H7N9 bird flu has killed 45 people on the Chinese mainland since the first human infection was confirmed in late March, a health official said Thursday.

A total of 134 cases of H7N9 infection have been confirmed on the Chinese mainland, said Wang Yu, director of the Chinese Center for Disease Control and Prevention.

Wang was speaking at an international symposium on the prevention and treatment of new types of influenza.

"There are still many unknown aspects of the H7N9 bird flu virus as well as diseases that might result from it, including the source of the virus, the infection routes and mutation," said Wang.

"In particular, the future development of the new bird flu virus is still unclear," he said.

Wang warned that the risks of pandemic influenza should not be underestimated.

Wang thought highly of the Chinese Government's response to the epidemic, noting its enhanced medical capabilities and information transparency.

Preliminary work of the manufacturing of the H7N9 vaccine has finished as scheduled, according to Zou Yong, quality director of the Beijing-based Sinovac Biotech Ltd., which is in charge of the H7N9 vaccine development.

Three batches of vaccine fluid have been prepared and are ready for rationing and vaccine formulation, followed by safety appraisal, stability study and clinic test, said Zou.

The vaccine has proven effective in animal tests and its suitability for humans will be further studied.

(Xinhua News Agency August 15, 2013)