The virus, which has infected 130 people in Asia and killed 67 of them since late 2003, is changing constantly and repeat infections by new strains could still leave a birdflu survivor in peril, just as people can catch a new form of the flu every year.

"Infected people with antibodies (to H5N1) should be protected for years against the same virus. But usually influenza viruses are changing constantly. You may not be protected from viruses with some changes," said Hitoshi Oshitani, a World Health Organisation Adviser on communicable diseases.

"This is why we have seasonal outbreaks of influenza every year and people can be infected with influenza every year," he wrote in an e-mail in reply to questions from Reuters.

Health experts are warning that the virus, which is spreading in poultry in parts of Asia and which has mostly jumped directly to humans from birds, will mutate into one that is easily passed between people and set off a pandemic, killing millions.

Influenza is an RNA virus, which is unsteady when it replicates. This results in frequent variation, or mutation, and finally one form will spread from person to person.

Samson Wong, a microbiologist with the University of Hong Kong, said no one knows if a birdflu survivor would be immune to future attacks of the virus.

"No one knows if a survivor will survive a repeat attack, because it is a new disease. They should have protective immunity, but like human influenza, H5N1 could mutate, and if the person is infected by a different strain, he may get sick all over again,' Wong told Reuters.

"If the person is infected by the same strain, he should have immunity, but nobody knows for how long."

How well survivors tackle repeat H5N1 attacks may also depend on how well their systems "remember" the virus strain.

Lymphocytes, the blood cells that make antibodies, have memory cells and this function allows them to remember a past encounter with the H5N1 strain, and produce antibodies to neutralise the virus in future encounters, experts say.

"If the survivor is infected by the same strain of H5N1, he should produce large amounts of antibodies to fight repeat infections," said William Chui, honorary associate professor at the department of pharmacology at the University of Hong Kong.