Officials in China have given permission for a Chinese drug maker to begin commercial production of a human vaccine against bird flu after clinical trials showed it was effective.

Officials from Sinovac Biotech, which jointly developed the vaccine with China's Center for Disease Control, say two clinical trials showed the vaccine worked.

The development is a promising sign for scientists who have worked for years for develop a vaccine to protect humans from the H5N1 strain of bird flu. The virus has infected nearly 400 people and killed at least 238 worldwide, most of them in Asia.

Experts worry that the H5N1 strain could cause a deadly flu pandemic in humans.

So far, most human victims have caught the virus from sick poultry. But there have been some cases of human-to-human exposure.

Malik Peiris teaches microbiology at Hong Kong University. He was among the scientists who first identified the virus when it appeared in the city 11 years ago, and was part of a team that provided the genetic map of the bird flu virus and its mutations.

"The concern of course is that if you give this virus enough time to keep on exposing the human population, sooner or later the virus might learn the trick of transmitting from human to human and then we would have a pandemic and actually quite a nasty pandemic at that," Peiris said.

Peiris says it is crucial to find an vaccine now because there is a strong chance that the infection rate could grow rapidly. He says the history of H5N1 appears similar to that of Severe Acute Respiratory Syndrome, which appeared in China in 2002, and spread around the world, killing nearly 800 people. In Hong Kong, 299 people died.

"If you go back to the story of SARS, again it was quite a similar situation as we can see now in retrospect, again it was a virus found in animals and animal markets in southern China and probably over a number of years it was repeatedly infecting humans," Peiris said. "But finally it did adapt and learn how to transmit from human to human and then it led to this global outbreak that we saw."

Since the H5N1 flu reappeared in 2003, the virus has spread worldwide, and has led to the deaths of hundreds of millions of chickens, ducks and other birds. Indonesia has the highest human bird flu fatalities, at 107 deaths, followed by Vietnam at 52.

Experts now consider the virus to be well entrenched in Southeast Asia and unlikely to be eradicated among poultry for years.

Vietnam also plans to start clinical trials for a human vaccine. A vaccine designed using a current strain of the virus might not offer full protection against other mutated strains in the future. But experts say developing vaccines now could at the very least shorten the time needed to make a more effective vaccine in the case of a pandemic.