The clinical trial in Turkey was much smaller than those of other major vaccine candidates, making the researchers’ claims for it less certain.
Turkish officials announced Thursday that a vaccine from the Chinese company Sinovac has an efficacy rate of 91.25 percent, but the finding was based on preliminary results from a small clinical trial and none of the data was published in a journal or posted online.
The announcement came a day after another ambiguous news conference, also about Sinovac’s vaccine, in Brazil. Officials there were expected to provide detailed results from another trial, but only reported that the vaccine had an efficacy rate over 50 percent.
A total of 7,371 volunteers were involved in the Turkish trial, but the efficacy data presented by Serhat Unal, an infectious diseases expert, was based only on 1,322 participants, 752 of whom got a real vaccine and 570 of whom received the placebo.
Dr. Unal said that 26 of the volunteers who received the placebo developed Covid-19, while only three of the vaccinated volunteers got sick. He and his colleagues did not share their data in written form.
“Now we are sure that the vaccine is effective and safe on Turkish people,” said Fahrettin Koca, the health minister.
Sinovac did not issue a public statement about the trial, nor did it comment on the trial in Brazil.
The small number of volunteers on which the Turkish researchers based their calculation of efficacy raised questions about the certainty of their conclusions. The more people who participate in a clinical vaccine trial, the stronger its statistical power becomes.
Pfizer and BioNTech, by contrast, presented data on 36,523 people to show that its vaccine has an efficacy rate of 95 percent. For their vaccine, 162 people who got the placebo developed Covid, compared to eight in the group that got the vaccine.
Turkey signed a deal with Sinovac for 50 million doses of the vaccine. The first three million doses are scheduled to arrive Monday in Turkey, Mr. Koca said. Mr. Koca said Turkey would also get 4.5 million doses of the Pfizer-BioNTech vaccine by the end of March. Around 1 million doses are expected to arrive at the end of January, he said.
CoronaVac, as Sinovac calls its vaccine, is made from killed coronaviruses. The method is one of the oldest ones for making vaccines, used by Jonas Salk in the 1950s to make a vaccine against polio. After viruses are inactivated with chemicals, they cannot make people sick, but they can provoke the immune system to make antibodies that can provide long-term protection against live viruses.
Sinovac developed CoronaVac in early 2020 and then carried out a series of clinical trials. They published their results in November. There they reported that the vaccine appeared safe and provoked an immune response against the coronavirus.
The company then moved on to Phase 3 trials in Brazil, Indonesia and Turkey, three countries with high Covid-19 rates.
Health officials in Brazil said on Wednesday that the Chinese vaccine had passed safety and efficacy tests that would pave the way for its use in Brazil, but they put off releasing detailed data from clinical trials in Brazil underlying those findings, citing a contractual agreement with Sinovac. Dimas Covas, the director of Butantan Institute, which ran the trials, said a joint announcement could happen within two weeks.
“Today is a historic day for science and for Brazilian health,” Jean Gorinchteyn, the health secretary for the state of São Paulo, told reporters at a news conference. “This will allow us to save the lives of millions of people, not just in Brazil but in the whole world.”
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