HONG KONG, Dec. 13 (Xinhua) -- A health official of Hong Kong said here Saturday laboratories in Hong Kong and the Chinese mainland will study carefully test results for the samples taken from a chicken farm where a recent avian flu outbreak occurred.

    Dr. Leung Ting-hung, Acting Controller of the Center for Health Protection in Hong Kong, said on a radio program here Saturday that health authorities will check whether there is any virus mutation in the outbreak and the government will enhance the vaccines' effectiveness when the situation warrants it.

    Hong Kong reported cases of the deadly H5N1 avian flu strain last week at a chicken farm in Yuen Long district, where dozens of mass abnormal deaths of chicken were found.

    It was the first outbreak of H5N1 bird flu at a Hong Kong farm in about six years.

    Expecting the preliminary test results to be ready in a week, Leung noted the six workers who had direct contact with the dead chickens at the farm and those involved in the recent chicken- culling operation are under close medical surveillance.

    "All of them are healthy," he said.

    Assistant Director of Agriculture, Fisheries and Conservation (Inspection and Quarantine) Dr. Thomas Sit said the department is investigating the outbreak and seek more information from some complainants claiming smuggled eggs was the cause.

    "Should the complaints be true, small chickens should have died instead of the older sentry chickens as happened this time," Dr. Sit said, adding the department will conduct a thorough check on all farms during the 21-day live chicken ban.