WASHINGTON (Reuters) - Bird flu is likely to cross over into people again and again if it even once acquires the ability to pass from human to human, experts predicted on Monday.

　　In theory, the virus only has to mutate once, in one person, to spark a pandemic. But the researchers argue that this could happen again and again, in several places around the world.

　　They said even if the current pandemic killing birds passes, no one should breathe a sign of relief because the threat to people will not be gone.

　　"At best, a containment policy will only postpone the emergence of a pandemic, 'buying time' to prepare for its effects," Dr. Marc Lipsitch and colleagues from the Harvard School of Public Health and Dr. Carl Bergstrom from the University of Washington wrote.

　　This is what officials hope they are doing now by culling birds when new outbreaks of H5N1 avian influenza occur. Public health experts agree the world is nowhere near ready to cope with a pandemic, but with a few years' preparations, some countries might be.

　　"We argue here that if a single introduction of a pandemic-capable strain is expected, multiple introductions should also be expected," Lipsitch's team wrote in the Public Library of Science Medicine, an online medical journal.

　　"Each containment effort would likely be more difficult than the last as manpower, antiviral stockpiles, and other scarce resources become depleted," they wrote.

　　H5N1 avian influenza has spread in chickens from Korea, across China, south into Indonesia, west across Turkey into western Europe and into the African continent.

　　It has killed or forced the culling of more than 200 million birds in 32 countries and Hong Kong. While it does not easily infect people yet, it has sickened 170 people and killed 92, according to the latest World Health Organization figures.

　　No one can say if or when it will happen, but if H5N1 acquires the ability to pass easily from human to human, it could spark a pandemic that could kill millions or even tens of million within a few months.

　　TEMPORARY CONTAINMENT

　　Some experts have published theoretical models showing that quick action with antiviral drugs, culling of birds and isolation of cases could quell such a pandemic before it started.

　　But it would require a lot of luck, noted Lipsitch and colleagues —— not the least identifying those cases right away, before they spread the disease.

　　Other experts have also noted this and also said there is no reason to believe that the mutations needed to make H5N1 a human disease would occur only once. Lipsitch's team ran some mathematical models based on known disease outbreaks.

　　Their article suggests that an H5N1 pandemic could only be contained temporarily. And the longer the virus is around, the harder it will be to stop it from spreading.

　　"Even if each successive containment effort is no more difficult than its predecessor, the chance of at least one failure increases with the number of introductions," they wrote.

　　"Since the last pandemic nearly 40 years ago, we have observed dramatic changes in social and ecological factors thought to facilitate emergence of a pandemic-capable strain," the researchers report.

　　"Surging human and bird populations in Asia have increased the frequency of contact between birds and humans —— and these changes might facilitate emergence by permitting 'crossing over' of a mutated avian influenza to humans, or by allowing human and avian influenza to (recombine) in the same animal host."