A large global trial for a leading Covid-19 vaccine candidate has been put on pause after a participant experienced an unexplained illness.

The trial was for a vaccine developed by pharmaceutical company AstraZeneca and the University of Oxford reported Stat News, a health news website produced by the Boston Globe.

A Sep. 8 statement from the company said that its "standard review process was triggered".

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"We voluntarily paused vaccination to allow review of safety data by an independent committee."

Participant expected to recover

The illness was reported in the United Kingdom, though details surrounding it have not been revealed.

According to Stat News, the affected participant is expected to recover.

"In large trials, illnesses will happen by chance," read the statement. "We are working to expedite the review of the single event to minimise any potential impact on the trial timeline."

Stat News reported that clinical holds are not uncommon in vaccine development, and it is not known how long AstraZeneca's will last.

Nine vaccines in Phase Three trials

The company's vaccine is among nine that are currently in Phase Three of trials — the phase before vaccines are typically approved.

The vaccine had been in Phase Two/Three trials in England and India, and Phase Three trials in Brazil, South Africa and the United States, according to the New York Times.

It is the first Phase Three Covid-19 vaccine trial known to have been put on hold.

Reuters reported that shares of AstraZeneca fell more than eight per cent after news of the halt broke.

In contrast, shares of rival vaccine developers — such as Moderna and Pfizer — rose.

All three pharmaceutical companies were among the nine leading U.S. and European vaccine developers who pledged on Sep. 8 to "uphold the integrity of the scientific process".

According to Reuters, the "historic pledge" came after a rise in concerns that safety and efficacy standards might slip as the world rushed to find a solution to the Covid-19 pandemic.

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