This week’s bird flu outbreak is the first incident of this type on a Hong Kong farm since 2003, when nearly a million birds were slaughtered after dozens died of the flu. No human cases were reported that year in Hong Kong.

Up to 60 dead chickens were discovered in a Hong Kong poultry farm and tests showed that the birds died after being infected with the H5 virus. The area surrounding the farm and the farm itself were declared as part of an infected area.

To prevent the spread of the deadly virus, more than 80,000 chickens within a three-kilometre radius of the infected farm will be culled.

Hong Kong will also suspend poultry imports for 21 days.

No humans are known to have been infected in the current outbreak, the officials said.

The outbreak is alarming because some of the chickens that were killed by the virus appear to have been vaccinated against it. Infectious disease experts say that Hong Kong uses an older version of the H5 vaccine than mainland China, where there are more frequent outbreaks and farmers vaccinate poultry specifically against the H5N1 strain of the virus.

Last year, in early September a bird flu outbreak in south China’s Guangzhou led to the death of more than 9,000 ducks. Then, the National Avian Influenza Reference Laboratory confirmed the outbreak as a sub-type of H5N1 bird flu and the government had culled more than 50,000 fowls in the infected region. In 1997, the H5N1 strain adapted to humans and killed six people – it was Hong Kong’s biggest outbreak of the bird flu.

Therefore, Hong Kong’s government said it was looking at whether there was a need to change the vaccine used to protect chickens against avian flu after the latest outbreak.

The bird flu virus has mutated over the last few years, although it may take some time before it turns into a human-to-human type of virus.

Speaking to reporters, health secretary York Chow said the H5N1 vaccine currently used on chickens is manufactured in the Netherlands and was considered to be effective, but because of the mutation of the virus, the government has asked the University of Hong Kong and the mainland to conduct research in order to see if there is a need to replace the vaccine or to look for a more appropriate one. However, he said it is too premature to decide on that.

According to the Word Health Organization, there have been 382 human cases of infection with the H5 virus since 2003, 241 of them fatal. The highly pathogenic Influenza A virus subtype H5N1 is causing global concern as a potential pandemic threat. Health experts fear that the co-existence of human flu viruses and avian flu viruses will provide an opportunity for genetic material to be exchanged, creating a new influenza strain that may cause fatal human infections.