Despite mass vaccinations of poultry in China, the bird flu virus continues to evolve. Samples collected from poultry markets in southern China over the last year show that a variant of the virus has spread outward from a single province and supplanted strains in the rest of the region, researchers report. The result calls into question the steps taken so far to contain the virus, which public health officials fear could give rise to a deadly worldwide flu pandemic.

In early 2004 the H5N1 bird flu virus spread from poultry in China to southern Asia and has since been identified in birds as far away as Europe and north Africa. In principle, vaccination of domestic chickens and other birds could limit the virus's transmission and thereby its ability to evolve into a more transmissible form. With that goal in mind, China announced last November that it would begin vaccinating 14 billion domestic chickens against H5N1.

Since that time, however, the virus seems to have become even more entrenched in domestic poultry, report Chinese and American researchers in a paper published online October 30 by Proceedings of the National Academy of Sciences USA. The team collected samples from over 50,000 seemingly healthy birds between July 2005 and June 2006 at live poultry markets in six provinces of southern China. They identified H5N1 in 2.4 percent of birds, primarily domestic ducks and geese, up from 0.9 percent the year before.

To identify the source of the ongoing transmission, the researchers selected 390 virus samples from infected birds, determined their genetic sequence and compared these sequences with known variants of the virus. One strain, hailing from the province of Fujian, appeared in only 3 percent of birds collected up to September 2005. Between April and June of 2006, however, Fujian-like viruses were responsible for 95 percent of infections. "It means that the virus is still evolving," says co-author Robert Webster of St. Jude Children's Research Hospital in Memphis. "It means that the problem is not under control." Offshoots of the Fujian variant were isolated in the 22 human cases of bird flu reported in China since last November, and the strain has sickened birds in Laos, Malaysia and Thailand, where it also infected people, the group notes.

A weakness in China's vaccine may have allowed the previously local variant to become widespread, the researchers surmise. They analyzed serum samples from 76 chickens for signs of antibodies against three H5N1 variants, including the Fujian-like strain. The presence of antibodies is a sign that a vaccine has taken effect. Almost all of the samples displayed two to four times more antibodies to the other two variants than to the Fujian virus, suggesting that the vaccine given to the chickens was less effective against that strain, the researchers point out. The result highlights the need to supplement vaccination with other measures, says veterinary researcher Richard Slemons of Ohio State University. The former can be effective if part of a broader program of monitoring vaccinated chickens with surveillance afterward, agrees Webster. Vietnam has vaccinated its poultry and saw no new human cases of bird flu this year.