HANOI, June 27 (Xinhuanet) -- Laboratory tests in Vietnam and abroad have indicated that the antigen structure of bird flu virus strain H5N1 in the country is changing, local newspaper Labor Monday quoted a Vietnamese Health Ministry report as saying.

    The change, which might increase the possibility of bird flu infections on human, explains why people in Vietnam have still been infected with H5N1 in recent months, although it has detected no major outbreaks.

    The mutation also makes it more difficult for health agencies to identify high-risk areas, since poultry infected with H5N1 with a change in the antigen structure do not exhibit the disease's symptoms, said the report.

    An international team, consisting of top virologists and epidemiologists from Chinese Hong Kong, Japan, the United Kingdom and the United States, is studying the possibility of more widespread H5N1 human transmission, changes in the virus strain, and the likelihood of increased person-to-person transmission. The experts are expected to make initial conclusions on the virus's mutation late this month.

    To minimize infections on poultry as well as humans, Vietnam will start vaccinating fowls against bird flu viruses in northern Nam Dinh province and southern Tien Giang province on a trial basis in early August, and then do the same in other localities with high risks of outbreaks in October if the vaccination proves effective.

    The country's Veterinary Department is asking localities to reduce the number of poultry flocks, raise and slaughter them on large scale in concentrated areas, and cull infected poultry as well as flocks of less than 30 fowls which are raised freely in gardens or fields, regardless they contract H5N1 or not.

    The total of bird flu patients confirmed in Vietnam since mid-December 2004 has increased to 59 cases, of which 18 were fatal. Enditem