AstraZeneca has "voluntarily paused" late-stage trials of the highly-anticipated Covid-19 vaccine it is developing with the University of Oxford after one of the study volunteers developed an unexplained illness, the company said on Wednesday, September 9, according to Al Jazeera.

The AstraZeneca-Oxford vaccine is seen as one of the most promising of the vaccines against coronavirus that are currently under development.

"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," said Michele Meixell, the company's spokeswoman.

"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 that in large trials, illnesses would sometimes happen by chance but the issue had to be reviewed independently.

The drug is in trials involving thousands of people in the United States, Brazil, South Africa and the UK where the patient fell ill.