SAO PAULO (AP) — Brazil's health regulator has halted clinical trials of the potential coronavirus vaccine CoronaVac, citing an “adverse, serious event.”

The decision posted on Anvisa's website Monday night elicited immediate surprise from parties involved in producing the vaccine.

The potential vaccine is being developed by Chinese biopharmaceutical firm Sinovac and in Brazil would be mostly produced by Sao Paulo's state-run Butantan Institute. Sao Paulo state’s government said in a statement it “regrets being informed by the press and not directly by Anvisa, as normally occurs in clinical trials of this nature.”

Butantan said in a statement that it was surprised by Anvisa's decision and Dimas Covas, who leads Butantan, said on TV Cultura late Monday that while a volunteer had died, it was not due to the shot.

“We found this Anvisa decision strange, because it is unrelated to the vaccine. There are more than 10,000 volunteers at this moment,” he said.

Sinovac issued a short statement in China on Tuesday saying it was in touch with Brazilian authorities. “Sinovac will continue to communicate with Brazil on this matter. The clinical study in Brazil is strictly carried out in accordance with GCP requirements and we are confident in the safety of the vaccine," it said, referring to Good Clinical Practice, a set of international standards for ethics and data quality in clinical research.

It did not offer further details.

Temporary halts of drug and vaccine testing are relatively common. In research involving thousands of participants, some are likely to fall ill. Pausing a study allows researchers to investigate whether an illness is a side effect or a coincidence. Last month, two drugmakers resumed testing of their prospective coronavirus vaccines in the U.S. after they were halted earlier.

The CoronaVac shot has already stirred controversy in Brazil, where President Jair Bolsonaro has cast doubt on its prospective effectiveness. He publicly rejected it last month, saying Brazilians would not be used as guinea pigs. The declaration followed news that his health minister, Eduardo Pazuello, had agreed to purchase CoronaVac doses produced locally by Butantan.

Bolsonaro has often expressed mistrust of China, particularly on the campaign trail in 2018, although he has softened his rhetoric somewhat in office. Also, the head of the state producing the vaccine, Sao Paulo Gov. João Doria, is a political rival and an outspoken critic of the president's pandemic response.

Bolsonaro took another jab at the Sinovac shot on Tuesday.

“Death, invalidity, anomaly. This is the vaccine that Doria wanted to force all in Sao Paulo to take,” he wrote on his Facebook page. “The president said the vaccine should never be mandatory. Another one that Jair Bolsonaro wins.”

Anvisa said in its statement that the event prompting the trial's suspension occurred Oct. 29, without saying what transpired.

“With the interruption of the study, no new volunteer can be vaccinated,” its statement said.

CoronaVac is being tested in seven Brazilian states, plus the federal district where the capital Brasilia lies.

Following the imbroglio last month surrounding the CoronaVac shot, Anvisa authorized the import from China of 6 million doses. The potential vaccine cannot be administered to Brazilians as it isn’t yet approved locally, the agency said at the time.

Earlier Monday, Sao Paulo state’s health secretary, Jean Gorinchteyn, said the first 120,000 CoronaVac shots would arrive at Sao Paulo’s international airport Nov. 20.

“We will keep following health protocols to give the shots. They will only be taken to the public after a final authorization from Brazil’s health regulator,” Gorinchteyn said.

The secretary added that nearly all of the volunteers who were given two doses of the vaccine produced antibodies thought to protect people from the virus.

Sao Paulo is also importing raw material for the production of 40 million CoronaVac shots, which is due to start arriving Nov. 27.

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Associated Press writer Huizhong Wu in Taipei, Taiwan, contributed to this story.