It is "very unlikely" that a trial for a COVID-19 vaccine in the UK will be cancelled after a patient reported side-effects this week.
Professor Luke O'Neill, immunologist and professor of biochemistry at Trinity College Dublin, said that it was common for trial participants to experience negative effects.
It comes after trials of a COVID-19 vaccine under development by AstraZeneca and Oxford University were put on hold owing to a reported side effect in a patient in the UK.
The participant developed transverse myelitis, which is an inflammation of the spinal cord.
Professor O'Neill told The Pat Kenny Show that the vaccine developers would not be surprised that a patient had reported a side-effect at this stage and they would be pausing the trial to look at it closely.
He said it could be "bad luck" that someone out of the first trance of trial participants had the symptom.
Professor O'Neill said: "It's common in trials to see adverse events, they'll look at this really closely, look at the person and see how severe it is, if it's mild, that's less of a concern, and then try and link it to the vaccine if they can."
He said the person with the reported side-effect could even have been in the control group.
He added: "If it's a very serious event that could well stop the trial but that is very unlikely."
Meanwhile, nine vaccine development companies issued a joint statement that they would not allow a vaccine to be released unless it was rigorously tested and safe.
Professor O'Neill said: "It's a really interesting development, you wouldn't have seen that before.
"I think it reflects the risk of safety, because everyone is conscious of the safety question, so they've got to make sure this is absolutely gold-plated, which will take time.
This means there would not be a vaccine before the US election in November, despite pressure from President Donald Trump to release one quickly.
Professor O'Neill added that when a vaccine is eventually ready to be released to the public, criteria will be listed which ensures it is distributed in the fairest possible way.
He said it was a "big worry" that the developing world will not get access to the vaccine and so that should be a top priority,
He said: "The fair distribution model makes a clear statement that when a vaccine is available it must be distributed as fairly as possible to the whole world."