A new type of swine flu with "pandemic potential" has been discovered by scientists in China.
The virus, known as G4, is a descendant of the H1N1 strain that originated in Mexico and caused a pandemic in 2009.
Writing in American science journal PNAS, researchers said the strain "possessed all the essential hallmarks of being highly adaptive to infect humans".
People do not have immunity against the virus, they said.
Researchers were especially concerned by blood studies that showed the virus appeared to have become increasingly infectious to humans.
But they said there was no evidence yet that it was capable of being transmitted from person to person.
More than 10 per cent of swine workers tested positive for the virus, especially participants aged from 18 to 35, of whom 20.5 per cent tested positive, "indicating that the predominant G4 EA H1N1 virus had acquired increased human infectivity", researchers wrote.
"Such infectivity greatly enhances the opportunity for virus adaptation in humans and raises concerns for the possible generation of pandemic viruses."
Kin-Chow Chang, a professor of veterinary molecular medicine at the University of Nottingham, was one of more than a dozen Chinese scientists who co-wrote the report.
"Right now, we are distracted by coronavirus and rightly so. But we must not lose sight of potentially dangerous new viruses," he said on BBC News.
While the new virus was not an immediate problem, Prof Chang said it should not be ignored.
The virus was discovered during swine flu surveillance studies that took place between 2011 and 2018.
Researchers took 30,000 nasal swabs from pigs and isolated 179 swine flu viruses.
The majority were of the new virus, G4, which scientists said had been the dominant strain among pigs since 2016.
Researchers infected ferrets with the virus, because they suffer symptoms similar to humans, to see how people may react to infection.
G4 proved to be highly infectious and caused more serious symptoms in the ferrets than other viruses.
The researchers called for the new strain to be monitored closely.
"Of concern is that swine workers show elevated seroprevalence for G4 virus," they wrote.
"Controlling the prevailing G4 EA H1N1 viruses in pigs and close monitoring in human populations, especially the workers in swine industry, should be urgently implemented."
Although the H1N1 strain that caused the 2009 pandemic was first feared to be a serious risk to health, it turned out to be a mild illness.
Estimates suggested that one in five people were infected with swine flu in the first year of the outbreak, but the mortality rate was 0.02 per cent – about five times lower than the seasonal flu.
The Centres for Disease Control in the US estimated that the 2009 swine flu outbreak caused the deaths of between 150,000 and 575,000 people worldwide.
About 80 per cent of swine flu deaths were of people under the age of 65, in contrast with seasonal influenza, a virus that typically claims the lives of older people.
Covid-19 has officially caused more than 500,000 deaths worldwide in the first five months of the pandemic, although the toll is thought to be higher.
Researchers estimated that its global mortality rate was about 1 per cent, although it varies from country to country.
That makes Covid-19 many more times deadlier than the flu.
