Sao Paulo’s Instituto Butantan, which partnered with Sinovac to produce the vaccine locally, said in a statement it was surprised by the decision and is looking into details of what happened in the study. Director Dimas Covas said in a TV interview that one volunteer in the trial has died, but that the death is not related to the vaccine.

Halts to investigate serious adverse events are not uncommon in large-scale drug trials and two western developers – AstraZeneca Plc and Johnson & Johnson — have paused their vaccine trials in recent months due to such incidents, only to re-start them after investigation.

But China has already started administering its vaccines, including Coronavac, to hundreds of thousands of people under an expansive emergency use approval, making the prospect of a safety issue being detected at this stage more concerning.