Beginning from late Wednesday, China's 13,300 volunteers who took part in clinical trials of domestically-developed vaccines against the H1N1 flu began to receive the final second shot, according to a notice issued by the Ministry of Health.

No case of severe adverse drug reaction has been reported so far, ministry officials said.

The volunteers will be studied by an expert team from the health ministry and further results will be compiled in mid August, said Wang Yu, director of the Chinese Center for Disease Control and Prevention (CDC).