Update:
NY Times is reporting that a vaccine trial participant "had received a diagnosis of transverse myelitis, an inflammatory syndrome that affects the spinal cord and is often sparked by viral infections."

According to Mayo Clinic website:

"Transverse myelitis interrupts the messages that the spinal cord nerves send throughout the body. This can cause pain, muscle weakness, paralysis, sensory problems, or bladder and bowel dysfunction... Most people recover at least partially. Those with severe attacks sometimes are left with major disabilities."

Although "immune system disorders that attack the body's tissues" are listed as a major cause, it's not known whether the vaccine was the trigger in this instance - hence the investigation.