Concern is mounting that China’s Covid-19 vaccines are less effective at quelling the disease, raising questions about nations from Brazil to Hungary that are depending on the shots and the country’s own mammoth inoculation drive.

While vaccines developed by Pfizer Inc., Moderna Inc. and even Russia’s Sputnik shot have delivered protection rates of more than 90%, Chinese candidates have generally reported much lower efficacy results. Research released Sunday showed the rate for Sinovac Biotech Ltd.’s vaccine- deployed in Indonesia and Brazil- was just above 50%, barely meeting the minimum protection required for Covid vaccines by leading global drug regulators. The other Chinese shots have reported efficacy rates of between 66% to 79%.

Anxiety over that disparity spilled into the open at the weekend when George Fu Gao, head of the Chinese Center for Disease Prevention and Control, said at a forum that something needed to be done to address the low protection rate of the Chinese vaccines.

The rare admission by a senior official appeared to go viral on social media before China’s censors swung into action, with posts and media reports about Gao’s comments quickly edited or taken down. Gao then backtracked, saying that his remarks were misinterpreted, and were only meant to suggest ways to improve the efficacy of vaccines.

Gao suggested that following up inoculations with additional booster shots and mixing different types of vaccines could help tackle the effectiveness issue. The concerns put a question mark over a vast swathe of the global vaccine rollout, particularly in the developing world, with richer countries’ domination of supplies of the highly effective mRNA vaccines seeing nations like Turkey and Indonesia turn instead to China’s shots.

Beijing, which is also donating vaccines to some nations, has been ramping up its own inoculation drive. It aims to vaccinate 40% of China’s population- or 560 million people- by the end of June, an ambitious effort that will require it to move at twice the pace of the U.S.

“They don’t really trust it themselves,” said Therese Hesketh, an expert on China’s healthcare system at University College London. “They really did a rush job on the vaccine and the clinical trials have never been properly scrutinized. I’m aware from colleagues in China that there’s huge vaccine hesitancy anyway.”

Chinese vaccine developers have been repeatedly criticized for a lack of transparency and lag foreign peers in publishing full trial data in peer-reviewed medical journals. The weekend study out of Sinovac vaccine’s late-stage trial in Brazil came three months after its first efficacy readouts, while state-owned Sinopharm has yet to publish full data from Phase III trials for its two inactivated Covid vaccines.

While a separate Sinovac study involving more than 10,000 people in Turkey put the vaccine’s efficacy at 83.5%, it just added to questions about the shot’s effectiveness. The company has said that differences in the severity of outbreaks, various Covid strains in circulation and the definition by which virus cases are identified in studies have all contributed to different results across several trial sites.

“The confusion that has arisen highlights the importance of full transparency with publication of results of trials in the peer-reviewed literature,” said Martin McKee, professor of European Public Health at the London School of Hygiene and Tropical Medicine. One reason for the low efficacy in the Brazil trial, according to the study’s researchers, was that the two doses of the vaccine were administered at a short interval of 14 days. The researchers noted “a trend to higher efficacy” among a limited number of participants who got their second dose in no less than 21 days.

Fearing a heavy-handed approach could draw a backlash, officials in China have so far refrained from making shots mandatory, and have spoken out against forced inoculation. Officials have instead dangled rewards and applied peer pressure among workers in the massive state sector to significantly raise vaccination rates, and are now issuing nearly 4 million doses a day from less than 1 million at the start of the year.