Gene segments of surface antigens of avian flu virus type A subtype H5N1 found in chickens and a number of patients in Vietnam are nearly the same, director of the country 's Hygiene and Epidemiology Institute told Xinhua on Wednesday.

"Gene segments of surface antigens HA, NA and NP are of respective similarity of 98.6 percent, 98.4 percent and 99.5 percent," said the director Hoang Thuy Long.

The findings by the US Center for Disease Control verifies the fact that affected chickens are a source for H5N1 to jump to humans,he said, adding that the transmission mechanism has not been identified yet.

"To cause flu in people, avian flu virus must mutate on certain conditions so that it can penetrate into human cells," Long stressed.

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 a mammal such as pigs, buffaloes, cows, dogs and cats to form a new strain," Long confirmed.

"No H5N1 has been found in pigs in Vietnam," he noted.

Recently, Vietnam's Biotechnology Institute announced that it has decoded N1 genes of H5N1. Earlier, the Pasteur Institute in Ho Chi Minh City decoded H5 genes. Their findings will help local medical officers early test samples for H5N1, and effectively use vaccines against flu type A.

All of local people, who are confirmed to have contracted H5N1, have had a direct contact to sick poultry, said Le Truong Giang, deputy director of Ho Chi Minh City's Health Department.

All of health workers, relatives and those who have contacted with bird flu patients are perfectly healthy without any signs of fever,he added.

Recently, the World Health Organization (WHO) corrected an initial statement that the viruses of two Vietnamese sisters suspected of contracting the disease from their brother have been tested. Accordingly, H5N1 has not passed from the brother to his two younger sisters, since a sample testing showed that the virus was entire avian in origin with no human genes.

However, genetic tests were not completed, WHO said, adding that testing on the other sister will be completed shortly.

By Tuesday afternoon, Vietnam reported 185 cases of flu type A infections and suspects, including 32 fatalities.