Brazil’s health regulator said Monday it had suspended clinical trials of a Chinese-developed Covid-19 vaccine after an “adverse incident” involving a volunteer recipient, a blow for one of the most advanced vaccine candidates.

The setback for CoronaVac, developed by Chinese pharmaceutical firm Sinovac Biotech, came on the same day US pharmaceutical giant Pfizer said its own vaccine candidate had shown 90 percent effectiveness, sending global markets soaring and raising hopes of an end to the pandemic.

The Brazilian regulator, Anvisa, said in a statement it had “ruled to interrupt the clinical study of the CoronaVac vaccine after a serious adverse incident” on October 29.

It said it could not give details on what happened because of privacy regulations, but that such incidents included death, potentially fatal side effects, serious disability, hospitalization, birth defects and other “clinically significant events.”

However, the public health center coordinating the trials of the vaccine in Brazil, the Butantan Institute, said it was “surprised” by the decision.

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The institute “is investigating in detail what happened,” and “is at the Brazilian regulatory agency’s disposal to provide any clarification necessary on any adverse incident the clinical trials may have presented,” it said.

It said it would hold a press conference on Tuesday at 11:00 am (1400 GMT).

CoronaVac has been caught up in a messy political battle in Brazil, where its most visible backer has been Sao Paulo Governor Joao Doria, a top opponent of far-right President Jair Bolsonaro.

The Sao Paulo state government said in a statement it “regrets that it learned of the decision from the press, instead of directly from Anvisa,” and was waiting along with the Butantan Institute for more information on “the real reasons for the suspension.”

– Pandemic politics –

Bolsonaro has labeled CoronaVac the vaccine from “that other country,” and pushed instead for a rival vaccine developed by Oxford University and pharmaceutical firm AstraZeneca.

Last month, he shot down a plan by his own health minister to buy 46 million doses of CoronaVac, saying, “The Brazilian people will not be anyone’s guinea pig,” and referring to it as “Joao Doria’s Chinese vaccine.”

Doria announced earlier Monday that the first 120,000 doses of CoronaVac would arrive in Sao Paulo on November 20.

His state has a deal with Sinovac to buy a total of 46 million doses — six million produced in China and the rest produced in Sao Paulo, which broke ground last week on a factory to make the vaccine domestically.

“Officials in the state government fear Bolsonaro will use technical decisions to delay the timeline of the vaccine for political reasons,” newspaper Folha de Sao Paulo reported, citing unnamed Doria allies.

Anvisa told AFP it had no comment beyond its statement announcing the suspension, which said that halting trials was standard procedure in such cases.

Bolsonaro faces criticism for his handling of the pandemic, which has included downplaying the virus, opposing lockdown measures and relentlessly promoting the drug hydroxychloroquine despite studies showing it is ineffective against Covid-19.

The Sinovac, Pfizer and Oxford vaccines are all in Phase 3 trials, the final stage of testing before regulatory approval.

All are being tested in Brazil, the country with the second-highest death toll in the pandemic after the United States, with more than 162,000 people killed by the new coronavirus.