HONG KONG, June 16 (AP) -- A recent bird flu case involving a man in southern China may indicate that the H5N1 virus has mutated so that the infection risk is as high in summer as in winter, Hong Kong's health chief said Friday.

China confirmed on Thursday that the 31-year-old man in the mainland city of Shenzhen, just across the border from Hong Kong, was critically ill with H5N1.

"We are concerned whether the virus has mutated so that the infection rate has become equally high all year around," said York Chow, secretary for Health, Welfare and Food.

The spread of the bird flu virus is believed to be most active in winter, when many birds migrate.

The virus "is now found in summer. Is it because the summer and winter seasons make no difference to it? Or is it that it is active in summer, but gets even stronger in winter? We still have to monitor the situation, and we can't say for sure now," Chow said.

The official Xinhua News Agency said Thursday that the Shenzhen man, hospitalized in critical condition, had visited a local market where live poultry was sold several times before developing a fever and pneumonia on June 3.

No bird flu outbreak has been reported in Shenzhen. Health experts say close contact with infected birds puts people at high risk for infection, but the World Health Organization has said that eating well-cooked chicken does not pose a bird flu risk.

The man is China's 19th reported human case of bird flu, Xinhua said. Twelve people have died.

Hong Kong said it would halt imports from the mainland for three weeks starting Friday.

Bird flu has killed at least 128 people worldwide since it started ravaging Asian poultry farms in late 2003.

Scientists fear the H5N1 virus will mutate into a highly contagious form, possibly sparking a global pandemic. So far, most human cases have been linked to infected birds.