The risk of H5N1 combining with seasonal flu to create a more dangerous strain of the virus will increase dramatically this winter, a major disease-tracking platform has predicted.
When two or more strains of a virus infect the same person, they can exchange genetic material and create new, more dangerous variants of a disease through a process called reassortment.
According to preliminary modelling by analytics firm Airfinity, the risk of H5N1 reassorting this winter will increase by five-fold compared with the summer months, due to the onset of seasonal flu.
Reassortment is a particular concern in the case of H5N1, the bird flu that has spread through cattle herds and poultry farms across the United States and infected several farm workers this year.
H5N1 has a high mortality rate but currently spreads inefficiently between humans.
But if it were to combine with a more transmissible influenza variant like seasonal flu it could become far better at transmitting from one person to another, which might cause major outbreaks or even a pandemic, experts fear.
The likelihood of dairy and poultry workers – those most at risk of catching H5N1– also being infected with seasonal influenza, rises in winter when flu infections surge naturally, owing largely to colder temperatures, indoor crowding, and weakened immune systems.
The 2009 H1N1 or ‘swine flu’ pandemic emerged as a result of a triple reassortment between avian, swine, and human influenza viruses, which led to a new variant which was able to spread rapidly, contributing to the deaths of an estimated quarter of a million people and infecting between 11 and 21 per cent of the global population.
The conditions for a reassortment event may be even better today, scientists argue.
Reassortment through coinfection
Early evidence suggests the effectiveness of the 2024 seasonal flu vaccine might be as low as 35 per cent, studies by the US Centre for Disease Control (CDC) of the recent flu season in the Southern Hemisphere have shown.
Coupled with a low uptake of the vaccine, which would leave more people at risk of catching the flu, these factors raise the risk of reassortment taking place, said Connor Browne, a biorisk consultant.
“The combination of a lower-than-expected efficacy for this year’s flu vaccine and the likelihood that uptake of the vaccine could well be lower than in previous years increases the chances of a H5N1 reassortment event occurring through coinfection,” he said on Twitter.
At least 19 people have been infected with H5N1 in the US this year – almost all of them poultry or dairy workers who had direct contact with sick animals.
But one case – a patient in Missouri – had no known contact with animals at all.
At least seven people who had contact with that patient came down with flu-like symptoms at a similar time, raising concerns the virus could be already spreading between people.
The CDC are investigating the potential cluster, but have not said whether or not they were infected with H5N1.
Experts have long considered avian influenza a top global health threat and have urged governments worldwide to fortify their pandemic preparedness plans in case the virus begins to spread between humans, something the World Health Organization has repeatedly stressed would be of “enormous concern”.
Dr Angela Rasmussen, a virologist at the Vaccine and Infectious Disease Organisation, told a press conference on Thursday that the US needs to focus on “access to testing, getting vaccines to people with high-risk occupational exposure, and answer some of the basic science questions – like how the virus is being transmitted between cows”.
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