A volunteer in the U.K. participating in the randomized, placebo-controlled trial had an unexplained event.By Dr. Maria Simbra

PITTSBURGH (KDKA) — Trials for one coronavirus vaccine are on hold.

“It was likely not a minor vaccine reaction, but something more significant,” says Dr. Marc Itskowitz, a primary care internist at the Allegheny Health Network.

A volunteer in the U.K. participating in the randomized, placebo-controlled trial had an unexplained event. According to the New York Times, it was inflammation of the spinal cord, a condition called transverse myelitis. It can occur after viral infections.

People can have pain, weakness, numbness and bowel and bladder problems.

“We expect a certain percentage of the population to develop that each year, even in the absence of the vaccine,” says Dr. Itskowitz, “However, if someone did actually receive the vaccine and developed this condition soon after, it is certainly something that requires further investigation.”

Vaccine maker AstraZeneca said in a statement: “A standard review has been triggered, leading to the voluntary pause of vaccination across all trials to allow an independent committee review of the safety data. This is a routine action.”

“If we can determine the patient received a placebo, then I think the vaccine trial could immediately restart,” Dr. Itskowitz says. “However, if the patient received the vaccine, then I think we need to start doing neurologic screenings on vaccine recipients to see if there’s any other signal that this vaccine is associated with a neurologic condition.”

This vaccine, also known as the Oxford vaccine, uses a weakened and typically harmless virus called adenovirus. It carries coronavirus genes to human cells to make an immune response against COVID-19. But sometimes adenovirus can set off its own immune response.

“In some ways, this event is what we hope to happen, in the sense that a clinical trial is designed to find problems with a vaccine before we deploy it in the general population,” Dr. Itskowitz said. “I think people who previously were hesitant to take any vaccine will likely use this information and amplify their own reaction even further. But I think if we can get the information out to the public with what the actual risk is, for example with transverse myelitis or any other neurologic complication, and understand that there are always going to be some risks associated with a vaccine. We have to balance that against the risk of COVID-19, and our hope that we can eradicate this disease in the next year.”

This is the second time the vaccine has been paused for transverse myelitis. With the prior episode, after a safety review, the trial resumed.

“If you look at the history of vaccine research in general, there are always stops and starts. It’s not usually a smooth road. But traditionally, it takes about 11 years for a vaccine to go from research and development to getting it out into the general population,” says Dr. Itskowitz.

In the U.S., 60 sites were participating in the trial, with the goal of enrolling 30,000 people.

Dr. Maria Simbra