Vietnam reported today five more suspected human cases of the bird flu, which experts said could have a far more devastating impact than SARS if it combines with a human influenza virus.
Dr Veronica Chan, head of the microbiology and parasitology department at the University of the Philippines' College of Medicine, said humans would have no protection against the resulting strain of flu and there would be an epidemic.
"We should worry. It kills. It kills," she told reporters.
How the bird flu has skipped to humans - killing three in Vietnam - is still a mystery. But its ability to combine with the human flu virus is not.
"The bird and the human influenza can re-assort their genetic components and come out with progenies, meaning products or a new virus," Dr Chan said. Dr Leo Poon, an assistant professor in microbiology at the University of Hong Kong, voiced similar fears.
"We don't rule out a large-scale outbreak if the virus mutates in pigs or humans in such a way as to raise man-to-man transmissibility," he said.
The World Health Organisation (WHO), which has confirmed that three of Vietnam's 18 suspected bird flu victims died of the disease, is just as worried.
Senior WHO experts have flown to Hanoi, including Dr Hiroshi Oshitani, who led the response to the SARS outbreak that killed several hundred people from China to Canada last year.
"That gives you a measure of how seriously we're taking this situation," said Mr Peter Cordingley, spokesman for the WHO's Western Pacific headquarters in Manila, which says there is no evidence yet the bird flu virus has passed from human to human.
State-run Vietnam radio said today that WHO in a meeting with health ministry officials had committed to introduce a new vaccine to fight the bird flu outbreak. The vaccine would arrive "in the next several weeks at the earliest," it said.