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 that it would hold a news conference Tuesday.

The CoronaVac shot has already stirred controversy in Brazil, where President Jair Bolsonaro has cast doubt on its prospective effectiveness. He sparked confusion last month when he publicly rejected it, 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, Sao Paulo state's Gov. João Doria has become a political rival and an outspoken critic of the president's pandemic response.

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.

Dimas Covas, who leads Butantan, said on TV Cultura late Monday that a volunteer had died, but that the person’s death 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.

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.

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.