Johnson & Johnson has paused its Covid vaccine trial to investigate why one participant in the study fell ill.
The company said an independent review would check if the person's unexplained symptoms were related to the jab.
It has suspended recruitment to the phase three trial as a precaution.
It said in large trials, with tens of thousands of volunteers, it was to be expected that some may become unwell during the study period.
The company said that for privacy reasons, it could not give more details about the participant.
"We're also learning more about this participant's illness, and it's important to have all the facts before we share additional information," it said in a statement.
It is not the first Covid vaccine trial to be suspended - a participant in the UK's Oxford University study had an unexplained illness too, but it has since been deemed safe to resume, except for in the US where regulators are still in discussions with manufacturer Astra Zeneca.
There are nearly 180 vaccine candidates being tested around the world, but none has yet completed clinical trials.
The Johnson & Johnson jab, like the Oxford University Astra Zeneca one, is in the advanced stage of testing.
Both use a modified common cold virus to prime the immune system against Covid.
The Johnson & Johnson trial had started recruiting participants in late September, with a goal of enrolling up to 60,000 volunteers across more than 200 sites in the US as well as in South America and South Africa.