China is seeking up to 500 volunteers for the second trial of a vaccine to protect humans against the deadly bird flu virus after first-phase tests indicated it was safe, state media said.

The second round of clinical tests would mainly aim to see for how long the vaccine could protect the human body against the H5N1 virus, the Beijing Youth Daily said, citing a doctor at the Sino-Japanese Friendship Hospital where the trial would take place.

Results from the first round of trials, which ended in June, showed that the 120 people who were vaccinated had no serious adverse reactions, company officials said this week.

The pool of volunteers for the second round was nearly four times the size of that for the initial trials, but officials did not explain why.

As with last time, the volunteers needed to be healthy people aged between 18 and 65, hospital officials were quoted as saying. No children, people older than 65, or pregnant women would be accepted, they said.

The vaccine must undergo three phases of clinical trials before being allowed on the market, Xinhua said.

The vaccine's manufacturer, Sinovac Biotech Limited, announced earlier this week that it planned to produce 20 million vaccines annually within the next few years, the Xinhua news agency said.

The company jointly developed the inoculation with China's Ministry of Science and Technology and the Center for Disease Control and Prevention.

About a dozen companies were conducting clinical trials on bird flu vaccines worldwide, Xinhua said.

The H5N1 bird flu virus is mainly spread among poultry and occasionally from poultry to humans. But experts fear it could mutate into a form easily transmitted between humans.

The bird flu virus has killed about 140 people, mostly in Asia, since 2003. It has infected 21 people in China since then, killing 14, according to official figures.