Health workers in masks culled tens of thousands of chickens in Hong Kong yesterday, a day after authorities raised the bird flu alert level to "serious" following a H5 bird flu outbreak at a farm.

The outbreak near the border with the mainland was the city's first in five years despite mass vaccination of the birds, prompting concerns that the virus might have mutated.

"Viruses change and since 1997, it has been changing. If we have been using the same vaccine since 2003, its efficacy will not be the same," Ho Pak-leung, a microbiologist at the University of Hong Kong, told Reuters in an interview.

Laboratories in the city were trying to determine the precise identity of the H5 virus that caused the farm outbreak.

Health workers slaughter all the chickens at a wholesale poultry market in Hong Kong yesterday. Three chickens tested positive for bird flu in Hong Kong, prompting the city to suspend poultry imports. [Shanghai Daily]

Another expert said it was likely to turn out to be the highly pathogenic H5N1 strain, which crops up regularly in flocks in Asia, parts of Europe and Africa.

Although H5N1 is mainly found in birds, it may mutate into a form that spreads easily among people. If that happens, it could trigger a pandemic and kill millions. Even in its current hard-to-catch form, H5N1 has infected 389 people since 2003, killing 246 of them.

Ho said scientists would be able to establish within a week if there were changes or characteristics in the virus that might allow it to spread easily among people.

"Genetic sequencing is being done. The key is whether it is still a fully avian virus," Ho said.

Scientists in Hong Kong and on the mainland are studying if new vaccines are needed to protect chickens against H5N1.

"If study suggests that a new vaccine may provide better protection for chickens, we will select some to try out for a test of the new vaccine," Hong Kong's Health Secretary York Chow said.

Guangdong Province has suspended all live poultry shipments to Hong Kong for 21 days to prevent any spread of the virus, Xinhua news agency reported.

(Shanghai Daily December 11, 2008)