A late-stage coronavirus vaccine trial has been paused in the US as the company investigates an “unexplained illness” developed by one of the participants.
Johnson & Johnson’s halting of its study marks at least the second such hold to occur among several vaccines that have reached large-scale final tests in the country.
The company said accidents and other so-called adverse events "are an expected part of any clinical study, especially large studies”.
Its physicians and a safety monitoring panel would try to determine what might have caused the illness, it said in statement.
The company declined to reveal any more details about the illness, citing the participant's privacy.
Temporary stoppages of large medical studies are relatively common.
Few are made public in typical drug trials, but the work to make a coronavirus vaccine has raised the stakes on these kinds of complications.
Companies are required to investigate any serious or unexpected reaction that occurs during drug testing.
Given such tests are done on tens of thousands of people, some medical problems are a coincidence.
One of the first steps the company said it would take would be to determine if the person received the vaccine or a placebo.
Final-stage testing of a vaccine made by AstraZeneca and Oxford University remains on hold in the US as officials examine whether an illness in its trial poses a safety risk.
That trial was stopped when a woman developed severe neurological symptoms consistent with transverse myelitis, a rare inflammation of the spinal cord, the company said.
That company's testing has restarted elsewhere.
