A new strain of the killer H5N1 avian flu virus that had emerged in early 2006 in China and southeast Asia appeared to be resistant to the vaccination programme and might even be aided by it.
It might have begun the third wave of transmission of H5N1 avian flu and could potentially spread throughout Eurasia, the scientists warned in the latest edition of the Proceedings of National Academy of Sciences.
The new strain had infected poultry in Hong Kong, Laos, Malaysia and Thailand, and affected people in China and Thailand, scientists from the Hong Kong University and the St. Jude Children's Research Hospital in the United States said.
Fortunately, the virus strain did not appear to have the ability to spread among humans as yet, the researchers added.
"There's no evidence in this paper of additional human-to-human transmission, which is the real bottom line we're all worried about, " said Robert Webster, a study co-author at the St. Jude Children's Research Hospital in Memphis.
"But so long as the virus is out there in such numbers, it is going to be a continuing pandemic threat."
The researchers also said that current poultry vaccination was ineffective against the new strain.
The report, based on China's ongoing flu surveillance programme, found that H5N1 was more prevalent from July 2005 to June 2006 compared with the previous 12 months.
The researchers tested 53,220 birds in poultry markets and 2.4 percent of them tested positive for H5N1 strain, up from 0.9 percent a year earlier.
It was found that ducks and geese were the most common carriers, and they were susceptible to bird flu year-round.