As AstraZeneca pauses COVID-19 vaccine trial over 'serious adverse reaction,' expert says the 'system is working as designed'

Abby Haglage
·7 mins read

Hopes that a COVID-19 vaccine may be imminent were dimmed on Tuesday when news broke that a “serious adverse reaction” in one of the participants of the AstraZeneca trial has triggered a pause in testing to conduct a safety review. The company’s vaccine, created in conjunction with Oxford University, began phase III trials this summer and has been considered one of the frontrunners in the race for a viable candidate.

In a statement to Stat, which broke the news, AstraZeneca characterized the review as standard. “This is a routine action which has to happen whenever there is a potentially unexplained illness in one of the trials ... ensuring we maintain the integrity of the trials,” the company wrote. “In large trials, illnesses will happen by chance but must be independently reviewed to check this carefully.”

A volunteer receives an injection at the Chris Hani Baragwanath Hospital in Soweto, in Johannesburg, South Africa. This is part of Africa's first participation in a COVID-19 vaccine trial developed at Oxford in conjunction with the pharmaceutical company AstraZeneca. (Siphiwe Sibeko/Pool via AP)
A volunteer receives an injection at the Chris Hani Baragwanath Hospital in Soweto, in Johannesburg, South Africa. This is part of Africa's first participation in a COVID-19 vaccine trial developed at Oxford in conjunction with the pharmaceutical company AstraZeneca. (Siphiwe Sibeko/Pool via AP)

Dr. Gregory Poland, a professor of medicine and infectious diseases at the Mayo Clinic and director of its Vaccine Research Group confirms that a safety review of this kind is not abnormal. “In any large trials there are always — not usually, always — things like this that come up,” Poland tells Yahoo Life. “This one just happens to be under a microscope.”

Still, with more than 6 million coronavirus cases in the U.S., any negative news about a vaccine — which many have already expressed resistance to getting — is a reason for further exploration. To help sort out what the news means, here’s what you need to know.

Multiple sources have suggested the adverse reaction was transverse myelitis

The New York Times and others have reported that the illness in question is likely a case of transverse myelitis, an inflammation of the spinal cord that causes symptoms such as lower back pain, weakness in the arms and legs, difficulty walking, incontinence and digestive problems. According to the National Institutes of Health, roughly 1,400 cases of it are diagnosed each year.

Experts are still unsure exactly what causes most cases of transverse myelitis, but some have been linked to autoimmune disorders like multiple sclerosis and lupus as well as viral and bacterial infections. One multi-analysis from 2009 found that vaccines “may be associated with autoimmune phenomena such as traverse myelitis,” but the Johns Hopkins Bloomberg School of Health contends that vaccines “have not been shown to cause transverse myelitis.” Poland says he is aware of instances where the polio vaccine has been linked to the condition but adds that the vaccine also prevents the illness. In either case, he says the COVID-19 vaccine candidates, which use RNA and not active virus, would likely be unable to trigger it. “Transverse myelitis due to vaccines would only occur with live attenuated vaccines — and even in that case is exceptionally rare.”

Cases of transverse myelitis have been reported in the wake of COVID-19 infections in some individuals, but it’s unclear whether the patient had contracted the virus previously. Most with transverse myelitis experience at least a partial recovery, and one-third experience a full recovery, although it can take months to years.

Stat shared another update on Wednesday, saying that the case reportedly involves a woman in the U.K. who had neurological symptoms “consistent with a rare but serious spinal inflammatory disorder called transverse myelitis.” The woman is said to be recovering and may be released from the hospital “as early as Wednesday.” AstraZeneca did not respond to Yahoo Life’s request for confirmation.

There are several possible explanations that do not involve the vaccine

Poland explains that when individuals enroll in a clinical trial for a vaccine, their health is tightly monitored and any change must be reported. Major developments like this one must be investigated by what’s called a Data and Safety Monitoring Board (DSMB). These individuals, separate from the company, are tasked with making an unbiased determination as to whether a reaction appears linked to the vaccine.

As a vaccinologist himself, Poland has experienced many safety reviews and says that often the reaction can turn out to be unrelated. “I ran a clinical trial once, gave the person their vaccine, and they left the office and got in a bad car accident. That's considered a serious adverse event,” says Poland. “You then do the detective work to say, ‘Well, wait, it doesn't seem like it would be related, but did the patient faint? Is there a link?’ And this case, no, it was unrelated.”

Poland says the DSMB will look at things like family history, as well as review baseline tests to see if the patient was already developing an autoimmune disease before the study began. Either one may allow the DSMB to rule out a connection. The DSMB will also be able to tell if the patient was in the placebo group or not, which the company (to keep it a blind study) is unable to confirm. If the patient received the placebo vaccine, the trial will continue unchanged.

The review process is quick but a sign that the “system is working as designed”

Although a pause in the phase III trials seems like a major detour, Poland says that the process is generally fast. “It typically takes days — but depending on what the adverse event was, it could take a week or two,” he says. After reviewing the details of the case, the DSMB will make a decision on whether the trial can continue.

Poland says Americans should feel encouraged by the thoroughness and transparency of this process. “I think it should build confidence in the public's mind. The system is working as designed,” he says. “Where I would have been concerned is if they rushed past it and didn't pause it for the sake of speed.”

Despite this, Poland is concerned that it may affect enrollment for trials

It remains to be seen whether the case — which Stat reported Wednesday is apparently mild — will be linked to the COVID-19 vaccine or not. But either way, Poland is worried that the safety review itself will further impede efforts to find the roughly 30,000 volunteers required for each phase III clinical trial. “My fear is actually that it will instill fear rather than confidence in the public and that it could cause a slowdown in enrollment in all of the vaccine trials,” says Poland. “And at this point, I think a reaction like that would be unwarranted.”

He emphasizes that the safety trials are a part of a larger system designed to protect the public and that the vaccine itself is critical to regaining normalcy in the U.S. — and elsewhere. “We have to continue these trials in order to know what vaccine or vaccines are going to turn out to be safe and effective,” he says. “Our only way out of this mid- and long-term is a vaccine.”

For the latest coronavirus news and updates, follow along at According to experts, people over 60 and those who are immunocompromised continue to be the most at risk. If you have questions, please reference the CDC’s and WHO’s resource guides.

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