Drugmaker Johnson & Johnson on Monday said it has temporarily paused the advanced clinical trial of its experimental Covid-19 vaccine because of an unexplained illness in one of the volunteers. The Phase 3 trial of its vaccine began last month, with a goal of enrolling up to 60,000 volunteers across more than 200 sites in the United States and around the world.

“Following our guidelines, the participant’s illness is being reviewed and evaluated by the ENSEMBLE independent Data Safety Monitoring Board as well as our internal clinical and safety physicians,” the company said in a statement.

The company added that illnesses, accidents and other adverse events were an expected part of any clinical study, especially large studies, but that its physicians and a safety monitoring panel would try to determine what might have caused the illness.

“Based on our strong commitment to safety, all clinical studies conducted by the Janssen Pharmaceutical Companies of Johnson & Johnson have prespecified guidelines. These ensure our studies may be paused if an unexpected serious adverse event (SAE) that might be related to a vaccine or study drug is reported, so there can be a careful review of all of the medical information before deciding whether to restart the study.” 

— Johnson & Johnson

The company did not reveal any more details about the illness, citing the participant’s privacy. “We’re also learning more about this participant’s illness, and it’s important to have all the facts before we share additional information,” it said.

This is the second Phase 3 coronavirus vaccine trial to be paused. Last month, pharmaceutical company AstraZeneca had also paused late-stage trials of its experimental coronavirus vaccine after a study participant developed an unexplained illness. However, four days later on September 12, the company resumed the clinical trials in the United Kingdom after getting the approval from the health regulatory authority. The vaccine, developed with University of Oxford, has been widely seen as one of the leading candidates against the infection. The trial was stopped when a woman developed severe neurological symptoms consistent with transverse myelitis, a rare inflammation of the spinal cord.

The other countries where the J&J trials were taking place are Argentina, Brazil, Chile, Colombia, Mexico, Peru and South Africa. It is one of six coronavirus vaccines being tested in the US, also one of four in the most advanced Phase 3 stage. It is a single-shot vaccine if proven effective could simplify distribution of millions of doses compared with leading rivals requiring two doses.

Over 100 vaccines are being developed around the world to tackle the coronavirus pandemic. The global coronavirus count has crossed 3.77 crore and the toll rose to 10,78,868, according to Johns Hopkins University. The number of recoveries is over 2.61 crore.