Brazil's health regulator said 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 Brazilian regulator, Anvisa, said in a statement it had “ruled to interrupt the clinical trial 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, hospitalisation, birth defects and other “clinically significant events”.

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