SÃO PAULO—China’s effort to boost its image by providing Covid-19 vaccines to the developing world suffered a setback after one of its leading inoculation candidates turned out to be 50% effective in late-stage trials in Brazil, significantly lower than earlier results showed.
While Sinovac’s CoronaVac efficacy rate still meets the 50% threshold the World Health Organization considers good enough for widespread use, scientists said a lack of transparency about the data risks damaging the credibility of a vaccine Brazilians and others world-wide are already reluctant to take.
Brazil’s Butantan Institute, a São Paulo-based public institute that is the first to complete late-stage trials of the CoronaVac vaccine, had said last week that it was shown to be 78% effective and offer total protection against severe cases of the disease.
But after rising pressure from Brazilian scientists, some of whom accused the trial’s organizers of misleading the public, Butantan said Tuesday those rates only included volunteers who suffered mild to severe cases of Covid-19. When data from all volunteers was considered—including those who contracted “very mild” cases of Covid-19 and required no medical assistance—the total efficacy rate fell to 50.4%, Butantan said.
Sinovac didn’t respond to requests for comment.