An infectious disease expert says it's normal for a vaccine trial to be paused when someone taking part falls ill

A volunteer has reacted while trialling a vaccine created by AstraZeneca - in partnership with Oxford University.

Stage three clinical trials, which involve up to 30-thousand people, are suspended while the issue is investigated.

Otago University's David Murdoch says it's very common for someone to coincidentally become ill in such a large trial.

He says there's probably many studies where we don't hear about people falling ill - and it's usually passed off as a chance event.