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Pressure to create a coronavirus vaccine is increasing by the day, but for a safe vaccine to enter the market, it takes time.

USA TODAY

Johnson & Johnson’s COVID-19 vaccine trial has been paused due to an unexplained illness in a volunteer. Now two of the four vaccine trials in the United States are now on hold.

The Phase 3 clinical trial was temporarily halted Monday, according to the company. It is not known if the unidentified illness is related to the vaccine, but clinical protocols require a pause while it is investigated.

Johnson & Johnson’s JNJ-78436735 vaccine is one of four large-scale, final-stage COVID-19 vaccine trials underway in the U.S. Another trial, run by AstraZeneca, was halted Sept. 8 after a second participant was diagnosed with a neurological condition.

Johnson & Johnson said Monday it has paused further dosing in its trial while the participant’s illness is reviewed and evaluated by an independent Data Safety Monitoring Board as well as the company’s clinical and safety physicians. 

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In any study the size of the Johnson & Johnson one, which will include up to 60,000 people when it is fully enrolled, some adverse events are expected among the participants. Such studies have clear, prespecified guidelines that say what events would trigger a shutdown. 

The independent Data Safety Monitoring Board, which oversees the trial, will review what happened and decide whether it is safe to resume the trial.

Labs are studying potential COVID-19 vaccines. (Photo: Chandan Khanna/ AFP via Getty Images)

Johnson & Johnson did not say what the illness was nor where the participant lived.  

“We must respect this 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,” the company said. 

Participants began enrolling in the trial on Sept. 23. 

The vaccine was developed by Janssen Research & Development, a subsidiary of Johnson & Johnson. It is delivered by a harmless virus called Adenovirus 26, a human virus uncommon in nature so most people have not developed immunity to it.

The 30,000-person AstraZeneca trial has been on hold since Sept. 8, about a week after launching.

A participant in a similar AstraZeneca trial in the United Kingdom developed a serious neurological condition that might have been caused by the vaccine, so AstraZeneca halted all its COVID-19 vaccine trials around the world while the case could be investigated.

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AstraZeneca also stopped its trials briefly in July because another participant developed symptoms of the neurological condition multiple sclerosis, although news of that stoppage was not made public until the second case. The first person was determined “to have an undiagnosed case of multiple sclerosis, which the independent panel concluded was unrelated to the vaccine,” the company said.

The AstraZeneca vaccine, based on technology