"It's quite possible ... H9N2 is infecting humans quite a lot, much (more) than we appreciate merely because it is beyond the radar," Malik Peiris, a Hong Kong-based microbiologist, told Reuters.
"In humans, it is very mild, so most of the time it's probably not even recognized or biologically tested," said Peiris, who has co-authored several papers on the strain in recent years.
So far, only a handful of human H9N2 cases have been documented worldwide, including four children in Hong Kong in 2003 who suffered from mild fevers and coughs -- as well as a batch in China's Guangdong province, where people often live in close proximity to poultry, Peiris said.
The Hong Kong cases were only picked up by chance given the city's rigorous influenza testing regime, Peiris said.
"It's quite a silent virus, it's not highly pathogenic ... and sometimes it causes some morbidity in poultry but by and large it is just there and it's unnoticed," Peiris said of the H9N2 strain.
The strain occurs mostly in birds, although it has also affected pigs and other animals in Europe and Asia.
Most influenza experts agree that a pandemic -- a deadly global epidemic -- of some kind of flu is inevitable.
No one can predict what kind but the chief suspect is the H5N1 bird flu virus, which has infected 385 people and killed 243 of them since 2003.
However, flu experts at the University of Maryland, St. Jude's Children's Research hospital in Memphis and elsewhere recently wrote in the Public Library of Science journal PLoS ONE that the H9N2 strain posed a "significant threat for humans."
They found that just a few mutations could turn it into a virus that people catch and transmit easily.
Peiris said that while the H9N2 strain might be more transmissible, its effects would be far less devastating than a possible H5N1 pandemic.
"There are other viruses out there besides H5N1 that could be the next pandemic," Peiris said. "But I suspect (H9N2) will not be so severe in its outcome."
Peiris pointed out that the last three major pandemics vastly differed in their severity, with the 1918 Spanish flu pandemic killing an estimated 50 million people worldwide, whereas the "Hong Kong" flu in 1968 killed around one million.
There are hundreds of strains of avian influenza virus but only four -- H5N1, H7N3, H7N7, and H9N2 - are known to have caused human infections, according to the World Health Organization.
(Reporting by James Pomfret; Editing by Paul Tait)