Brazil National Health Surveillance Agency Anvisa has suspended a clinical trial of Chinese biopharma firm Sinovac’s Covid-19 vaccine, CoronaVac, owing to a severe adverse reaction.

According to Anvisa, the incident happened on 29 October and the suspension of the trial surprised organisers, who said that the death during the trial was not due to the vaccine.

After communicating with its Brazilian partner Butantan Institute, Sinovac has given a clarification saying that the serious adverse event is not related to the vaccine.

Sinovac said in a statement: “The clinical study in Brazil is strictly carried out in accordance with GCP requirements and we are confident in the safety of the vaccine.”

No new volunteers will be vaccinated due to interruption of the trial and the data obtained so far will be evaluated to analyse the risk/benefit of continuing the trial, Anvisa said.

Sao Paulo Butantan Institute head Dimas Covas told TV Cultura: “As there are more than 10,000 volunteers at this moment, deaths can occur. It’s a death that has no relation with the vaccine and as such it is not the moment to interrupt the trials.”

Earlier, Brazilian President Jair Bolsonaro criticised the Sinovac vaccine dismissing it as lacking credibility.

Adverse events are common in clinical trials and the number of cases can be on the higher side in large-scale studies.

Temporary suspension of trials allows organisers to investigate whether they are related to the drug.

Late-stage trials of CoronaVac are being carried out in Indonesia and Turkey.

Last month, Anvisa announced that a volunteer in the Brazilian trial of AstraZeneca and Oxford University’s investigational Covid-19 vaccine had died.