Evidence that the bird-flu virus has evolved into distinct subgroups is challenging the wisdom of relying on a single strain of the disease to make a vaccine.

Dr. Robert G. Webster one of the co-authors of the study, is a leading American influenza expert.

A study to be published this week in the US Proceedings of the National Academy of Sciences, and produced by up to 30 researchers, analysed thousands of bird-flu samples taken from across southern China.

The authors say that they have shown that H5N1 virus has persisted in its birthplace, southern China, for almost 10 years and has been repeatedly introduced into neighbouring (e.g., Vietnam) and distant (e.g., Indonesia) regions, establishing ‘‘colonies’’ of H5N1 virus throughout Asia that directly exacerbate the pandemic threat. They say that the best approach to avert the threat is to control H5N1 virus infection at its source, domestic poultry. Experience in Hong Kong, Japan, and South Korea has shown that early detection and large-scale culling of infected poultry, combined with other measures, is effective in controlling this HPAI H5N1 influenza.

The authors says that control measures in China, Indonesia, Thailand, and Vietnam have been less effective, allowing the establishment of virus endemicity and repeated interspecies transmission to humans. They say that results indicate that H5N1 virus has been introduced into Vietnam from southern China on multiple occasions; in 2001, 2003, and 2005. Therefore, control of this regional epizootic and its attendant pandemic threat requires that the source of virus in southern China be contained.

The authors say that "the antigenic diversity of viruses currently circulating in Southeast Asia and southern China challenges the wisdom of reliance on a single human vaccine candidate virus for pandemic preparedness; the choice of candidate viruses for development of human vaccines must reflect the antigenic diversity observed across this wider region. Furthermore, antigenic drift observed over time within those H5N1 sublineages highlights the necessity of continually updating the candidate virus chosen for future H5N1 vaccines. These concepts are critical for the control of this pandemic threat."

Source: Paper - Establishment of multiple sublineages of H5N1 influenza virus in Asia: Implications for pandemic control - - Map of Eastern Asia showing Fujian, Guangdong, Guangxi, Guizhou, Hong Kong, Hunan, and Yunnan where influenza surveillance was conducted and the locations of Poyang and Qinghai Lakes. Following are the genotypes of H5N1 viruses tested in poultry from each Province in our surveillance program in southern China since 2004, plus data from migratory birds at Poyang and Qinghai Lakes: Guangxi, genotypesW(15), Z (5), andG(3); Hunan, genotypes Z (13) and G (3); Fujian, genotype Z (2); Yunnan, genotype Z (4); Guangdong, genotype Z (7); Poyang Lake, genotypes Z (3) and V (3); Qinghai Lake, genotype Z (20). (Scale bar: 500 km.)

The World Health Organization has not yet changed its recommendation that vaccine manufacturers focus on an older strain of the virus to make their bird-flu vaccines. It is however reported that the WHO,  is considering whether to revise that guidance. Some strains isolated from both birds and humans are being tested at several WHO laboratories around the world to check which might serve best as the basis of a vaccine.