The final clinical trials of vaccine developed by AstraZeneca and Oxford University are put on hold after a participant had an adverse reaction.
“In large trials, illnesses will happen by chance but must be independently reviewed to check this carefully”, an Oxford University spokesperson said.
All international trials have been paused until an investigation reviews the safety data before the regulator decided to restart the trial.
This is the second time that the Oxford coronavirus vaccine trial has been put on hold.
The outcome of vaccine trials is being closely watched around the world.
The AstraZeneca-Oxford University vaccine is seen as a strong contender among dozens being developed globally.
Hopes have been high that the vaccine might be one of the first to come on the market, following successful phase 1 and 2 testing.
Its move to Phase 3 testing in recent weeks has involved some 30,000 participants in the US as well as in the UK, Brazil and South Africa. Phase 3 trials in vaccines often involve thousands of participants and can last several years.
The World Health Organization (WHO) says nearly 180 vaccine candidates are being tested around the world but none has yet completed clinical trials.