LONDON: Clinical trials on one of the most advanced experimental Covid-19 vaccines, which is being developed by pharmaceutical company AstraZeneca and Oxford University, were “paused” Tuesday after a volunteer developed an unexplained illness.

With billions of people around the world still suffering with the fallout of the pandemic and the global death toll nearing 900,000, a worldwide race for a vaccine is underway, with nine companies already in late-stage Phase 3 trials.

Worldwide infections to date now stand at more than 27 million, and more than 890,000 people have died from the disease.

Russia has already approved a vaccine, and research published in The Lancet medical journal last week said patients involved in early tests developed antibodies with “no serious adverse events”, although scientists cautioned the trials were too small.

A spokesperson for the AstraZeneca vaccine said in a statement “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 said that in large-scale trials, illnesses will sometimes happen by chance, but must be reviewed independently.

AstraZeneca did not offer further details, but David Lo, a professor of biomedical sciences at the University of California, Riverside, said the pause may not necessarily be a setback.

“Probably right now it’s just being cautious — it’s a pause, it’s not the same thing as saying, ‘We can’t move forward’,” said Lo.

“In some ways I’m quite relieved, it means they’re really paying attention.”

The volunteer may have experienced an adverse reaction already seen in earlier patients such as fever and soreness, but in a more severe form, Lo added.