Brazil is one of the world’s top COVID-19 vaccine testing grounds. Now officials there say that CoronaVac, the experimental COVID-19 vaccine from Chinese developer Sinovac, is the safest of the coronavirus immunizations evaluated in the country so far.

“The first results of the clinical study conducted in Brazil prove that among all the vaccines tested in the country, CoronaVac is the safest, the one with the best and most promising rates,” São Paulo Gov. João Doria told reporters in Brazil on October 19.

Doria’s comments are especially striking since Brazil has more extensive and varied exposure to COVID-19 vaccine candidates than other countries. Brazil is conducting Phase III trials for at least five of the 10 vaccine makers that have reached the final stage of testing. In addition to Sinovac, Brazil is the site of large, human trials of COVID-19 vaccines developed by American vaccine makers Johnson & Johnson and Pfizer, the U.K.’s AstraZeneca, and China's Sinopharm. Brazil's struggles to contain COVID-19—it has the third-highest number of cases globally—have made it fertile ground for vaccine makers that need high rates of infection to adequately test their candidates.

Brazilian authorities pegged Sinovac as the top contender after reviewing preliminary data from Sinovac’s ongoing clinical trials. Sinovac hasn't released the data to the public, but Dimas Covas, director of the Butantan Institute, a vaccine research center, said that Sinovac's CoronaVac candidate had proved safe among 9,000 volunteers.

No volunteers in Sinovac’s trials experienced severe adverse reactions to the vaccine; 20% reported mild pain from the injection site; and less than 15% said they had headaches, nausea, or tiredness after receiving the shot, Covas told reporters on October 19.

Covas and Doria did not provide specifics on how Sinovac's candidate stacked up against rivals; their statements represent the first preliminary result from any Phase III trial of a COVID-19 vaccine.

The Brazilian authorities said the Sinovac data indicated that the vaccine provided protective antibodies among volunteers, but a final determination of CoronaVac’s effectiveness will come once the 13,000-volunteer trial is complete, which could occur as early as mid-November.

Brazilian President Jair Bolsonaro has, at times, promoted "miracle cures" and other dubious science related to COVID-19. But the Butantan Institute, which is affiliated with the São Paulo government, is known globally as a reputable biomedical research institution and is Latin America's largest vaccine manufacturer.

Even before October 19's announcement, Brazil had a lot riding on the success of Sinovac's candidate.

In late September, Brazil’s São Paulo state agreed to purchase 46 million doses of CoronaVac for $90 million. Days later, Doria said that he had asked Brazil’s health regulator to register CoronaVac for potential distribution and announced that São Paulo would be “one of the first places in the world to vaccinate the public.”

Brazil’s vote of confidence in Sinovac’s vaccine comes as vaccine makers like Johnson & Johnson and AstraZeneca have paused trials amid safety concerns. Johnson & Johnson’s Phase III clinical trials in the U.S., Brazil, and Argentina remain temporarily suspended after the company reported an unexplained illness in a trial participant. AstraZeneca’s trials also remain paused in the U.S. after the vaccine maker discovered a similar unexplained illness in a U.K. volunteer in September, but the company has resumed testing in the U.K., Brazil, and South Africa.

Barring any holdups, Brazilian authorities said on October 19 that they will aim to approve CoronaVac before the end of 2020 and begin distributing the vaccine to the public by the start of next year.

Other vaccine makers have put forth roughly similar timelines.

Pfizer said in a statement on Oct. 16 that it may be ready to begin rolling out its vaccine for emergency use in the U.S. by the third week of November. Moderna has provided a similar emergency use timeline in the U.S. but says its vaccine might not be ready for widespread distribution until the spring of 2021.