News broke earlier this week that AstraZeneca, a pharmaceutical company running one of the leading COVID-19 vaccine trials, had stopped their work because of possible medical complications in a participant. The health news site STAT reported this morning that the participant who got sick was a woman in the United Kingdom. Although it wasn’t officially confirmed, she appears to have a condition called transverse myelitis. STAT reports she is likely to be discharged from the hospital soon.

Myelin is a substance that covers nerve fibers, the body’s version of fiber-optic cables, that carry signals around the body. Transverse myelitis occurs when the spinal cord becomes inflamed. According to The National Institute of Health, transverse myelitis has many causes, including infections from bacteria and viruses as well as immune disorders.

In some cases, transverse myelitis can be a rare reaction to vaccination. But, there many other explanations.

This has not been the first pause in the AstraZeneca vaccine trials. They were halted once before in July when another participant became ill. That person was eventually diagnosed with multiple sclerosis, a degenerative disease unlinked to the vaccine.

As to this case of transverse myelitis, more investigation will be necessary before the trials can start again.