Brazil’s health regulator said Monday it has suspended the clinical trials for China’s Sinovac coronavirus vaccine due to a ‘severe adverse effect’ to a volunteer on October 29.

The health regulator however did not disclose further details on the vaccine trial suspension.

Sao Paulo’s Instituto Butantan, which partnered with Sinovac to produce the vaccine locally, said in a statement it was surprised by the decision.

The health regulator did not clarify if the incident took place in Brazil or in other countries who are also conducting clinical trials for the vaccine.

Sinovac has yet to address the issue. So far, no other country announced that they will be suspending the trial.

Last phases of the trial are now being done in Indonesia and Turkey.

Sinovac’s vaccine is among the frontrunners in the world’s race to find the vaccine for the coronavirus disease.

The company was able to obtain emergency approval to start clinical trials to hundreds of thousands of individuals in different countries.