A Chinese-made bird flu vaccine has been approved by the State Food and Drug Administration (SFDA) for a second phase of clinical trials.

The second phase of clinical trials will further test the vaccine's tolerance, safety, and immunogenicity to the H5N1 virus strain, according to Yin Weidong, President of Sinovac Biotech, the Beijing-based pharmaceutical company behind the trials.

The first phase of trials ended last June with none of the 120 people injected with the vaccine experiencing any serious side-effects.

The SFDA has also approved trials for a H5N1 split virus vaccine, with less reported side effects than the full-strain vaccine.

A researcher with Sinovac announced that some 880 volunteers are expected to participate in the trials for the two vaccines.

Zhang Jiansan, vice President of Sinovac said that the company is planning to expand its production facilities to produce massive quantities of the bird flu vaccine after the drug has completed the SFDA-led trials.

In China, vaccines must undergo three phases of clinical trials before being allowed on the market.

It is still hard for people to catch the H5N1-strain of the bird flu virus, but experts fear it may mutate into a form that spreads easily among people, potentially sparking a pandemic.

China has reported a total of 24 human cases of bird flu since 2003. On March 28, the Ministry of Health confirmed that a 16 year-old boy had died from bird flu in eastern Anhui Province, bringing the total number of people in China who have been killed by the virus to 15.