Final clinical trials for a coronavirus vaccine, developed by AstraZeneca and Oxford University, have been put on hold after a participant had an adverse reaction in the UK.
AstraZeneca described it as a "routine" pause in the case of "an unexplained illness".
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.
What have the developers said?
All international trial sites have now been put on pause while an independent investigation reviews the safety data before regulators decide whether the trial can restart, the BBC's Medical Editor Fergus Walsh reports.
"In large trials, illnesses will happen by chance but must be independently reviewed to check this carefully", an Oxford University spokesperson said.
This is the second time the Oxford coronavirus vaccine trial has been put on hold, our correspondent notes. Such events are routine in major trials, and happen any time a volunteer is admitted to hospital when the cause of their illness is not immediately apparent.
It is thought the trials could resume in a matter of days.
Stat News, the health website which first broke the story, said details of the UK participant's adverse reaction were not immediately known, but quoted a source as saying they were expected to recover.
Where are we in the search for a vaccine?
US President Donald Trump has said he wants a vaccine available in the US before 3 November's election, but his comments have raised fears that politics may be prioritised over safety in the rush for a vaccine.
On Tuesday, a group of nine Covid-19 vaccine developers sought to reassure the public by announcing a "historic pledge" to uphold scientific and ethical standards in the search for a vaccine.
AstraZeneca is among the nine firms who signed up to the pledge to only apply for regulatory approval after vaccines have gone through three phases of clinical study.
Industry giants Johnson & Johnson, BioNTech, GlaxoSmithKline, Pfizer, Merk, Moderna, Sanofi and Novavax are the other signatories.
They pledged to "always make the safety and well-being of vaccinated individuals our top priority".
The World Health Organization (WHO) says nearly 180 vaccine candidates are being tested around the world but none has yet completed clinical trials.
The organisation has said it does not expect a vaccine to meet its efficacy and safety guidelines in order to be approved this year because of the time it takes to test them safely.
Similar sentiments have been shared by Thomas Cueni, director-general of the International Federation of Pharmaceutical Manufacturers. The industry body represents the companies that signed the pledge.
Despite this, China and Russia have begun inoculating some key workers with domestically developed vaccines. All of them are still listed by the WHO as being in clinical trials.
Meanwhile, the US national regulator, the Food and Drug Administration (FDA), has suggested that coronavirus vaccines may be approved before completing a third phase of clinical trials.
Last week it also emerged that the US Centers for Disease Control and Prevention had urged states to consider waiving certain requirements in order to be ready to distribute a potential vaccine by 1 November - two days before the 3 November presidential election.
Although President Trump has hinted that a vaccine might be available before the election, his Democratic rival Joe Biden has expressed scepticism that Mr Trump will listen to scientists and implement a transparent process.