The health regulator, Anvisa, said on Monday the event took place on Oct. 29 but did not specify if the incident took place in Brazil or in another country. It also did not give an indication of how long the suspension of the large late-stage trial might last.
Dimas Covas, the head of Sao Paulo's medical research institute Butantan which is conducting the trial said the decision was related to a death but added he found the regulator's announcement strange "because it's a death unrelated to the vaccine."
"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," Covas told local broadcaster TV Cultura.
Butantan plans to hold a news conference on Tuesday at 11 a.m. local time (1400 GMT).
Sinovac did not immediately respond to requests for comment.
The Sinovac vaccine has been criticized by Brazil's president, Jair Bolsonaro, who has dismissed it as lacking credibility. Bolsonaro, who regularly expresses anti-Chinese sentiment, has previously said the federal government will not buy the vaccine.
Earlier on Monday he appeared to row back on those comments, saying the government would buy any vaccines that were approved by the Health Ministry and regulator Anvisa.
Bolsonaro's stance has, however, set a clear political battleline with the Governor of Sao Paulo, Joao Doria, who has said his state will both import and produce the vaccine.
Doria, who is widely expected to challenge Bolsonaro at the next presidential election in 2022, said a public inoculation program in Sao Paulo with the Sinovac vaccine would likely be rolled out as early as January.
It is not uncommon for clinical trials to be suspended temporarily after a volunteer takes ill so that trial organizers can check whether it is related to the drug being tested.
Sinovac's vaccine is among the three experimental COVID-19 vaccines that China has been using to inoculate hundreds of thousands of people under an emergency use program. A Chinese health official said on Oct. 20 that serious side effects have not been observed in clinical trials.
The Brazil trial was the first of Sinovac's large late-stage trial to get underway. Late-stage trials are also being conducted in Indonesia and Turkey. Indonesia's state-owned Bio Farma said on Tuesday that its Sinovac vaccine trials were "going smoothly."
Brazil has seen over 160,000 people die from COVID-19 and had more than 5.6 million confirmed cases.