AstraZeneca has temporarily put the brakes on its late-stage, global COVID-19 vaccine trial after a participant came down with what experts call an "adverse event."
"Whilst it has been reported that one trial volunteer has become ill, this may be due to an issue related to the vaccine. It also may not," a spokesperson from Oxford University told ABC News this morning.
The promising vaccine candidate created by researchers at Oxford had been marching through rounds of clinical trials before word of the voluntary pause came down Tuesday night.
Trial participants in the U.S had started injections just last week.
Specialists ABC News spoke with said they welcomed the pause.
"It is very reassuring that in this political era, with very significant political pressure on the pace of vaccine development, that the regular order here prevailed," Ruth Faden, M.P.H., Ph.D., founder of the Johns Hopkins Berman Institute of Bioethics, told ABC News.
"This is an example of how the conduct of clinical trials is supposed to work," said Anna P. Durbin, M.D., a vaccine researcher and Professor at John Hopkins Bloomberg School of Public Health.
"In pausing the trial, AstraZeneca is taking the time to thoroughly review the event and in doing so, ensuring the safety of the product is of highest priority," Durbin told ABC News.
Experts say that in large trials like this, illnesses occur in some participants by chance. The trial must then be halted to allow for further investigation into whether or not the vaccine is related to any serious adverse event.
"An adverse event is defined as any untoward reaction that occurs following administration of a vaccine, and it may or may not be related to that vaccine," said Durbin.
Adverse events may be anything from redness and soreness at the injection site to more serious complications. What's important to the clinical trial is to see if it has to do with the vaccine.
STAT News reported Wednesday morning that AstraZeneca CEO Pascal Soriot told investors that the adverse event involves a woman in the United Kingdom who was injected with the vaccine as part of the trial, and has symptoms consistent with a rare neurological disease called transverse myelitis.
An AstraZeneca spokesperson said, in part, "There is no final diagnosis and there won’t be one until more test are carried out."
This is the second time that AstraZeneca's trial has been halted. In July, the trial was also paused for possible safety concerns, but the issue was resolved quickly and the trial resumed shortly thereafter.
Pausing the trial again for investigation "is absolutely not unheard of," Durbin said.
"It is important to note that this is a normal process in vaccine studies. This is how the safety review is supposed to work," she said.
The Data and Safety Monitoring Board, made up of independent reviewers who are experts in the field of vaccine research, will be helping with the investigation.
To determine whether this case of suspected transverse myelitis is related to the AstraZeneca vaccine, investigators may look into the timing between vaccine administration and the onset of neurologic symptoms. The results may take days or weeks depending on lab results.
They may also review the frequency of similar adverse events in other trial participants, and look at any previous reports of transverse myelitis with other vaccines.
"Vaccines have been a reported cause of transverse myelitis, but it has never been definitively proven," Jonathan E. Howard, M.D., a neuroimmunologist at NYU Langone Health and chief of neurology at New York City's Bellevue Hospital, told ABC News.
Transverse myelitis is an inflammatory disease of the spinal cord caused by an autoimmune process whereby the body attacks its own nervous system tissue. This autoimmune process may be triggered by infection, underlying autoimmune disease, or unknown causes.
Viruses including the flu, HIV, herpes, and others are known culprits. Bacterial infections such as Lyme disease and syphilis are also associated with it. In some cases, transverse myelitis is associated with underlying autoimmune disorders like multiple sclerosis or lupus.
"There's a lot more information that needs to be reviewed with this adverse event," Durbin said. "You have to look at all the data in total and evaluate that against the risk of the disease you are trying to prevent."
In an email to ABC News, Dr. Paul Goepfert, professor of medicine and director of the Alabama Vaccine Research Clinic, said that the implications of this investigation could be serious for the future of the vaccine program.
"If transverse myelitis is found to be due to the vaccine, it certainly could be problematic," he said.
For now, the delay in the AstraZeneca trial doesn't appear to have any ripple effects in the other trials for a safe and effective vaccine.
An official with Moderna, whose vaccine is one of the three candidates currently in large-scale, late-stage trials in the United States, told ABC News, "While Moderna has no comment on the AstraZeneca trial, we can confirm that we are not aware of any impact on the Moderna COVE study at this time."
Pfizer officials told ABC News they are on schedule and that they continually monitor and evaluate the safety of their participants, including evaluation by an independent data monitoring committee (DMC) made up of vaccine safety experts.
"To date, no safety signal has been identified, and the DMC has recently recommended continuing the trial as planned," the official said.
Durbin said it is good clinical practice to thoroughly investigate the safety of a vaccine before it is given to more people.
"In this atmosphere, I commend [AstraZeneca] for pausing the study," she said.
"There is no shortcuts when it comes to safety when we’re developing these vaccines," CDC Director Robert Redfield said Wednesday during a Research America's 2020 Virtual National Health Research Forum discussion.
"I think for the American public, what I'd like for them to take away from this -- and the global public -- is that safety is paramount," he added. "That these vaccines are going to be developed, and they're going to be evaluated critically for safety, and if there's a safety issue, we're going to basically slow down and take a look at this to make sure that we stay in that climate that safety is the number one objective of all these initiatives."
Leah Croll, M.D., is a neurology resident at NYU Langone Health and a contributor to the ABC News Medical Unit.