China is working on a bird flu vaccine for humans and initial test results suggest it is safe and effective, but a leading expert is treating the news with caution, saying more details and independent checks are needed.

Government-backed Beijing Sinovac Biotech Co. said it had developed a H5N1 vaccine for humans, using a strain of the virus that had been found in recent years in Vietnam.

The Chinese vaccine was jointly developed by the Chinese Centers for Disease Control and Prevention and supported by China?s Ministry of Science and Technology.

Pharmaceutical companies around the world are trying to develop a vaccine for the H5N1 virus, which experts fear could mutate into a form that can pass easily among humans, possibly killing millions. Questions surround the positive test results from China.