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AstraZeneca halts phase 3 trials of Covid-19 vaccine after participant develops adverse reactions

The company said that it is a routine action that is taken whenever there is a potentially unexplained illness in one of the trials

AstraZeneca and its partner Oxford University have temporarily halted the phase 3 testing of their potential Covid-19 vaccine after a participant reported adverse reactions. The reasons behind the illness are unclear at the moment.

It is one of nine vaccine candidates that entered the final stage of Covid-19 vaccine testing. The goal is to test the vaccine safety and efficacy: if it can keep participants from developing the disease. The company said that a standard review process triggered the pause on vaccination, prompting them to get an independent committee to review the data.

"This is a routine action which has to happen whenever there is a potentially unexplained illness in one of the trials, while it is investigated, ensuring we maintain the integrity of the trials," AstraZeneca said in a statement.

"In large trials, illnesses will happen by chance but must be independently reviewed to check this carefully. We are working to expedite the review of the single event to minimize any potential impact on the trial timeline. We are committed to the safety of our participants and the highest standards of conduct in our trials," the company said in a statement to CNN.

"We are working to expedite the review of the single event to minimize any potential impact on the trial timeline. We are committed to the safety of our participants and the highest standards of conduct in our trials," the company added.

Recently, Astranzeca and eight other companies signed a pledge to not seek approval or emergency use authorization until their vaccine candidates have been thoroughly vetted for safety and efficacy. "We believe this pledge will help ensure public confidence in the rigorous scientific and regulatory process by which Covid-19 vaccines are evaluated and may ultimately be approved," they said in a statement.

 AstraZeneca's vaccine candidate is a part of the US government's initiative called Operation Warp Speed. The company received $1.2 billion in funding from the US in May. The company began phase 3 trials in late August after showing encouraging results from preliminary tests. During testing, some participants can report adverse reactions that would require emergency room visits and hospitalization.

"Serious reactions do occur in vaccine trials," Dr Gregory Poland, an infectious diseases expert and director of the Mayo Clinic's Vaccine Research Group in Rochester, Minnesota, said in an email to NBC News. "Generally, when these events occur, trials are paused, data collected, and an independent data monitoring and safety board reviews the details to make a determination whether to resume the trial or alter it in some way." He added that these events are often coincidental, but precautions are necessary to ensure the safety of the volunteers.

AstraZeneca's vaccine candidate contains a weakened version of a type of virus called adenovirus, which causes the common cold in chimpanzees. It is combined with a coronavirus protein. The vaccine — dubbed ChAdOx1 nCoV-19 — is designed to only activate the body's defenses to fight infection and is incapable of growing or infecting human cells.