SAO PAULO — 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 it was surprised by Anvisa’s decision and it would hold a news conference Tuesday.

Sinovac issued a short statement in China on Tuesday saying it was in touch with Brazilian authorities. “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 standards.