Johnson & Johnson JNJ.N has paused clinical trials of its coronavirus vaccine candidate due to an unexplained illness in a study participant, delaying one of the highest profile efforts to contain the global pandemic.

The move comes around a month after AstraZeneca AZN.L also suspended trials of its experimental coronavirus vaccine - which uses a similar technology - due to a participant falling ill.

J&J said on Monday the illness was being reviewed by an independent data and safety monitoring board as well as the U.S. group’s clinical and safety physicians.

The company, which reports quarterly financial results on Tuesday morning, said such pauses are normal in big trials, which can include tens of thousands of people.

It added the voluntary “study pause” in giving doses of the vaccine candidate was different from a “regulatory hold” imposed by health authorities.

AstraZeneca last month 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.

While AstraZeneca’s trials in Britain, Brazil, South Africa and India have since resumed, its U.S. trial is still on hold, pending a regulatory review.

The J&J and AstraZeneca vaccines are both based on a so-called adenovirus, a harmless modified virus that instructs human cells to produce vaccine proteins.

They are both also part of the U.S. 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.

AstraZeneca and medical experts say trial suspensions to look into the cause of a participant’s illness are not uncommon. Underwriters of clinical trial insurance have said premiums for coronavirus vaccines studies are only marginally higher than for pre-pandemic vaccines.

J&J on Sept. 22 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.”Everybody is on the alert because of what happened with AstraZeneca,” Dr. William Schaffner, a professor of infectious diseases at the Vanderbilt University School of Medicine, said by email, adding it could take a week to gather information.

“This is likely to be a neurological event,” he said. Last month, J&J said its vaccine candidate produced a strong immune response 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 some participants in studies get placebos, and it was not always clear whether a person suffering a serious adverse event in a trial received a placebo or the treatment.

Stat News reported here the pause earlier, citing a document sent to outside researchers, which stated that a "pausing rule" had been met, the online system used to enroll patients in the study had been closed and the data and safety monitoring board would be convened.