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. It is not yet known whether the reaction was directly caused by the company’s vaccine or was coincidental.
The pause will allow AstraZeneca 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.