The fast-spreading H5N1 virus is mutating quickly enough to spur WHO into action and begin pre-pandemic vaccine development.
Since its first known human infections in 1997, the H5N1 bird flu virus –also known as avian influenza– has been a known threat, primarily affecting birds but occasionally infecting humans through direct exposure.
While sporadic human infections have occurred, the virus has never been able to spread through human-to-human transmission.
However, recent developments are causing concern among scientists as the virus spreads to new mammal species, adding to fears about its ability to adapt further.
The World Health Organization (WHO) has already begun pre-pandemic vaccine development, selecting candidate H5N1 variants that closely match the viruses detected in the US and South America.
The hope is that, if needed, vaccines can be rolled out quickly to curb a potential outbreak before human-to-human transmission occurs.
The latest wave began in 2021, when a strain of H5N1 known as clade 2.3.4.4b arrived in North America via migratory birds.
By 2022, it had led to an outbreak among minks in Spain, which was contained through mass culling.
In the past ten months, at least 68 human infections have been recorded in North America, with one fatality.
Although most cases have been mild, researchers cautions that each spillover into mammals increases the risk of mutations that could enhance the virus’s ability to spread among humans.
While there is currently no confirmed human-to-human transmission, at least 40 cases of human infection from dairy cows have been reported in the US.
Dr Joseph Sriyal Malik Peiris, chair of Virology at the University of Hong Kong and co-director of the WHO H5 Reference Laboratory, emphasises the importance of vigilance.
“Whenever an avian virus establishes sustained transmission in a mammalian species other than humans, there is an opportunity for mammalian adaptation of the virus to take place, which may also increase risk to humans,” he explains. While the overall human health risk remains low for now, he stresses that the situation must be closely monitored.
Vaccine development is already underway for pre-pandemic preparedness, but how concerned should the public be, and what steps can be taken to mitigate the risks?
How much has bird flu changed?
Since the 1990s the virus has caused over 860 human infections worldwide, with a fatality rate of more than 50 percent in those cases.
It first infected humans in 1997 in Hong Kong, when it infected 18 people, 6 of these being fatal.
“It has repeatedly spilled over to domestic poultry, causing poultry outbreaks and has occasionally infected humans,” says Dr Peiris.
He mentions that there was even a 2022 outbreak at a mink farm in Spain, which was contained through depopulation.
However, those past avian flu outbreaks – including the H1N1 pandemic in 2009 – emerged when viruses evolved to efficiently transmit between humans.
“The risk has increased as we’ve gone on—especially in the last couple of months, with the report of [some] severe infections,” says Seema Lakdawala, an influenza virologist at Emory University School of Medicine.
Dr Peiris explains that the virus has undergone adaptive mutations in both marine mammals and US dairy cows, but these changes have not yet allowed it to spread between humans.
She warns for continued surveillance, particularly as researchers look for mutations that could allow the virus to bind more effectively to human cells.
How can we protect ourselves?
While the virus has not yet become a pandemic-level threat, experts stress that precautionary measures are very important, particularly for those working in high-risk environments.
Dr Peiris emphasises the importance of safety measures for workers in the US dairy industry, as infected cows have been found to carry high levels of the virus in their milk.
“People working closely with dairy cows in the US (especially those in affected states) need to use personal protective equipment, which reduces the risk of infection,” he advises.
Additionally, consumption of raw milk can be highly risky, increasing the chance of exposure to harmful pathogens. Studies show that pasteurisation effectively kills the virus, making such milk safe for consumption, while unpasteurised dairy products should be strictly avoided.
Beyond the dairy industry, experts urge the public to avoid contact with sick or dead wild birds and marine mammals.
“People should also avoid touching dead or sick wild birds anywhere in the world and marine mammals washed up on the shores in South America because there is the possibility that these may be carrying H5N1 or related viruses,” Peiris warns.
The virus’s ability to spread into mammals does not automatically mean a human pandemic is on the horizon.
However, with H5N1 continuing to evolve, public health officials stress that early precautions, surveillance, and vaccine preparedness remain key tools in preventing another global crisis.
