A new study warns that the strain of H1N1, common on China’s pig farms since 2016, should be “urgently” controlled to avoid another pandemic.

A new strain of the H1N1 swine flu virus is spreading silently in workers on pig farms in China and should be “urgently” controlled to avoid another pandemic, a team of scientists says in a new study.

H1N1 is highly transmissible and spread around the world in 2009, killing about 285,000 people and morphing into seasonal flu.

The newer strain, known as G4 EA H1N1, has been common on China’s pig farms since 2016 and replicates efficiently in human airways, according to the study published on Monday. So far, it has infected some people without causing disease, but health experts fear that could change without warning.

“G4 viruses have all the essential hallmarks of a candidate pandemic virus,” the study said, adding that controlling the spread in pigs and closely monitoring human populations “should be urgently implemented.”