CNN  — 

A “serious adverse event” that led Brazil’s health authorities to halt clinical trials of a Chinese-developed Covid-19 vaccine was not related to the vaccine trial itself, the director of Brazil’s Butantan biomedical institute said on Tuesday.

Brazil’s National Health Surveillance Agency (Anvisa) suspended the trial on Monday after an incident on October 29. A note from Anvisa said the trial had been paused in order to better evaluate the data and assess the risk.

But Dimas Covas, the institute’s director, told reporters at a news conference in Sao Paulo that the incident was in no way linked to the vaccine trial.

“The serious adverse event observed in a volunteer in the trial has no relation to the vaccine,” Covas said. “It was analyzed and determined that there was no relation.”

Covas said there was no reason to interrupt the trial, and added he is hoping it will restart soon.

China’s Sinovac began Phase 3 clinical trials of its CoronaVac vaccine in collaboration with the Butantan Institute and the state of Sao Paulo in late July, with an aim to recruit 130,000 volunteers.

Phase 3 trials represent the final and most important testing stage before regulatory approval is sought.

News that the trial had been paused came as US drugmaker Pfizer announced that early data from its own coronavirus vaccine showed it was more than 90% effective.

Covas said he could not provide more details about the adverse event or the volunteer for privacy reasons, but insisted that regulators had all of the information showing that the event was not related to the vaccine.

Sinovac is also conducting Phase 3 trials in Indonesia and Turkey. Sinovac did not immediately responded to CNN’s request for further comment.

In a statement posted online Tuesday, Sinovac said it was “confident in the safety of the vaccine,” without giving a reason for the reported suspension.

“We have communicated with our Brazilian partner, the Butantan Institute, and the head of the institute believes the incident has nothing to do with the vaccine. Sinovac will continue to communicate with the Brazilian side on this matter. Work related to our clinical research in Brazil will continue to be carried out in strict accordance with GCP (Good Clinical Practice) requirements,” the statement said.

A Chinese Foreign Ministry spokesman read the company’s statement to reporters when asked to comment on the development at the ministry’s daily press briefing Tuesday.

The Chinese vaccine has been at the heart of a political feud between Brazilian President Jair Bolsonaro and the governor of Sao Paulo, Joao Doria, who is expected to run against him in the country’s next presidential elections in 2022.

Doria has said he hopes to eventually vaccinate Sao Paulo state residents with CoronaVac, but Bolsonaro has disparaged the vaccine, instead backing a trial by Oxford University and AstraZeneca.

Last month, Doria called CoronaVac “the safest, the one with the best and most promising rates” among all the vaccines tested in Brazil, after Butantan Institute said the vaccine had proved safe in a trial involving 9,000 volunteers, according to Reuters.

A Brazilian volunteer receives a coronavirus vaccine produced by Chinese company Sinovac Biotech at the Sao Lucas Hospital in Porto Alegre, southern Brazil on August 8.

Pausing a clinical trial is not unusual. In September, drug giant AstraZeneca paused global trials of its coronavirus vaccine because of an unexplained illness in one of the volunteers.

Just last month, Chinese officials and vaccine developers declared that no serious side effects had been observed or reported in the clinical trials of any of China’s rapidly produced vaccines. China currently has four coronavirus vaccines in Phase 3 clinical trials, mostly conducted in developing countries across the world.

An official with China’s Ministry of Technology said at a press conference on October 20 that a total of 60,000 participants had been injected with the four Chinese vaccines in Phase 3 trials – and no serious adverse effects had been reported.

Sinovac General Manager Gao Qiang said at the same event that while some mild adverse effects had been detected in its Phase 3 trials, no serious reactions had been reported in Brazil, Indonesia or Turkey. He said its collaborators in the three countries had established a “thorough detection system for adverse effects.”

Vaccines, like any medicine, may cause side effects, but most are usually minor and short-lived, such as pain, rashes, headaches or fever.

It is unclear if Brazil’s suspension of the Sinovac trials will have any impact on the company’s ongoing trials in Indonesia and Turkey.

Last week, Indonesia said it planned to vaccinate 9 million people with Sinovac’s CoronaVac in the third week of December, under an Emergency Use Authorization approval from the country’s drug regulator.

CNN has reached out to Indonesian and Turkish health authorities for comment in light of the decision to pause testing in Brazil.

In China, drug makers have been administering experimental vaccines on tens of thousands of people since July under a government-approved emergency uses program, which allows vaccine candidates to be used on a limited scale before their safety and efficacy have been fully proven by clinical trials.

The move is controversial as some experts worry that fast-tracking the normal approval process could potentially expose participants to unknown side effects, and Chinese drug makers are taking risks that contravene international ethical and safety norms.