Share on Pinterest
AstraZeneca is currently in phase 3 trials for its COVID-19 vaccine. Getty Images
  • Drugmaker AstraZeneca halted its global COVID-19 vaccine trial over a suspected adverse reaction, but experts say this type of pause is “not uncommon.”
  • According to reports, a trial participant developed symptoms associated with a condition called transverse myelitis.
  • Phase 3 trials enroll thousands of people to catch more rare side effects. But this increases the chance that a participant will have an unrelated illness while they’re enrolled.

All data and statistics are based on publicly available data at the time of publication. Some information may be out of date.

Drugmaker AstraZeneca, which is developing a COVID-19 vaccine in conjunction with the University of Oxford, halted its global phase 3 study on Tuesday after a participant at a site in the United Kingdom developed a suspected adverse reaction, STAT reports.

“Our standard review process triggered a pause to vaccination to allow review of safety data,” said a spokesperson for AstraZeneca in a statement to STAT.

The spokesperson added that this is a “routine action” that occurs during a clinical trial whenever a participant develops a “potentially unexplained illness.”

The company is also “working to expedite the review of the single event to minimize any potential impact on the trial timeline.”

In a follow-up statement, the company said it initiated the hold on the study voluntarily.

The drugmaker’s CEO, Pascal Soriot, said in a conference call that the study participant, a woman who received the candidate vaccine at a U.K. site, experienced neurological symptoms consistent with transverse myelitis, reports STAT.

This is an inflammatory condition that affects the spinal cord and can be triggered by viral infections.

Soriot added that the woman’s diagnosis hasn’t been confirmed, but that she’ll likely be released from the hospital soon.

He also confirmed that the woman had received the company’s COVID-19 vaccine and not the inactive placebo.

This is the second time that AstraZeneca has paused its trial in the United Kingdom because of neurological symptoms, according to Nature.

Earlier in the summer, another participant developed symptoms of transverse myelitis and was later diagnosed with an “unrelated neurological illness.”

Dr. Jon Andrus, adjunct professor of global vaccinology and vaccine policy at the Milken Institute School of Public Health of George Washington University, says the fact that AstraZeneca paused its trial shows that the “process is working.”

“When you have a flag and the responsible parties recognize that flag and pause to investigate it, that’s a good sign,” he said. “Hopefully it’ll be nothing, but it deserves attention.”

In an interview on “CBS This Morning,” Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said pausing a clinical trial over a single adverse event is “not uncommon.”

Many times the adverse event isn’t related to the drug or other treatment, he said, but occurs around the time that the participant was enrolled in the study.

But you can’t assume that, he added, which is why unexpected adverse events undergo a careful safety review.

“That’s the reason why you have various phases of trials, to determine if in fact these candidates are safe,” he said. “[Pausing a study is] really one of the safety valves that you have on clinical trials such as this.”

Researchers running all types of clinical trials, including smaller phase 1 and 2 clinical trials, monitor participants for negative side effects.

“Even in a phase 1 study, you may see a red flag that would alert you to stop and pause,” Andrus said.

In the phase 1 and 2 trials for AstraZeneca’s COVID-19 candidate vaccine, participants experienced only mild or moderate side effects, such as fever, chills, muscle aches, and headache.

All of these resolved quickly and none were life threatening, so the candidate vaccine moved on to the phase 3 trial.

Phase 3 trials, though, enroll thousands of people. This is needed to catch more rare side effects.

But it also increases the chance that a participant will have an unrelated illness while they’re enrolled in the study.

Many clinical trials have a data and safety monitoring board (DSMB), an independent group of experts who monitor data on the effectiveness and safety of a treatment throughout the trial.

How often a DSMB reviews the clinical trial data varies, depending on the size of the study, potential risks to participants, and other factors.

The work done by these groups goes largely unnoticed, because the media doesn’t usually report on clinical trials until the data is published. This has shifted with COVID-19 trials.

“The attention the media has given these new COVID-19 vaccines probably ramps up the accountability aspect of these trials,” Andrus said.

The pause of AstraZeneca’s vaccine trial comes a day after nine vaccine-makers released a joint pledge to listen to the science in their development of COVID-19 vaccines.

AstraZeneca is one of three companies whose candidate vaccines are in phase 3 trials in the United States.

Some experts think the vaccine-makers’ statement was an attempt to boost public confidence in the development and regulatory review of these vaccines — even as President Donald Trump continues to push for the approval of a vaccine by Election Day on Nov. 3.

Andrus thinks ongoing media coverage of the vaccine trials can help depoliticize the process.

“I think that this attention is good,” Andrus said. “The public should know [about these studies], because at the end of the day, it will be their confidence in the product that determines whether or not they decide to get vaccinated.”