Why AstraZeneca just paused its coronavirus vaccine trial

gloved woman drawing vaccine from vial
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Editor's Note: This story was updated at 5:30 p.m. E.D.T. to include new information on the serious event that paused the trial.

Clinical trials for the COVID-19 vaccine developed by AstraZeneca and Oxford University have been put on hold after a participant developed a suspected adverse reaction, Stat News first reported.

But don't worry; this is exactly how clinical trials are meant to work. The decision to pause the trial also doesn't mean the vaccine caused the adverse event; the side effect could have occurred by chance in someone who received the vaccine.

The pharmaceutical company AstraZeneca "voluntarily paused vaccination to allow review of safety data by an independent committee," a company spokesperson said in a statement. "This is a routine action that has to happen when there is a potentially unexplained illness in one of the trials." In large clinical trials with thousands of participants, "illnesses will happen by chance," but each illness must be independently reviewed to uncover whether the reaction was at all related to the vaccine, the spokesperson added.

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The participant — a woman in the U.K. — showed neurological symptoms often associated with transverse myelitis, a condition where the spinal cord becomes inflamed, AstraZeneca CEO Pascal Soriot told investors during a private call on Sep. 9, according to Stat News. These symptoms can include pain, muscle weakness, paralysis and bladder problems. Soriot confirmed that the patient did receive the real COVID-19 vaccine and not a placebo shot, but her diagnosis of transverse myelitis has yet to be confirmed and may or may not be linked to the vaccine itself. 

"This is the whole point of doing these phase 2, phase 3 trials," Dr. Phyllis Tien, an infectious disease physician at the University of California, San Francisco, told The Times. "I think halting the trial until the safety board can figure out whether or not this was directly related to the vaccine is a good idea."

The AstraZeneca trials were put on pause in July, as well, when a different patient developed neurological symptoms, Soriot said during the conference call, according to Stat News. However, that patient was later diagnosed with multiple sclerosis and the company determined that the illness was not related to the vaccination, he said. 

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AstraZeneca is one of nine drug companies to publicly pledge that they will not release a coronavirus vaccine without adequate safety and efficacy data from "large, high-quality clinical trials," The New York Times reported Sep. 8. In recent weeks, President Donald Trump has teased that a COVID-19 vaccine could be available before election day, raising concerns from scientists and health officials that a vaccine might be released without clear proof that it's safe and actually works. 

"I love the fact that the nine big vaccine manufacturers ... said they would not do anything premature," Dr. Judith Feinberg, the vice chairwoman for research in medicine at West Virginia University, told The Times. "I think there's enormous pressure to do something premature." 

Originally published on Live Science. 

Nicoletta Lanese
Channel Editor, Health

Nicoletta Lanese is the health channel editor at Live Science and was previously a news editor and staff writer at the site. She holds a graduate certificate in science communication from UC Santa Cruz and degrees in neuroscience and dance from the University of Florida. Her work has appeared in The Scientist, Science News, the Mercury News, Mongabay and Stanford Medicine Magazine, among other outlets. Based in NYC, she also remains heavily involved in dance and performs in local choreographers' work.