The emergence of new variants of the H5N1 avian flu virus doesn't appear to raise or lower the risk the virus poses to humans, officials of the World Health Organization and the Food and Agriculture Organization said Tuesday.

Representatives of the UN agencies charged with animal and human health issues held a teleconference Tuesday to discuss the discovery of the new subgroup of viruses. The new variant, called a Fujian-like virus, was reported Monday in the scientific journal The Proceedings of the National Academy of Sciences.

The newly described subgroup of viruses could pose challenges for containing H5N1 in poultry because vaccines being used in China — where it emerged — may not protect domestic birds against this variant.

But the pattern of human cases with these viruses is similar to that seen with viruses spreading in Indonesia, or those that caused human infections in Vietnam in 2004 and 2005, said Michael Perdue of the WHO. The variant is responsible for recent human cases in China and Thailand.

"If you look at the mortality rate and the disease, the Fujian-strain infections are no different," said Perdue, a senior scientist with the WHO's global influenza program.

"So there's no reason to lead us to believe that this sublineage is acting any differently than any of the other sublineages in terms of affecting humans."

Dr. Juan Lubroth, head of infectious diseases for the FAO's animal health service, said the variant bears watching, but doesn't raise the level of alarm.

"The virus isn't necessarily more pathogenic, more virulent. The fact that you have a virus that has mutated or has changed somewhat and that humans have become infected with the more dominant virus when it has occurred is logical, because that's the virus that is circulating," he said.

The study shows the importance of conducting regular surveillance to look at what viruses are infecting poultry, as well as the need to update poultry vaccines so that they protect against the viruses in circulation.

He said, however, that the FAO is not certain Chinese manufacturers are taking this important step.

"We don't have enough data from China to say that this is what is happening."

The researchers, from Hong Kong University and St. Jude Children's Research Hospital in Memphis, Tenn., discovered the new variant as part of an ongoing surveillance program looking for H5N1 infection in the live poultry markets of southern China.

The authors say the new variant appears to have emerged in Fujian province in 2005 and has since become the dominant virus type in southern China. It has spread to Hong Kong, Laos, Malaysia and Thailand.