The diagnosis of Hong Kong's first case of H7N9 avian flu on Monday realized the worst fears of the health care community that the potentially lethal flu strain would return this winter.

An Indonesian maid is believed to have caught the virus after killing a chicken bought in Shenzhen, bringing the total number of confirmed cases of influenza A (H7N9) in the country to six in the last two months. The five other cases occurred on the mainland in October and November.

According to the World Health Organization, the virus infected 135 people, of which 45 died, between February and July this year. The virus appeared to disappear in August and September, before making a comeback in October.

Professor Benjamin Cowling, part of a team of scientists who carried out research on H7N9 published July in the Lancet medical journal, said he expected more cases in the coming months, the peak season for this type of virus.

"We predicted H7N9 would go away in summer and then reappear in winter. The only caveat is that there are many new viruses in birds and when new viruses come along they tend to push older ones out. But these recent cases mean H7N9 has stuck around."

Cowling, head of the Division of Epidemiology and Biostatistics at the University of Hong Kong's School of Public Health, said research suggested the confirmed cases were only the tip of the iceberg and many milder infections had gone undetected.

However, he warned that although the undetected cases may mean H7N9 was less severe than the confirmed cases indicated, it also meant the virus was more widely circulated. That raised the risk that H7N9 could mutate, making it more infectious to humans.

"One of the biggest fears with H7N9 is that it could adapt and spread from human to human. Every time it infects a human there is the chance the virus will mutate and acquire that ability," he said.

There is also the risk that avian flu could mix with seasonal flu within one person to become a strain more infectious among humans. This is why those working with poultry and pigs should be vaccinated against seasonal flu, said Cowling.

This concern is echoed by Hong Kong Medical Association vice-president Dr Alvin Chan Yee-shing, who believes the presence of avian flu strengthens the case for seasonal flu vaccinations.

"Although the seasonal influenza vaccination cannot prevent infection from any bird flu virus, it could at least decrease the chance of mutations caused by the swapping of the DNA between the seasonal human, avian and swine flu viruses," said Chan.

The Department of Health said it had implemented several measures to safeguard Hong Kong against a pandemic of H7N9 and as a result of the new case had raised the response level of its Preparedness Plan for Influenza Pandemic to "serious" and suspended the import of live chickens from Shenzhen.

"The Department of Health will continue to maintain close liaison with the World Health Organization (WHO), the mainland and overseas health authorities to monitor the latest development," it said in a statement.

(HK Edition 12/06/2013 page1)