Vietnam's Ho Chi Minh City Pasteur Institute has found that the bird flu virus strain H5N1 in the country has mutated to make it more dangerous, local newspaper Youth reported Saturday.

There are some changes in gene segments of surface antigens HA and NA and some other gene segments of bird flu virus type A subtype H5N1 in Vietnam in early 2005, which indicates that the virus has been mutating to reproduce effectively in cell of mammals, and increase its attacking capability, according to results of the institute's research on 24 virus samples from infected poultry and people in the southern region between December 2003 and March 2005.

Besides, all the viruses researched by the institute in southern Ho Chi Minh City are resistant to the two antiviral medicine agents of Amantadine and Rimantadine. Therefore, it is necessary to keep close surveillance on H5N1's resistance to Oseltamivir (active agent contained in bird flu medicine Tamiflu),the report stated, warning that people will be likely to have to face the human-to-human transmission of H5N1.

The institute has decoded completely or partly genes of 24 virus samples. Five samples from humans and 16 from fowls have been decoded completely.

However, the institute has been unable to define which kinds ofmutations allow human-to-human transmission, and which conditions lead to the mutations.

"To cause flu in people, the bird flu virus must mutate on certain conditions so that it can penetrate into human cells," Hoang Thuy Long, a leading epidemiologist in Vietnam, told Xinhua recently.

This leads to two hypotheses: H5N1 passes from chickens to humans on certain conditions, and the virus transmits to humans via a mammal, he said.

To successfully penetrate into a person's cells, H5N1 must mutate, while the victim must have defective immune systems. "Otherwise, the virus in poultry must combine with flu viruses in amammal such as pigs, buffaloes, cows, dogs and cats to form a new strain," Long confirmed.

Up to 92 Vietnamese people from 32 cities and provinces have been infected with bird flu virus strain H5N1 since the disease started to appear in the country in late 2003, according to the Vietnamese Ministry of Health.

The World Health Organization, on Nov. 9, confirmed a total of 125 human cases of bird flu infections, including 64 deaths, in Southeast Asia since December 2003. Of the fatalities, 42 are fromVietnam, 13 from Thailand, 5 from Indonesia and 4 from Cambodia.