In a rare admission of the weakness of Chinese coronavirus vaccines, the country's top disease control official says their effectiveness is low and the government is considering mixing them to get a boost.

Chinese vaccines ``don't have very high protection rates,'' said the director of the China Centers for Disease Control, Gao Fu, at a conference Saturday in the southwestern city of Chengdu.

Beijing has distributed hundreds of millions of doses abroad while trying to promote doubt about the effectiveness of the Pfizer-BioNTech vaccine made using the previously experimental messenger RNA, or mRNA, process.

``It's now under formal consideration whether we should use different vaccines from different technical lines for the immunization process,'' Gao said.

Officials at a news conference Sunday didn't respond directly to questions about Gao's comment or possible changes in official plans. But another CDC official said developers are working on mRNA-based vaccines.

Experts say mixing vaccines, or sequential immunization, might boost effectiveness. Researchers in Britain are studying a possible combination of Pfizer-BioNTech and the traditional AstraZeneca vaccine.