In a rare admission of the weakness of coronavirus vaccines developed in China, the country’s top disease control official says their effectiveness is low and the government is considering mixing them to get a boost. China’s vaccines “don’t have very high protection rates,” Gao Fu, the director of China’s Centers for Disease Control, said 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, Wang Huaqing, said developers are working on mRNA-based vaccines. Gao did not respond to a phone call requesting further comment. “The mRNA vaccines developed in our country have also entered the clinical trial stage,” Wang said. He gave no timeline for possible use.