A new strain of the killer H5N1 avian flu virus had emerged and become the dominant strain in southern China and southeast Asia by early this year, displacing previous ones, according to a study published yesterday.

The strain appeared to be resistant to the current chicken vaccination program 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 sickened people in China and Thailand, reported the scientists from the Hong Kong University and the St. Jude Children's Research Hospital in the United States.

Fortunately, as yet, the virus strain did not appear to have improved its ability to spread among humans, 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 these numbers, it's 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 program, found that H5N1 became more prevalent from July 2005 to June 2006 compared with the previous 12 months. The researchers tested 53,220 birds in live poultry markets and found that 2.4% of them tested positive for any strain of H5N1, up from 0.9% a year earlier.

They found that ducks and geese were the most common carriers, and they were susceptible to bird flu year-round. Chickens tend to succumb only in the winter, but the researchers discovered cases in 11 out of the 12 months of their study, up from four out of 12 months the previous year.

Overall, the peak flu season of October to March had been extended until June, the researchers found. The strain had steadily risen in prevalence over the last year, accounting for 103 of the 108 samples tested from April to June.

Now it is likely a third wave has already started, according to the researchers led by Yi Guan, a professor at the University of Hong Kong.

The emergence of a dominant strain has some advantages, in that it is easier to control a single dominant lineage than a number of smaller ones, the researchers said. But on the negative side, a dominant strain is also more likely to spread widely.