London — The late-stage COVID-19 candidate vaccine trials run by AstraZeneca have been paused after a participant fell ill.

The company is investigating whether the vaccine recipient’s “potentially unexplained” illness is a result of receiving the shot.

In a statement, AstraZeneca said the “standard review process triggered a pause to vaccination to allow review of safety data” without revealing what the potential side effect of the vaccine was, AP writes.

AstraZeneca is trialling the Oxford University vaccine which was developed using a chimpanzee adenovirus. The adenovirus was then genetically modified so that it cannot grow in humans.

Typically, the virus causes the common cold in chimpanzees.

The news about the trial’s pause was first reported by health news site STAT, which reported the participant was located in the United Kingdom.