A frontrunner in the race to produce a coronavirus vaccine has paused its widespread human trial after its product possibly caused a serious reaction in one participant.

AstraZeneca and University of Oxford have put their combined Phase 2 and 3 trial on standby after one United Kingdom participant reportedly developed severe neurological symptoms.

The volunteer's condition and what brought it on remain unclear.

But University of Arizona immunologist Deepta Bhattacharya said such questions are the point of large human trials.

"As we start to expand out the numbers into tens of thousands of people, will we start to see some rare events, and what is the risk-reward proposition once you start to see that data?"

Proof linking the vaccine to the illness won't end development as long as the reaction is rare. But it could reopen another case involving neurological symptoms that was previously ruled unrelated.

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