Pharmaceutical company AstraZeneca said Tuesday it had "voluntarily paused" a randomized clinical trial of its coronavirus vaccine in what it called a routine action after a volunteer developed an unexplained illness.

The company, which is developing the drug alongside the University of Oxford, is a frontrunner in the global race for a Covid-19 vaccine. 

"As part of the ongoing randomized, controlled global trials of the Oxford coronavirus vaccine, our standard review process was triggered and we voluntarily paused vaccination to allow review of safety data by an independent committee," a spokesperson said.

"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."

It added that in large trials, illnesses will sometimes happen by chance but must be reviewed independently.

"We are working to expedite the review of the single event to minimise any potential impact on the trial timeline," the spokesperson added.

It was not immediately clear where the patient was, or the nature and severity of their illness.

Holds during clinical trials are not uncommon, but this is thought to be the first time it has happened for a Covid-19 vaccine trial.

AstraZeneca is one of nine companies currently in late-stage Phase 3 trials for their vaccine candidates.

In the US, the company began enrolling 30,000 volunteers across dozens of sites on August 31. 

The vaccine, called AZD1222, uses a weakened version of a common cold causing adenovirus that has been engineered to code for the spike protein that the novel coronavirus uses to invade cells.

After vaccination, this protein is produced inside the human body, which primes the immune system to attack the coronavirus if the person is later infected.