Anvisa, the country’s health regulator, suspended the trials late on Monday saying the event occurred on October 29.
The state government of Sao Paulo, where the trial is being run, said the death of a trial volunteer had been registered as suicide and was being investigated.
A police report of the incident was seen by Reuters.
The news of the trial being suspended has delighted Brazilian President Jair Bolsonaro, who has repeatedly criticised the vaccine’s credibility and said it would not be purchased by his government.
This suspension further inflamed tensions between Bolsonaro and Sao Paulo Governor Joao Doria, who has pinned his political ambitions on the Chinese vaccine which he aims to roll out in his state as early as January, with or without federal assistance.
On Tuesday, Anvisa said it would maintain the suspension and did not give any indication of how long it might last, adding it required more information on the incident.
It dismissed any suggestion the move was politically motivated, saying the decision was purely technical.
The trial’s organisers criticised Anvisa’s decision, saying they had not been notified in advance and that there was no reason to stop the trial.
Although a trial volunteer had died, it had nothing to do with the vaccine, Jean Gorinchteyn, Health Secretary for the state of Sao Paulo told a news conference on Tuesday.
“We had an external event that led to the regulator being notified,” Gorinchteyn said. “This vaccine is safe.”
Dimas Covas, the head of Sao Paulo’s medical research institute Butantan, which is conducting the Sinovac trial, said the vaccine had shown no serious adverse effects.
Speaking at the same news conference, he said he hoped the trial would be resumed later on Tuesday or Wednesday.
Anvisa said the initial information it received from Butantan had not specified that the death was a suicide.
“We had no choice but to suspend the trials given the event,” the head of the agency Antonio Barra Torres said.
The setback to Sinovac’s efforts contrasts with welcome news from Pfizer Inc, which said on Monday its experimental Covid-19 vaccine is more than 90% effective based on initial trial results.
A World Health Organization spokeswoman sought to play down the politics. “I don’t think you need to try to find reasons or explanations other than the fact that people who are looking for a vaccine … are very cautious,” Fadela Chaib said.
Sinovac said in a statement on its website on Tuesday it was confident in the safety of its vaccine and will continue to communicate with Brazil on the matter. It has previously said it expects interim results of late-stage trials this year.
It is not uncommon for clinical trials to be suspended temporarily after a subject dies or becomes ill so that independent monitors and trial organisers can check whether it is related to the drug being tested.
Late-stage trials are also being conducted in Indonesia and Turkey. Indonesia’s state-owned Bio Farma said on Tuesday its Sinovac vaccine trials were “going smoothly”.
Four Covid-19 vaccines are being tested in Brazil, including one being developed by Oxford University with AstraZeneca Plc and another from Johnson & Johnson.
Pfizer’s vaccine, developed in partnership with Germany’s BioNTech SE, is undergoing late-stage tests involving 3,100 volunteers in Sao Paulo and Bahia states.
Worldwide, there are at least 10 experimental coronavirus vaccines in late-stage clinical trials, according to the World Health Organization. Four of them are from China.