Hanoi (AsiaNews/Agencies) – The avian or bird flu might strike without patients showing any symptoms—the disease might therefore be more widespread than thought before.

Today Vietnamese health authorities have released information about two such cases. Nguyen Tran Hien, director of the National Institute for Hygiene and Epidemiology, confirmed that lab tests showed that a 61-year-old woman from northern Thai Binh province had contracted the H5N1 virus but was not showing any symptoms.

The woman's husband died on February 23 from the bird flu but thus far she has not shown any of the disease's typical symptoms such as respiratory problems and high fever. She remains in good health but has been placed in isolation.

Nguyen Van Thieu, director of the Kien Xuong District Medical Centre in Thai Binh, explained that the "source of infection is still unclear". The woman's family raised chickens but none was infected.

Tests conducted at the National Institute for Hygiene and Epidemiology in Hanoi showed that an 80-year-old man had also contracted the virus but he, too, had not fallen ill. By contrast, his 21-year-old grandson and his 14-year-old granddaughter had been hospitalised earlier in Hanoi with the bird flu.

Health officials suspect the three, who are also from Thai Binh province, might have become infected eating an infected duck a month ago.

So far there is no evidence of person-to-person contagion. However, the two latest cases are raising fears that the bird flu might be more widespread than thought before.

"It's quite possible that some people are falling sick and their symptoms are very light and they don't end up in hospital," Peter Cordingley, regional spokesman for the World Health Organisation, said earlier this week.

Since 2003 the bird flu has killed 46 people in South-East Asia: 33 in Vietnam, 12 in Thailand and 1 in Cambodia. It has also forced the authorities to cull millions of birds.