After a study participant in the United Kingdom fell ill, AstraZeneca said in a statement that it would review safety and efficacy data. Then the company released a second statement, saying it initiated the pause even though the study participant is expected to make a full recovery, according to STAT News.

Despite the pause, adverse reactions are very common in clinical trials as is pausing for a safety and efficacy review. It also does not mean the vaccine candidate is a failure. However, it does make it increasingly unlikely that the vaccine will be approved and ready for distribution by Election Day, as President Trump has often said a vaccine would be.

In a statement, according to The New York Times, AstraZeneca described the pause as 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 was not clear about how long the pause in the vaccine trial will last. The late-stage vaccine trial so far has included 17,000 people in Europe, the U.S., Brazil and South Africa, according to The Guardian. Phase 3 trials include a large number of study participants for exactly this reason; only by testing a large number of people do rare side-effects emerge.

"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, as The New York Times reported. "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."

While the exact condition that the study participant suffered has not been released, The New York Times is reporting that sources are saying the person received a diagnosis of transverse myelitis, an inflammatory syndrome that affects the spinal cord and is often sparked by viral infections. It's unclear if the diagnosis is directly related to the vaccine candidate or not.

Robert Booy, a University of Sydney professor of vaccinology, told The Guardian that if the sickness is unrelated to the vaccine candidate then the trial might be paused for just one week, but it may extend to one month. He said that the pause "is more likely the product of being super careful" rather than a problem with the vaccine.

"This could have just been a high fever and it turns out they've got appendicitis," he said. "My bet would be that they find something in this participant that is not causal [from the vaccine]. It'll take them about one to two weeks to ascertain this, and then they will probably restart the trial in a month."

AstraZeneca said in a statement that it was "working to expedite the review of the single event to minimize any potential impact on the trial timeline," and did not waver from its commitment "to the safety of our participants and the highest standards of conduct in our trials," as The New York Times reported.

AstraZeneca was one of nine companies that signed a pledge Tuesday to assure the public that a vaccine would not be rushed due to politics. In a rare move, nine companies working on a vaccine made a joint pledge to "stand with science" and insisted that they would not be rolled out prematurely without proving beyond a doubt that they are safe and effective, according to The New York Times.