A volunteer taking part in clinical trials into an Oxford University and AstraZeneca coronavirus vaccine has died.
Brazil's health authority has announced the tragedy, which has been confirmed by the university - but scientists said no safety concerns have been identified.
It later emerged that the volunteer had been given a placebo drug.
Trials of the drug will not be halted.
The Federal University of Sao Paulo, which is helping coordinate phase 3 clinical trials in Brazil, said the volunteer was Brazilian - but did not say where the person lived.
No further details about the circumstances have been released, and it is not known if the person's death was in any way related to the trial.
"Following careful assessment of this case in Brazil, there have been no concerns about safety of the clinical trial and the independent review in addition to the Brazilian regulator have recommended that the trial should continue," a spokesman for the university said in a statement.
The drug is one of the frontrunners as scientists across the world attempt to find a way out of the coronavirus nightmare gripping the planet.
Professor Jonathan Van-Tam, England’s deputy chief medical officer, has said the jab could be available around the turn of the year.
It was paused last month after a participant became ill, but following a review was allowed to continue.
In a statement at the time, AstraZeneca said: "In large clinical trials, illnesses will happen by chance and must be independently reviewed.
"AstraZeneca is working to expedite the review of the single event to minimise any potential impact on the trial timetable.
"We are committed to the safety of our participants and the highest standard of conduct in our trials."
Britain has stockpiled 340 million doses from six prototype vaccines - the most of any country - but taskforce boss Kate Bingham admits there are still plenty of uncertainties.
The head of the UK Vaccine Taskforce said a "partially effective vaccine is better than no vaccine at all" and that any vaccine would likely require more than one dose.
"Flu vaccines are 50% effective, but they are widely used and have a big impact on reducing the clinical impacts of flu in the population," Ms Bingham told Sky News.
* The original version of this story has been updated to include information which subsequently emerged to make it clear that the volunteer in the trial was given a placebo drug.
