How likely is a human bird flu pandemic?

The worst-ever outbreak of bird flu has led to the disease becoming endemic in some birds, inflicted huge costs on the poultry industry, spread into wild and captive mammals and, in some rare instances, infected humans.

The World Organisation for Animal Health this month said a rising number of cases had been reported in mammals, “causing morbidity and mortality” in species such as otters and seals. Reports of infections in farmed mink in Spain had increased concerns, WOAH said, because cases involving large numbers of animals kept close to one another exacerbated the risk of wider transmission.

If H5N1, the strain mainly responsible for the latest outbreak from late 2021, develops mutations that make it easier to transmit to humans, experts fear the emergence of a pandemic that carries more risks for global health than the Covid-19 outbreak. Though bird flu has infected relatively few humans, its fatality rate is about 50 per cent, according to the European Centre for Disease Prevention and Control.

H5N1 is “a big worry”, said Jeremy Farrar, flu expert and outgoing director of the Wellcome Trust. “You would hate to look back in the midst of an H5N1 pandemic and say: ‘Hold on, didn’t we watch this avian population die all over the world and we started to see mammals dying and what did we do about it?’,” he told a briefing in London this week.

He said more vigorous action was needed both to build up H5N1 vaccine stocks and prevent circulation of the virus among mammals. “If there was an outbreak tomorrow of H5N1 in humans, we wouldn’t be able to vaccinate the world within 2023,” added Farrar, who will become chief scientist at the World Health Organization in May.

How likely is a human H5N1 pandemic?

Infectious disease experts say the risk remains largely contained to the animal population — 50mn birds, including poultry, have been killed by the virus or culled in this outbreak, according to the ECDC. Large-scale culling has been carried out in dozens of countries including Japan, France and the US.

The WHO this month said avian influenza was “still an avian virus” and the threat, including human-to-human transmission, “is low”.

Bird flu is a “recurrent problem” typified by sporadic seasonal outbreaks, according to Gregorio Torres, head of the science department at WOAH. But many countries with outbreaks were now reporting cases throughout the year, he said. “The seasonality we [used to] observe isn’t there anymore.”

Changes to poultry farming methods and rising global temperatures that force wild birds to alter migration patterns, as well as increased movement between countries of both animals and humans, were behind the rise in cases, he said.

“The virus can be transmitted by contaminated feed, contaminated food, also contaminated trucks,” added Torres.

One crucial factor is how H5N1 evolves, which scientists say can happen in two ways: either through a linear series of mutations that makes the virus more efficient at spreading through a particular animal species; or through recombination, when two different strains of the virus affect host cells at the same time and swap genes — leading to greater evolutionary leaps.

“Nobody can predict when or where [a mutation] will happen,” said David Heymann, a professor in infectious disease epidemiology at the London School of Hygiene and Tropical Medicine. “What’s important is you keep your eye on infections.”

What is being done to stem the spread?

Experts agree that the surveillance measures in place have been adequate for identifying and isolating disease, despite the huge cost to food producers and other groups.

Torres said the focus should continue to be on “biosecurity”, or actively seeking out diseased animals to isolate them from healthy ones.

“It’s definitely the most effective tool to avoid animals getting infected,” Torres said. He added that the risk to humans, while still low overall, had “probably never been so high”.

He warned that the public should be wary of handling dead birds in the wild, one of the usual routes of transmission to humans. In London and many other cities, councils have warned against feeding birds in parks.

And what other measures should be taken?

Richard Ebright, professor of chemistry and chemical biology at Rutgers University, said it was “clear” that further measures should be taken to reduce the zoonotic potential of H5N1.

He urged an end to two activities that could enable avian influenza viruses to make the jump to humans — fur farming and gain-of-function research, in which pathogens are manipulated to help scientists understand how they behave.

“The former, in effect, selectively breeds for new H5N1 influenza viruses that can transmit efficiently in mammals,” Ebright said. “The latter, by definition, deliberately creates new H5N1 influenza viruses reasonably anticipated to transmit efficiently in mammals.”

“Both are unnatural. Neither offers benefits that offset the risks,” he added.

What counter-measures are available?

According to the WHO, vaccines for use in humans against H5N1 infection have been developed, but have not been widely used.

“Several manufacturers have developed prototype H5 vaccines that can be authorised when and if a pandemic H5N1 strain emerges,” it said, adding the most important tool was the identification of positive cases and the monitoring of their contacts as part of routine outbreak investigations. Influenza antivirals are also available.

Only a handful of countries such as Russia, China and Egypt have vaccinated animals against avian flu in the past three years, according to WOAH. But with vaccines unable to offer immunity or complete protection against disease, many experts say the safest measure is the separation of sick and healthy animals.

To protect humans in the long term, Farrar advocates building up a global reserve of vaccines “for every single strain of influenza that exists in the animal kingdom through at least phase 1 and phase 2 studies [in humans], so that you know the vaccines are safe and immunogenic and that you could manufacture them well”.