During a COVID-19 vaccine trial, a previously healthy 37-year-old woman in the U.K. reportedly experienced a rare neurological condition that left her hospitalized, an internal document obtained by CNN said.

The internal safety report by pharmaceutical powerhouse AstraZeneca explains how during the trial the study volunteer "experienced confirmed transverse myelitis,” which is a disorder caused by inflammation of the spinal cord.

The document details symptoms the woman experienced 14 days after receiving her second dose of the vaccine, like difficulty walking and pain and weakness. She was hospitalized on Sept. 5.

Media reports of the confirmed case of rare condition were initially dismissed by AstraZeneca in the days following her hospitalization. Last week, however, the company paused vaccine trials worldwide to review the study volunteer’s “unexplained illness.” Trials resumed in the U.K. Saturday.

The participant’s illness in the U.K. trial comes as Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said it is "just a matter of time" before trials resume in the U.S.

The lead infectious disease expert said the U.K. participant’s rare illness could be a "one-off" and that the symptoms in the case are something U.S. doctors will look for during trials.

"You have to be extra special careful and watch out to see if it happens again, and then if it does, it becomes an entirely different situation," Fauci said.

Astrazeneca's partnership with the University of Oxford is one of the COVID-19 vaccine development efforts furthest along, with President Donald Trump’s administration investing $1.2 billion in the efforts thus far. Trump has continued to express optimism that a vaccine will be available this fall, with some experts saying a vaccine will only be more available to all in the U.S. later in 2021.