Influenza is the pathogen most likely to trigger a new pandemic in the near future, according to leading scientists.

An international survey, to be published next weekend, will reveal that 57% of senior disease experts now think that a strain of flu virus will be the cause of the next global outbreak of deadly infectious illness.

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 Cologne University’s Jon Salmanton-García, who carried out the study.

“Each winter influenza appears,” he said. “You could describe these outbreaks as little pandemics. They are more or less controlled because the different strains that cause them are not virulent enough – but that will not necessarily be the case for ever.”

Details of the survey – which involved inputs from a total of 187 senior scientists – will be revealed at European Society of Clinical Microbiology and Infectious Diseases (ESCMID) congress in Barcelona next weekend.

The next most likely cause of a pandemic, after influenza, is likely to be a virus – dubbed Disease X – that is still unknown to science, according to 21% of the experts who took part in the study. They believe the next pandemic will be caused by an as-yet-to-be-identified micro-organism that will appear out of the blue, just as the Sars-CoV-2 virus, the cause of Covid-19, did, when it started to infect humans in 2019.

Indeed, some scientists still believe Sars-CoV-2 remains a threat, with 15% of the scientists surveyed in the study rating it their most likely cause of a pandemic in the near future.

Other deadly micro-organisms – such as Lassa, Nipah, Ebola and Zika viruses – were rated as serious global threats by only 1% to 2% of respondents. “Influenza remained – by a very large degree, the number one threat in terms of its pandemic potential in the eyes of a large majority of world scientists,” added Salmanton-García.

Last week, the World Health Organization raised fears about the alarming spread of the H5N1 strain of influenza that is causing millions of cases of avian flu across the globe. This outbreak began in 2020 and has led to the deaths or killing of tens of millions of poultry and has also wiped out millions of wild birds.

Most recently, the virus has spread to mammal species, including domestic cattle which are now infected in 12 states in the US, further increasing fears about the risks to humans. The more mammalian species the virus infects, the more opportunities it has to evolve into a strain that is dangerous to humans, Daniel Goldhill, of the Royal Veterinary College in Hatfield, told the journal Nature last week.

The appearance of the H5N1 virus in cattle was a surprise, added virologist Ed Hutchinson, of Glasgow University. “Pigs can get avian flu but until recently cattle did not. They were infected with their own strains of the disease. So the appearance of H5N1 in cows was a shock.

“It means that the risks of the virus getting into more and more farm animals, and then from farm animals into humans just gets higher and higher. The more the virus spreads, then the chances of it mutating so it can spread into humans goes up and up. Basically, we are rolling the dice with this virus.”

To date, there has been no indication that H5N1 is spreading between humans. But in hundreds of cases where humans have been infected through contact with animals over the past 20 years, the impact has been grim. “The mortality rate is extraordinarily high because humans have no natural immunity to the virus”, said Jeremy Farrar, chief scientist of the World Health Organization.

The prospect of a flu pandemic is alarming, although scientists also point out that vaccines against many strains, including H5N1, have already been developed. “If there was an avian flu pandemic it would still be a massive logistical challenge to produce vaccines at the scale and speed that will be needed. However, we would be much further down that road than we were with Covid-19 when a vaccine had to be developed from scratch,” said Hutchinson.

Nevertheless, some lessons of preventing disease spread have been forgotten since the end of the Covid pandemic, said Salmanton-García. “People have gone back to coughing into their hands and then shaking hands with other people. Mask-wearing has disappeared. We are going back to our old bad habits. We may come to regret that.”