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Oxford vaccine trial on hold because of potential safety issue

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Clinical trials for the University of Oxford's COVID-19 vaccine have been put on hold, drug maker AstraZeneca said Tuesday. The decision to pause the trials comes after a study participant in the U.K. reportedly developed a spinal cord injury.

"Our standard review process was triggered and we voluntarily paused vaccination to allow review of safety data by an independent committee," AstraZeneca, which is developing the vaccine in partnership with the U.K.'s University of Oxford, said in a statement. "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."

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National Institutes of Health director Dr. Francis Collins addressed the AstraZeneca trial pause during a hearing Wednesday before the Senate Health, Education, Labor and Pensions committee.

"To have a clinical hold, as has been placed on AstraZeneca as of yesterday because of a single, serious adverse event, is not at all unprecedented," Collins said. "This certainly happens in any large-scale trial where you have tens of thousands of people invested in taking part, and some of them may get ill, and you always have to try to figure out is that because of the vaccine or were they going to get that illness anyway?"

"In large trials illnesses will happen by chance but must be independently reviewed to check this carefully," the AstraZeneca statement continued. "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 next steps are for the researchers to investigate carefully to see if anybody else who received the vaccine might have experienced a similar adverse reaction — in this particular case, "a spinal cord problem," Collins said. Oxford has not confirmed details of the patient's injury.

The clinical trial pause "ought to be reassuring to everybody listening when we say we are going to focus first on safety and make no compromises," Collins said. "Here is exhibit A of how that is happening in practice."

And if the investigation finds that the adverse event was in fact caused by the vaccine, "then all the doses that are currently being manufactured for that will be thrown away, because we do not want to issue something that is not safe," Collins said.

Related: Despite pressure from the White House, researchers may not be able to say whether the vaccine works until after the election.

AstraZeneca, which is working with the University of Oxford on a coronavirus vaccine, began its phase 3 clinical trials in the U.S. last week. NBC News has confirmed that the pause has affected trial sites in the U.S.

The trials were also halted in Brazil, according to a statement from the Federal University of São Paulo, which is coordinating the trials in the country.

Dr. Charlie Weller, Head of Vaccines Programme at the U.K.'s Wellcome Trust foundation, which is supporting the research, said safety was "the most important consideration when developing any vaccine, and it is right for the trial to be paused while an investigation takes place."

She said that it was "often a normal part of the process in vaccine trials, which involve tens of thousands of people," adding that vaccines were "among the most rigorously tested and monitored products we have in society, and the COVID-19 vaccines should be no different."

Putting a trial on hold while researchers determine whether a serious adverse event was caused by a vaccine is "uncommon, but not unheard of," Dr. Paul Offit, director of the Vaccine Education Center at Children's Hospital of Philadelphia, said.

"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."

"Often these events are coincidental, but these precautions are necessary to ensure the safety of the trial participants," Poland said.

"Presumably, we should hear more about what the problem was in a few days," said Offit, who is on the Food and Drug Administration's vaccine advisory committee.

The Oxford trial was the third phase 3 trial to begin in the U.S. Pfizer and Moderna began their phase 3 trials in late July and have already enrolled about 30,000 volunteers. Moderna said that it's not aware of any effect of the Oxford pause on its own phase 3 trial.

The Oxford vaccine uses a type of virus called an adenovirus to teach the immune system how to make antibodies to attack the coronavirus' so-called spike protein. It's that spike protein that allows the virus to infect human cells.

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