Late-stage studies of AstraZeneca's Covid-19 vaccine candidate are on temporary hold while the company investigates if a report of a patient with a serious side effect is linked to the shot.

In a statement issued on Tuesday evening, the company said its "standard review process triggered a pause to vaccination to allow review of safety data".

AstraZeneca did not reveal any information about the possible side effect except to call it "a potentially unexplained illness".

The news site Stat first reported the pause in testing, saying the possible side effect occurred in the United Kingdom.

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An AstraZeneca spokesperson confirmed the pause in vaccinations covers studies in the US and other countries.

Late last month, AstraZeneca began recruiting 30,000 people in the US for its largest study of the vaccine.

It also is testing the vaccine, developed by Oxford University, in thousands of people in Britain, and in smaller studies in Brazil and South Africa.

Two other vaccines are in huge, final-stage tests in the United States, one made by Moderna Inc and the other by Pfizer and Germany's BioNTech.

Those two vaccines work differently than AstraZeneca's, and the studies already have recruited about two-thirds of the needed volunteers.

Temporary holds of large medical studies are not unusual, and investigating any serious or unexpected reaction is a mandatory part of safety testing.

AstraZeneca pointed out that it is possible the problem could be a coincidence; illnesses of all sorts could arise in studies of thousands of people.

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

The development came the same day that AstraZeneca and eight other drugmakers issued an unusual pledge, vowing to uphold the highest ethical and scientific standards in developing their vaccines.

The announcement follows worries that President Donald Trump will pressure the US Food and Drug Administration to approve a vaccine before it is proven to be safe and effective.

The US has invested billions of dollars in efforts to quickly develop multiple vaccines against Covid-19.

The UK Government also placed an order for 90 million doses in July following the results of initial human trials which showed T cell and antibody responses without significant adverse effects.