A new strain of flu with 'pandemic potential' has been discovered in pigs in China, scientists have confirmed.
The new flu strain 'G4 EA H1N1', which can infect humans, is similar to the swine flu that spread globally in 2009, researchers say.
Carried by pigs, the virus has 'all the essential hallmarks of being highly adapted to infect humans'.
Writing in the US journal Proceedings of the National Academy of Sciences, the Chinese scientists highlighted their concerns that it could mutate further.
But while the virus is not seen as an immediate threat because it has not, so far, infected humans, they called for measures to monitor people working in the pig sector.
The strain can grow and multiply in the cells that line the human airways, and current vaccines do not appear to protect against it.
As part of the study which discovered the strain, scientists worked from 2011 to 2018 taking 30,000 swabs from pigs in Chinese abattoirs, and another 1,000 swabs from pigs with respiratory symptoms at a veterinary hospital.
The researchers found 179 virus strains in Chinese pig populations, but G4 EA H1N1 stood out.
The scientists said this strain is "distinct from current human influenza vaccine strains, indicating that preexisting immunity derived from the present human seasonal influenza vaccines cannot provide protection."
Prof Kin-Chow Chang, who works at Nottingham University, told BBC News that the world is 'distracted' with the Covid-19 pandemic and 'rightly so'.
"But we must not lose sight of potentially dangerous new viruses... We should not ignore it," he said.