On Tuesday, drug maker AstraZeneca said clinical trials for Oxford’s experimental coronavirus vaccine have been put on hold due to safety concerns.

The British-Swedish multinational pharmaceutical company said in a statement, “Our standard review process was triggered and we voluntarily paused vaccination to allow review of safety data by an independent committee.”

“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,” the company added.

AstraZeneca continued, “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.”

AstraZeneca and Oxford University have been working on their coronavirus vaccine candidate, which began its Phase III clinical trials in the United States last week, which is the late-stage trials before approval.

The trial sites in the United States have been affected by the hold, according to NBC News.

Moderna and Pfizer began their Phase III trials in late July and have already enrolled nearly 30,000 volunteers.

The AstraZeneca vaccine candidate is based on a type of virus known as adenovirus to help the immune system to make antibodies to attack the virus’ so-called spike protein, a type of protein that allows the virus to invade and infect the human cells. Globally, the virus has affected more than 27 million and killed over 901,000 people. The United States officials have reported over 6.5 million confirmed cases, with more than 194,000 deaths.