Pharmaceutical giant AstraZeneca and Oxford University have resumed their Covid-19 vaccine trials, they announced on Saturday, after a pause due to an illness in one volunteer.
In a statement, Oxford said that "in large trials such as this, it is expected that some participants will become unwell and every case must be carefully evaluated to ensure careful assessment of safety."
The university did not give details about the patient who reported the side effect, but said that an "independent review process" concluded it was safe to resume the trials.
The late-stage testing will now be resumed across all trial sites in Britain.
According to medical news site Stat, there was a suspected serious adverse reaction in a participant in the Britain-based trial.
The firm voluntarily paused trials of the Oxford vaccine on September 6 to review safety data and investigate, in what it called a routine action in such a trial.
Vaccines can take years to develop and test but many countries have accelerated efforts in a bid to slow the coronavirus pandemic.
Earlier trials involving more than 1,000 people found the vaccine to be safe and produce immunity.
Hopes are high for the Oxford vaccine - the EU has already secured the right to purchase up to 400 million doses in case of authorization.
The vaccine could then be acquired for all 27 EU states and distributed according to population and need. Britain has signed a separate deal.