The medical community believes anti-virals such as Tamiflu are only effective in fighting the H5N1 bird flu virus if they are administered within 48 hours of the onset of symptoms. But virologist Menno de Jong said this assumption might only hold true for established human flu viruses.

The H5N1 strain behaves differently from human flu viruses and has been observed to be replicating in human hosts even on the seventh or eighth day, he said.

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"In my experience, there is a clear suggestion that there was still virus replication [when there was] a late start in treatment," he said in Singapore during a conference on avian flu and other diseases. "In four of my patients, there was very rapid clearance of the virus from the throat, and all four survived."

If the "viral load" can be decreased with drugs, he said, "you can have a good outcome. Even those who are treated late had good results."

Anti-virals such as Tamiflu were designed to counter common human flu viruses and are thought to be superfluous after 48 hours, when the average person's immune system would have begun fighting the flu attack and blocking the flu virus from replicating.

But de Jong, head of virology at the Hospital for Tropical Diseases in Ho Chi Minh City, said Tamiflu could still be used to fight bird flu many days after the onset of symptoms because the avian virus would still be multiplying.

The H5N1 virus remains a scourge for birds but experts fear it can cause a pandemic and kill millions of people if it learns to pass efficiently between humans.

De Jong agreed with many experts that an early start to treatment was still best - something easier said than done in places such as Vietnam, Indonesia and China.

Most of the 258 people known to have been infected with H5N1 worldwide since late 2003 lived in rural areas. Of those infected, 154 died because healthcare was unavailable, too far away or the infection was too far advanced by the time they were admitted to hospital.

"You have to get very effective treatment as early as possible because you will prevent direct viral damage" to lung and other tissues, de Jong said.

Early treatment could also prevent what is known as the "cytokine storm response" - when a human's immune system launches such a heavy counter- attack that it destroys not only the invading avian flu virus but the person's own surrounding tissues as well - de Jong added.

Indonesia is struggling to curb H5N1 deaths in people. So far, 57 people have died, the highest toll for any nation. REUTERS