In the third in a series of conflicting announcements about a Chinese-made COVID-19 vaccine candidate, Brazilian researchers reported today that the results of their efficacy trial were less impressive than they claimed last week. When analyzed by stricter criteria than used earlier, the vaccine’s efficacy against all forms of COVID-19, including mild cases, dropped from about 78% to 50%.
The vaccine still appeared to give nearly 100% protection against disease severe enough to require hospitalization, although the trial amassed too few of those cases for that result to reach statistical significance.
Commenting at the press conference today, microbiologist Natália Pasternak Taschner, president of the Question of Science Institute, a Brazilian nonprofit that aims to support public policy based on scientific evidence, said it was a “clear and clean study” and stressed that she wants th vaccine for herself and her parents. “We do not need to say this is the best vaccine in the world,” said Pasternak, who was not involved with the trial. “We have to say that this is our vaccine and it is a good vaccine to start the process of pandemic control.”
The vaccine, made by China’s Sinovac Biotech, contains a chemically inactivated version of SARS-CoV-2, the virus that causes COVID-19. At last week’s press conference that announced the 78% efficacy, Dimas Tadeu Covas, head of the Butantan Institute—a state-owned vaccinemaker in São Paulo that is conducting the study with Sinovac—offered few details. (Information was even sketchier in a press conference last month, when the team claimed only that the efficacy was greater than 50%.) But when pressed by journalists, he responded that this number came from about 220 cases of symptomatic disease. Today, researchers revealed that the 78% figure actually derived from analyzing COVID-19 cases in which people sought assistance for symptoms and tested positive for the virus—31 such cases among placebo recipients in the trial and seven in the vaccinated group. ...
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