Here's What We Know So Far About the AstraZeneca Vaccine Trial Halt

An AstraZeneca building photographed Aug. 19, 2020, in Sydney, Australia.
An AstraZeneca building photographed Aug. 19, 2020, in Sydney, Australia.
Photo: Lisa Maree Williams (Getty Images)

Pharmaceutical company AstraZeneca formally announced on Wednesday that its Phase III trial in the UK testing out a vaccine for the coronavirus that causes covid-19 was put on pause, following a report of a serious adverse event among its participants. More details concerning the report have since emerged, but there are still many unanswered questions about whether this incident is truly connected to the vaccine.


On Tuesday night, STAT News reported that the trial was suddenly put on hold due to the adverse event, though it wasn’t immediately clear what exactly had happened. By late Tuesday, other outlets reported that the event involved a UK woman in the treatment group who is thought to have developed transverse myelitis, a neurological condition characterized by inflammation of the spinal cord. On Wednesday afternoon, STAT News reported that AstraZeneca CEO Pascal Soriot told investors earlier in the morning that the woman had been hospitalized with serious neurological symptoms, but she was already recovering and could be discharged as early as today.

In its statement addressing the hold, the company said that “a standard review process has been triggered, leading to the voluntary pause of vaccination across all trials to allow an independent committee to review the safety data of a single event of an unexplained illness that occurred in the UK Phase III trial.”

Transverse myelitis is a rare condition that affects motor and sensory functions regulated by both sides of the spinal cord, though the symptoms associated with it aren’t always evenly distributed on both sides of the body. These symptoms include pain and weakness of the limbs, trouble controlling your bowels and bladder, and sensations of numbness or tingling. The majority of the time, there’s no clear cause for transverse myelitis. But it’s often been linked to a wide range of infectious diseases, such as Zika, West Nile, and Lyme disease.

Given what we know about the condition, it’s certainly plausible the woman’s symptoms could have been caused by the AstraZeneca vaccine, which is being developed in conjunction with the University of Oxford. But right now, it’s simply too early to know. Though adverse events reported during a clinical trial are often seen as synonymous with a treatment’s “side effects,” the term is more limited than that. It just means that these symptoms happened while someone was taking the treatment—they may or may not be related to the treatment. It’s only after further investigation, typically by scientists not directly involved in carrying out the research, that we can more clearly tie an adverse event to the drug or medical device being tested.

According to STAT News, this isn’t the first time that a potential red flag has shown up during one of AstraZeneca’s trials for a coronavirus vaccine. During the investor call, Soriot reportedly revealed that a trial was put on hold sometime in July after a male participant began developing neurological symptoms. The man was eventually diagnosed with multiple sclerosis, and his case was deemed to be unconnected to the vaccine, but no single virus has emerged as a clear cause. Transverse myelitis is sometimes the first sign of what’s later recognized as MS.

A spokesperson for AstraZeneca told Gizmodo via email that a final diagnosis for the woman has not yet been made and that more tests will be carried out. An independent safety committee will review these tests and determine her final diagnosis and whether it is connected to the vaccine. This same independent panel reviewed the earlier adverse event and ruled that it was unrelated to the vaccine, according to AstraZeneca.


Demoralizing as this news may be, it’s also an indication that the clinical trial process is working as intended. That this possible complication was noticed seemingly almost immediately after it happened is a good sign that researchers are keeping a close eye on volunteers. If this illness does turn out to be related to the vaccine, that’s also something for researchers working on other covid-19 vaccines—especially those similarly based on using a neutered adenovirus as the delivery method for the vaccine dose—to look out for in their studies.

If this does lead to the failure of the AstraZeneca vaccine, that wouldn’t be a surprise, either. Unfortunately, clinical research is rarely a smooth ride, and there’s a wide margin of failure for potential treatments, even for those that make it to Phase III, the last stage needed to secure approval by government agencies like the U.S. Food and Drug Administration. Of course, fingers crossed that this isn’t the case.


As for the woman at the center of it all, the prognosis for transverse myelitis can vary widely. People do recover to an extent within the first three months, though two-thirds are still left with moderate to severe lingering symptoms. The faster someone is diagnosed and receives supportive treatment such as steroids and physical therapy, the better their outcome is generally thought to be.

This article has been updated with comments from AstraZeneca.


Born and raised in NYC, Ed covers public health, disease, and weird animal science for Gizmodo. He has previously reported for the Atlantic, Vice, Pacific Standard, and Undark Magazine.



No surprise. There are going to be many failures. That’s how it goes. At least they were smart enough to halt it.