The participant's illness is being reviewed and evaluated by an independent data and safety monitoring board as well as the company's clinical and safety physicians, the company said in a statement.
J&J said that such pauses are normal in big trials, which can include tens of thousands of people. It said the "study pause" in giving doses of the vaccine candidate was different from a "regulatory hold" required by health authorities. The current case is a pause.
However, J&J's move follows a similar one by AstraZeneca . In September, the British group paused late-stage trials of its experimental coronavirus vaccine, developed with the University of Oxford, due to an unexplained illness in a British study participant.
Both candidates are based on a so-called adenovirus, a harmless modified virus that instructs human cells to produce vaccine proteins, and both are part of the US government's Operation Warp Speed programme to support vaccine development.
"This could be a second case of adenoviral vaccine to spur safety concerns," said Bryan Garnier analyst Olga Smolentseva.
J&J on September 22nd became the fourth Warp Speed participant to enter the final stage of testing on humans, with the aim of enrolling 60,000 volunteers in the United States and abroad.
While Astra's trials in Britain, Brazil, South Africa and India have resumed, the US trial is still on hold, pending a regulatory review.
Dr William Schaffner, a professor of infectious diseases at the Vanderbilt University School of Medicine, said by email that "Everybody is on the alert because of what happened with AstraZeneca," adding that it could take a week to gather information.
"It would have to be a serious adverse event. If it was something like prostate cancer, uncontrolled diabetes or a heart attack - they wouldn't stop it for any of those reasons. This is likely to be a neurological event," he said.
Last month, J&J said its experimental Covid-19 vaccine produced a strong immune response against the novel coronavirus in an early-to-mid stage clinical trial. This prompted the company to start the large scale trial, with results expected by the end of this year or early 2021.
J&J declined to elaborate on the illness due to privacy concerns. It did say that some participants in studies get placebos, and it was not always clear whether a person suffering a serious adverse event in a clinical trial received a placebo or the treatment. -Reuters
