The British-Swedish pharmaceutical firm AstraZeneca says it has suspended trials of the Covid-19 vaccine it is developing with the University of Oxford after one of the trials’ volunteers fell ill with an unexplained illness.

The company is the frontrunner in the race to develop Covid-19 vaccines.

The pause on its clinical trial could delay the prospects to get a vaccine.

AstraZeneca said in a statement that the company’s "standard review process triggered a pause to vaccination to allow review of safety data.”

"This is a routine action which has to happen whenever there is a potentially unexplained illness in one of the trials,” the company said in a statement.

The nature of the adverse reaction and when it happened were not immediately known, though the participant is expected to recover, according to Stat News which first reported the news.

AstraZeneca’s statement said that "in large trials, illnesses will happen by chance but must be independently reviewed to check this carefully.”

Trials of the vaccine, called AZD1222, are underway at different stages in Britain, the United States, Brazil, South Africa and India. Trials are also planned in Japan and Russia.

Clinical holds are not unusual, and it’s not known how long AstraZeneca’s might last. However, the progress of the company is being closely monitored given the persistent need for new ways to curtail the global pandemic.

There are currently nine vaccine candidates in Phase 3 trials. AstraZeneca’s is the first Phase 3 Covid-19 vaccine trial known to have been put on hold.

AstraZeneca only began its Phase 3 trial in the U.S. in late August.

The U.S. trial is currently taking place at 62 sites across the country, according to clinicaltrials.gov, a government registry, though some have not yet started enrolling participants.