Johnson & Johnson has announced it is pausing its trials of a Covid-19 vaccine while the company investigates a study participant's "unexplained illness".

In a statement this morning, the pharmaceutical giant said an independent committee overseeing the trial would review and evaluate the individual's illness.

The pause is at least the second to occur among several vaccines that have reached large-scale final tests in the US.

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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.

The halt was first reported by the health news site STAT.

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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 has said. That company's testing has restarted elsewhere.