Brazil halted a late-stage trial of a Chinese vaccine on Monday that 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 decision provoked a political firestorm on Tuesday after President Jair Bolsonaro, who has been critical of China and spoken dismissively of its vaccine, CoronaVac, called the suspension a political victory.

Instituto Butantan, the Brazilian institute assisting with the CoronaVac trial, called the suspension unwarranted, arguing that it had been triggered by the death of a trial participant, but that the death was unrelated to the vaccine.

A police report obtained by The New York Times says the participant’s death, which occurred in late October, is being investigated as a suicide. A senior government official for the State of São Paulo, which runs Instituto Butantan, said that fact led health experts at the institute to conclude that it did not raise red flags about the safety of the vaccine.

São Paulo state, the largest in Brazil, is led by Gov. João Doria, a political rival of Mr. Bolsonaro who has criticized the president’s cavalier handling of the pandemic.

Gustavo Mendes, the director of pharmaceuticals at Anvisa, Brazil’s health regulatory agency, said in an interview on Tuesday that regulators have yet to conclude that the volunteer’s death was unrelated to the vaccine.

“It was a precautionary measure,” he said. “Unfortunately, it’s being politicized.” He added: “Halting a study until there is more information is what is expected of a regulatory agency.”

CoronaVac, which is being developed by the Chinese company Sinovac, is one of 11 experimental vaccines produced by some of the world’s foremost pharmaceutical companies that are currently in Phase 3 trials. As the world grapples with 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.

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

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Credit…Andre Penner/Associated Press

The Chinese vaccine study has inflamed a political rivalry between Mr. Bolsonaro and Mr. Doria, who is widely expected to run for president in 2022. If the vaccine is approved, it would be manufactured by the institute.

Mr. Bolsonaro had voiced skepticism about the Chinese vaccine previously, and on Tuesday he gloated about the setback in a message on Facebook.

“This is the vaccine Doria wanted everyone in São Paulo to take,” the president wrote. “Yet another victory for Bolsonaro.”

Last month Mr. Bolsonaro reacted angrily when he learned the health ministry intended to buy 46 million doses of the vaccine.

“I ordered that it be canceled,” he said. “It appears no country in the world is interested in that Chinese vaccine.”

Mr. Doria’s party, the Brazilian Social Democracy Party, said in a statement that the race to develop a vaccine “is not a political contest and cannot be treated like one.”

It accused Mr. Bolsonaro of “putting his political aspirations above anything else, showing disregard for the lives of Brazilians.”

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.

Credit…Amanda Perobelli/Reuters

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 Mr. Bolsonaro, who has downplayed the threat and blamed China for the pandemic, have criticized them.

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.

Vaccine experts have said that they considered 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.

Credit…Thomas Peter/Reuters

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 on 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 Centers 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.”

Reporting was contributed by Manuela Andreoni, Letícia Casado, Elsie Chen and Muktita Suhartono. Amber Wang contributed research.