appears that in Europe and other places where it hasn’t been, it will become endemic, at least in wild species. It’s possible that it develops immunity in the birds and that the effects between them are less, but it’s not really known.
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