AstraZeneca temporarily halted late-stage studies for a COVID-19 vaccine candidate to determine whether the vaccine caused a “potentially unexplained” illness in one of the recipients.
The company said in a statement on the evening of Sept. 8 that its “standard review process triggered a pause to vaccination to allow review of safety data.” The statement did not provide further details. The health news site STAT reported that the pause was prompted by a potential side effect which occurred in the United Kingdom.
The pause applies to the studies AstraZeneca is running in the United States and other countries. The company began recruiting 30,000 participants in the United States in August. Thousands of people in Britain, Brazil, and South Africa are also part of the trial.
Moderna and Pfizer are also conducting large-scale final-stages trials in the United States for vaccines that work differently from AstraZeneca’s. Moderna and Pfizer have recruited roughly two-thirds of the needed participants.
President Donald Trump has suggested that at least one of the vaccines may be approved as soon as October.
The pause is part of a mandatory protocol built into the study, which requires that any serious or unexpected reaction be investigated. The company pointed out that illnesses of all sorts could arise in studies of thousands of people.
“We are working to expedite the review of the single event to minimize any potential impact on the trial timeline,” the company statement said.
It’s likely the unexplained illness was serious enough to require hospitalization and not a mild side effect such as fever or muscle pain, said Deborah Fuller, a University of Washington researcher who is working on a different COVID-19 vaccine that has not yet started human testing.
“This is not something to be alarmed about,” Fuller said. Instead, it’s reassuring that the company is pausing the study to figure out what’s happening and carefully monitoring the health of study participants, she added.
Angela Rasmussen, a virologist at Columbia University in New York, said on Twitter that the illness may be unrelated to the vaccine, “but the important part is that this is why we do trials before rolling out a vaccine to the general public.”
During the third and final stage of testing, researchers look for any signs of possible side effects that may have gone undetected in earlier patient research. Because of their large size, the studies are considered the most important study phase for picking up less common side effects and establishing safety.
The trials also assess effectiveness by tracking who gets sick and who doesn’t between patients getting the vaccine and those receiving a dummy shot.
Representatives for the FDA did not immediately respond to requests for comment Tuesday evening.
The Associated Press contributed to this report.