AstraZeneca Pauses Vaccine Trial for Safety Review

The company halted late-stage trials of its coronavirus vaccine because of a serious suspected adverse reaction in a participant.

By Katherine J. Wu and Katie Thomas

The pharmaceutical company AstraZeneca halted large, late-stage global trials of its coronavirus vaccine on Tuesday because of a serious suspected adverse reaction in a participant, the company said. It is not yet known whether the reaction was directly caused by the company’s vaccine or was coincidental.

The pause, which was first reported by STAT, will allow AstraZeneca, a British-Swedish company, to conduct a safety review and investigate whether the vaccine caused the illness. How long the hold will last is unclear.

Drug companies are racing to complete a coronavirus vaccine that could bring an end to a pandemic that has already claimed more than 890,000 lives globally. AstraZeneca is a front-runner, with late-stage clinical trials underway around the world, and has said it hoped to have a vaccine ready before the end of the year. If the cause of the reaction turns out to be related to the vaccine, those efforts could be derailed.

Late-stage vaccine testing remains crucial, as large trials can turn up rare but serious side effects that would surface only if many thousands of people received a vaccine.

“This is the whole point of doing these Phase 2, Phase 3 trials,” said Dr. Phyllis Tien, an infectious disease physician at the University of California, San Francisco. “We need to assess safety, and we won’t know the efficacy part until much later. 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.”

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