Johnson & Johnson pauses its coronavirus vaccine trial because of a volunteer’s ‘unexplained illness.’

Johnson & Johnson has paused the large late-stage clinical trial of its coronavirus vaccine because of an “unexplained illness” in one of the volunteers, the company said on Monday.

The company did not say whether the sick participant had received the experimental vaccine or a placebo. The pause was first reported by the health news website Stat. On Tuesday morning, shares of Johnson & Johnson fell about 2 percent on the S&P 500.

Johnson & Johnson, which just began the so-called Phase 3 trial of its vaccine last month, was behind several of its competitors in the vaccine race, but its vaccine had some advantages over others. It does not need to be frozen, and it could need just one dose instead of two. It would also be the largest trial, with a goal of enrolling 60,000 volunteers.

“Adverse events — illnesses, accidents, etc. — even those that are serious, are an expected part of any clinical study, especially large studies,” the company said in a statement. “We’re also learning more about this participant’s illness, and it’s important to have all the facts before we share additional information.”

Trial pauses are often lifted quickly after the illness is investigated and deemed not to be a serious safety risk.

“It’s actually a good thing that these companies are pausing these trials when these things come up,” said Dr. Phyllis Tien, an infectious disease physician at the University of California, San Francisco, a trial site for Johnson & Johnson and AstraZeneca. “We just need to let the sponsor and the safety board do their review and let us know their findings.”

“It doesn’t mean that the adverse event is related to the vaccine, but it needs to be investigated thoroughly,” said Dr. Luciana Borio, who oversaw public health preparedness for the National Security Council under President Trump and was acting chief scientist at the Food and Drug Administration under former President Barack Obama. Depending on the findings, she said, additional data may be collected, trial rules may be modified, or other steps may be taken to ensure safety.

Johnson & Johnson’s is not the first trial to be paused because of safety concerns. Two participants in AstraZeneca’s trial became seriously ill after getting its vaccine. That trial was halted twice — most recently last month — and has still not resumed in the United States, though locations abroad swiftly broke the pause.

Both Johnson & Johnson and AstraZeneca developed their vaccines using adenovirus vectors, modified viruses engineered to carry coronavirus genes into human cells. AstraZeneca’s vaccine used an adenovirus that causes common colds in chimpanzees. Johnson & Johnson’s used another adenovirus, called Ad26, which has also been used in its approved vaccine for Ebola.

Experts say safety issues must be investigated and are one reason late-stage trials cannot be rushed — even for a president who has repeatedly claimed that a vaccine will be ready before the U.S. election on Nov. 3. “This kind of event epitomizes why vaccine development can’t be influenced by artificial timelines such as an election,” said John Moore, a virologist at Weill Cornell Medicine.

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