The Covid-19 vaccine developed by China's Sinovac Biotech is just 50.4% effective in a Brazilian trial — barely meeting the threshold for regulatory approval and well below the initially reported efficacy rate, according to several media reports.
Brazil is the first to complete late-stage trial of the vaccine CoronaVac. The state-run Butantan Institute has been criticized by scientists and health experts for a lack of transparency in making public its trial data.
The institute reportedly delayed releasing trial results three times — which it blamed on a confidentially clause in its contract with Sinovac — and last week announced partial data that showed an efficacy rate that was later revised much lower.
Butantan said last week that CoronaVac was 78% effective among volunteers with mild to severe infections, reported the Wall Street Journal. But the institute said Tuesday that the overall efficacy rate fell to 50.4% when including "very mild" cases that did not require medical assistance, the Journal reported.
Sinovac did not immediately respond to CNBC's emailed request for comment.
Brazil and other developing nations have pinned their hopes on Chinese vaccines as wealthier countries snap up vaccines developed in the West.
Sinovac's CoronaVac vaccine is also cheaper and easier to transport because it can be stored in ordinary refrigerators — unlike the vaccines from Pfizer-BioNTech and Moderna that must be kept at subfreezing temperatures.
The South American country has reported more than 8.1 million Covid infections in total, the third highest number globally, according to data compiled by Johns Hopkins University. The country's death toll of over 204,000 is the second highest in the world, Hopkins data showed.
Butantan has requested for CoronaVac to be authorized for emergency use, reported Reuters. Brazil's health regulator Anvisa, which has stipulated a minimum 50% efficacy rate for vaccines in the pandemic, said it will meet on Sunday to decide, the news agency said.
In addition to Brazil, other developing countries including Turkey and Indonesia are also conducting trials on CoronaVac.
Indonesia on Monday approved the vaccine for emergency use — the first country outside China to do so after interim data from a late-stage trial showed that CoronaVac was 65.3% effective, reported Reuters.
Meanwhile, Turkish researchers said their interim analysis showed the vaccine to be 91.25% effective, Reuters reported last month.
Experts quoted by the news agency said it's not unusual for differential trials of a vaccine to show differing efficacy rates. But fragmental releases of CoronaVac trial data could add to skepticism and undermine confidence in the vaccine, Reuters said.