The drug company AstraZeneca halted global trials of its coronavirus vaccine on Tuesday as a result of a significant and unexpected adverse reaction in a participant, the company said.

The trial’s halt, which was first reported by Stat News, will allow the British-Swedish company to conduct a security review. How long the hold will last is unclear.
In a statement, the company described the halt as a “routine action which has got to happen whenever there’s a potentially unexplained illness in one in all the trials, while it’s investigated, ensuring we maintain the integrity of the trials.”

In large trials just like the ones AstraZeneca is overseeing, the company said, participants do sometimes become sick accidentally, but such illnesses “must be independently reviewed to see this carefully.”

The company said it was “working to expedite the review of the only event to attenuate any potential impact on the trial timeline” which it had been “committed to the security of our participants and therefore the highest standards of conduct in our trials.”

A person acquainted with the situation, and who spoke on the condition of anonymity, said that the participant had been enrolled during a Phase 2/3 trial based within the united kingdom. The individual also said that a volunteer within the U.K. trial had been found to possess transverse myelitis, an inflammatory syndrome that affects the medulla spinalis and is usually sparked by viral infections. However, the timing of this diagnosis, and whether it had been directly linked to AstraZeneca’s vaccine, is unclear.

AstraZeneca’s vaccine, referred to as AZD1222, relies on a chimpanzee adenovirus that has been modified to hold coronavirus genes and deliver them into human cells. Although the adenovirus is mostly thought to be harmless, the coronavirus components of the vaccine are intended to incite a protective immunologic response that might be roused again should the particular coronavirus attempt to infect a vaccinated individual.

Adenoviruses, however, can sometimes trigger their own immune responses, which could harm the patient without generating the intended sort of protection.
AstraZeneca’s vaccine is currently in Phase 2/3 trials in England and India, and in Phase 3 trials in Brazil, South Africa, and quite 60 sites within the U.S. The company intended for its U.S. enrollment to succeed in 30,000.

AstraZeneca is one of three companies whose vaccines are in late-stage clinical trials within the U.S.