Beijing -- Preliminary clinical tests show that a bird flu vaccine for human use developed by Chinese researchers is safe and effective, researchers said on Monday.

The vaccine was jointly developed by China's Ministry of Science and Technology, Center for Disease Control and Prevention and Beijing Sinovac Biotech Co., a Beijing-based pharmaceutical company.

They said on Monday that the first phase of clinical trials has proved the vaccine is safe and effective for humans.

Six volunteers took part in the clinical tests last November at the Beijing China-Japan Friendship Hospital, after the State Food and Drug Administration granted the vaccine developers the green light for clinical trials.

Results from the first phase trails, which ended in June, showed that the four antigens worked at different levels in stimulating the production of antibodies, according to the company.

It said the 10 microgram dosage of the vaccine proved most effective, stimulating 78.3 percent protective antibodies, exceeding the European Union standard of 70 percent for a flu vaccine.

The 120 participants who were vaccinated have shown no serious adverse reactions, researchers said. Blood tests and urine tests all indicate that the vaccine is safe for human use.

The vaccine was developed from the virus's NIBRG-14 strain which was provided by the World Health Organization (WHO) and protects against the deadly H5N1 strain of avian influenza.

The vaccine can be mass produced, according to researchers.

Beijing Sinovac Biotech Co. said it is ready to apply for the second phase of clinical trials

In China, a vaccine is allowed to enter the market after it completes three phases of clinical trials.

Bird flu remains essentially an animal disease, but experts fear that the H5N1 virus could mutate into a form that could pass easily among humans.

Worldwide, about a dozen companies are currently conducting clinical trials on bird flu vaccines.

According to the WHO, the H5N1 virus has proven difficult to predict, and as drug companies move forward with their pandemic vaccine development, they may be gambling on which virus they think is most likely to mutate into a killer strain.

The virus has killed 14 people in China since 2003 and 21 Chinese have contracted the virus.

The latest case was a 62-year-old man in Xinjiang Uygur Autonomous Region, who died on July 12.