BEIJING (BLOOMBERG) - 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, Moderna and even Russia's Sputnik have delivered protection rates of more than 90 per cent, Chinese candidates have generally reported much lower efficacy results.
Research released on Sunday showed that the rate for Sinovac Biotech's vaccine - deployed in Indonesia and Brazil - was just above 50 per cent, barely meeting the minimum protection required for Covid-19 vaccines by leading global drug regulators.
The other Chinese shots have reported efficacy rates of between 66 per cent and 79 per cent.
Anxiety over that disparity spilled into the open at the weekend when Dr George Fu Gao, head of the Chinese Centre 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, according to local news outlet the Paper.
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 on Dr Gao's comments quickly edited or taken down.
Dr Gao then backtracked, telling state-backed newspaper Global Times on Sunday that his remarks were misinterpreted, and were only meant to suggest ways to improve the efficacy of vaccines.
He suggested that following up inoculations with additional booster shots and mixing different types of vaccines could help tackle the effectiveness issue, according to Global Times.
The concerns put a question mark over a vast swathe of the global vaccine roll-out, particularly in the developing world, with richer countries' domination of supplies of the highly effective mRNA vaccines has seen countries such as Turkey and Indonesia turning to China's shots.
Beijing, which is also donating vaccines to some nations, has been ramping up its own inoculation drive, aiming to vaccinate 40 per cent 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 United States.
"They don't really trust it themselves," said Dr 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 scrutinised. I'm aware from colleagues in China that there's huge vaccine hesitancy anyway."
Chinese vaccine developers have been repeatedly criticised for a lack of transparency and lag foreign peers in publishing full trial data in peer-reviewed medical journals.
The weekend study of the 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 three trials for its two inactivated Covid-19 vaccines.
While a separate Sinovac study involving more than 10,000 people in Turkey put the vaccine's efficacy at 83.5 per cent, it just added to questions about the shot's efficacy.
The company has said that differences in the severity of outbreaks, various Covid-19 strains in circulation and the definition by which virus cases are identified in studies have all contributed to different results across several trial sites.
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 fewer than 21 days.
The stakes are high for Brazil's vaccination roll-out. The country is relying on both the Sinovac shot, known as CoronaVac, and the booster from AstraZeneca and Oxford University which has encountered controversy after some people experienced blood clots.
At home, China is already walking a tightrope in trying to keep its vaccination rates on par with some other countries, especially the US, to avoid a delay in lifting border restrictions and resuming international travel.
While China is working on more effective vaccines, including shots that deploy mRNA technology, it should continue to roll out those that have been approved for now, said Professor Benjamin Cowling, head of epidemiology and biostatistics at the University of Hong Kong.
"They can provide a high level of protection, particularly against severe Covid," he said.
Fearing a heavy-handed approach could draw backlash, the central government in China has so far refrained from making shots mandatory and has 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 four million doses a day from less than one million at the start of the year.