The tepid results could also be problematic for Chinese officials, as they touted the effectiveness of the Sinovac and Sinopharm vaccines. Although the vaccines had not been approved by regulatory authorities and data from late trials had not been made public, Beijing gave them to thousands of Chinese under an emergency policy; it plans to vaccinate 50 million people by the middle of next month.
State media in China downplayed the news from Brazil. The Global Times, a state nationalist tabloid, headlined a headline saying the Sinovac vaccine was "100 percent effective in preventing serious cases and could reduce hospital admissions by 80 percent."
The new data could increase skepticism among people around the world who are already wary of vaccines made in China, as the country has a history of vaccine quality scandals. A study by the Chinese University of Hong Kong has discovered this only 37.2 percent of respondents in Hong Kong were ready to be vaccinated.
Scientists had already questioned the patchy way in which efficacy data on the Chinese vaccines had been released. Indonesia said Monday that its interim analysis showed that CoronaVac has an efficacy rate of 65.3 percent. Last month, Turkey said it had an efficacy rate of 91.25 percent, but that was based on preliminary results from a small clinical trial.
The vaccine had long taken on a political dimension in Brazil. President Jair Bolsonaro had spoken derisively about CoronaVac, sparking a growing anti-vaccination movement in the country, where more than 200,000 people have died from Covid-19. The vaccine was defended by São Paulo Governor João Doria, who is widely expected to run for president in 2022 and is one of Mr Bolsonaro's most outspoken critics.
In Brazil, officials say the higher rate of efficacy previously announced for CoronaVac was related to the protection it provided against developing Covid-19 symptoms significant enough to require treatment. Although officials claimed last week that the vaccine provided absolute protection against moderate to severe symptoms, they had not disclosed another group that had "very mild" infections despite vaccination.
Denise Garrett, a Brazilian-American epidemiologist and vaccine expert, said there was no reason to doubt the safety of CoronaVac, adding that the data presented so far suggested it would provide a satisfactory level of protection. But Dr. Garrett said the vague and sometimes misleading way in which information about the vaccine was made public could shake people's confidence in its reliability and spark the political battle over the vaccine.