The National Institutes of Health has told Kaiser Health News that the agency is “very concerned” about the AstraZeneca’s COVID-19 trial that was put on pause after a volunteer became ill following the second dose.

“The highest levels of NIH are very concerned,” Avindra Nath, MD, told Kaiser Health. “Everyone’s hopes are on a vaccine, and if you have a major complication the whole thing could get derailed.” Dr. Nath is the intramural clinical director and a leader of viral research at the National Institute for Neurological Disorders and Stroke, an NIH division.

The pharmaceutical company has not released any more information about the woman who was ill, but news reports indicated that she may have developed transverse myelitis, a neurological disorder that occurs when the spinal cord is inflamed. The most common symptoms are weakness in the arms and legs, numbness or tingling, pain, and difficulty with bowel and bladder. However, the agency that oversees clinical trials in the United Kingdom allowed AstraZeneca to resume its trial there. Nature reported that the trial in Brazil has restarted; at this writing, it is unknown about the status of the trials in South Africa.

The FDA says it will not allow the US trial to resume until it has more information. Researchers have not yet obtained any tissue or blood samples from the affected volunteer, although FDA scientists could look at samples from US volunteers to check for antibodies that could attack the tissue around the brain and spinal cord.

Vaccine safety is a big concern, particularly given the push for a COVID-19 vaccine sooner than later. However, the AstraZeneca vaccine candidate is not the only one in a later stage trial, so if this particular vaccine is held up, there are many other trials in play. The Lancest is keeping track of them, Click here.