Brazil said on Monday that it had halted a late-stage trial of a Chinese vaccine, which had been considered a global front-runner in the race to develop a protective shot for the coronavirus, after a “serious adverse” reaction in a participant.

The Brazilian health regulator provided little information on its decision, including whether the reaction was related to the vaccine, called CoronaVac and produced by the Chinese company Sinovac, or coincidental.

The participant who had the reaction became ill on Oct. 29, according to the authorities. They did not divulge where in Brazil the vaccine had been administered or what had happened to the volunteer, citing patient confidentiality. In a statement, they said that such a “serious adverse incident” might include death, disability, hospitalization, birth defects or other “clinically significant events.”

CoronaVac is one of 11 experimental vaccines, produced by some of the world’s foremost pharmaceutical companies, currently in Phase 3 trials. As the world braces for another major wave of coronavirus infections, the race for a vaccine has intensified and been made all the more competitive by fractious geopolitics. On the same day that Brazil suspended the Sinovac trial, the American company Pfizer announced that an early analysis of its coronavirus vaccine trials suggested that its drug was more than 90 percent effective in preventing the virus that causes Covid-19.

Sinovac’s drug was seen in China as a leading candidate. But in Beijing’s push to get a Chinese vaccine to be the first on the global market, officials stretched the definition of “emergency use.” They have permitted tens of thousands of people to receive the Sinovac vaccine and two other locally made vaccines, despite having not yet concluded Phase 3 trials.

Adverse effects are not unusual in Phase 3 trials. AstraZeneca and Johnson & Johnson both paused their trials after several volunteers fell seriously ill, resuming them six weeks later, in October, after concluding that the illnesses were not related to the vaccines

The details of why the Brazilian health regulator had paused the trials were uncertain.

In a statement issued on Tuesday, Sinovac said Instituto Butantan, the medical center coordinating the Brazilian trials, had deemed the “serious event” not related to the vaccine. The company said it was “confident in the safety” of its vaccine.

According to news reports, the institute confirmed that a volunteer had died, but officials there said they were “surprised” by the government’s decision to halt the trials.

Dimas Covas, head of the institute, told a Brazilian television network that he found the government regulator’s decision strange “because it’s a death unrelated to the vaccine.”

Prof. Kim Mulholland, a pediatrician at the Murdoch Children’s Research Institute in Melbourne, Australia, called it “alarming” that Instituto Butantan appeared to have no idea why the trial had been stopped. “It leaves me wondering who had done that and why,” Professor Mulholland added. “That’s the question that really needs to be answered because this is a violation of the normal process.”

It is rare for a drug that has not been fully tested to be so widely administered, and scientists from around the world have repeatedly warned that the Chinese government was risking the health of its people before the completion of late-stage trials.

Sinovac has said previously that more than 10,000 people in Beijing have been injected with its vaccine. Separately, it said that nearly all its employees — around 3,000 in total — and their families had taken it. Vaccine experts said it was important to conclude the third and final phase of human testing before making the drug available. Phase 3 trials involve tens of thousands of people and may detect uncommon but potentially severe side effects.

The company began Phase 3 trials in Brazil and in Indonesia in August, and in Turkey in September, testing its vaccine in about 27,000 people. Sinovac said it had hoped to complete the trials by the end of the year, but the suspension of the trial in Brazil could affect the process in other countries, potentially derailing that timeline.

The Indonesian state-owned pharmaceutical company Bio Farma, which is conducting trials for Sinovac, said that it had no plans to cancel its trials, according to Iin Susanti, head of Bio Farma’s business planning and strategy division.

The suspended Brazilian trial is a reminder of the formidable challenges facing Chinese vaccine companies when they go abroad. Few of the companies have experience operating overseas, much less navigating potential political minefields. All of them had to test their vaccines in places with active outbreaks because the virus had largely been stamped out in China.

In Brazil, the trials have been politically fraught as supporters of President Jair Bolsonaro, who has downplayed the threat and blamed China for the pandemic, have criticized them.

The trials had mostly taken place in São Paulo, the country’s most populous state, backed by its governor, João Doria, a rival of Mr. Bolsonaro. Last month, Mr. Bolsonaro contradicted his own health minister and rejected the announced purchase of 46 million doses of the Sinovac vaccine, saying, “The Brazilian people will not be anyone’s guinea pig.”

Prof. Raina MacIntyre, who heads the biosecurity program at the Kirby Institute of the University of New South Wales in Sydney, Australia, said serious side effects in a vaccine trial were “extremely rare” but added, “We can expect to see this scenario happen again with different vaccines. If you’ve got 45,000 people in a trial, it’s quite likely that one person will have a serious health event in that time.”

Professor MacIntyre said that the pause would allow a safety monitoring board to investigate whether the vaccine had caused the illness.

How long the suspension will last is unclear. Professor MacIntyre said that if the event had occurred in a volunteer given a placebo, it could take a few days. But if not, it could take weeks or even months.

Vaccine experts have said that they consider data from Sinovac’s early-stage trials to be promising. Results from the company’s Phase 1 trials showed no adverse effects, and Phase 2 trials showed 90 percent protection against SARS-CoV-2, the virus that causes Covid-19.

Seeing a company from China develop a vaccine first has been a priority for the country’s leader, Xi Jinping. Mr. Xi has staked his personal reputation on the effort, which is seen as a way to erase some of the blame that several countries have placed on China for its initial missteps when the virus first emerged in the city of Wuhan last year.

Sinovac’s treatment is an inactivated vaccine, meaning that it is made of a coronavirus that has either been weakened or killed by chemicals.

Tao Lina, a vaccine expert in Shanghai, said he believed that the Brazilian suspension was not based on science but because of politics.

“The technology of inactivated vaccines is very mature, and the possibility of this incident being related to vaccines is very slim,” said Mr. Tao, a former immunologist at the Shanghai Center for Disease Control and Prevention. He said the Chinese government was unlikely to halt the emergency-use program because “it will cause a lot of panic.”

Elsie Chen and Muktita Suhartono contributed reporting. Amber Wang contributed research.



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