New York: Late-stage studies of AstraZeneca's COVID-19 vaccine candidate are on temporary hold while the company investigates whether a recipient's "potentially unexplained" illness is a side effect of 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 didn't reveal any information about the possible side effect except to call it "a potentially unexplained illness".

The health news site STAT first reported the pause in testing, saying the possible side effect occurred in the United Kingdom. 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 aren't unusual, and investigating any serious or unexpected reaction is a mandatory part of safety testing. AstraZeneca pointed out that it's possible the problem could be a coincidence; illnesses of all sorts could arise in studies of thousands of people.

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

It's likely the unexplained illness was serious enough to require hospitalization and not a mild side effect such as fever or muscle pain, said Deborah Fuller, a University of Washington researcher who is working on a different COVID-19 vaccine that has not yet started human testing.

"This is not something to be alarmed about," Fuller said. Instead, it's reassuring that the company is pausing the study to figure out what's happening and carefully monitoring the health of study participants.

Dr Ashish Jha of Brown University said via Twitter that the significance of the interruption was unclear but that he was "still optimistic" that an effective vaccine will be found in the coming months.

"But optimism isn't evidence," he wrote. "Let's let science drive this process."

Angela Rasmussen, a virologist at Columbia University in New York, tweeted that the illness may be unrelated to the vaccine, "but the important part is that this is why we do trials before rolling out a vaccine to the general public".

During the third and final stage of testing, researchers look for any signs of possible side effects that may have gone undetected in earlier patient research. Because of their large size, the studies are considered the most important study phase for picking up less common side effects and establishing safety.

The trials also assess effectiveness by tracking who gets sick and who doesn't between patients getting the vaccine and those receiving a dummy shot.

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's proven to be safe and effective.

The US has invested billions of dollars in efforts to quickly develop multiple vaccines against COVID-19. But public fears that a vaccine is unsafe or ineffective could be disastrous, derailing the effort to vaccinate millions of Americans.

Representatives for the FDA did not immediately respond to requests for comment Tuesday evening.

AstraZeneca's US-traded shares fell more than 6 per cent in after-hours trading following reports of the trial being paused.

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Abu Dhabi (PTI): Kolkata Knight Riders splurged a record-breaking Rs 25.20 to land top Australian all-rounder Cameron Green even as Indian stars Prithvi Shaw and Sarfaraz Khan went unsold in the Indian Premier League players' auction here on Tuesday.

Green surpassed compatriot Mitchell Starc (Rs 24.75 crore) to become the most expensive overseas player at an IPL auction. This was after Kolkata Knight Riders and Chennai Super Kings were involved in an intense bidding war for him before the latter emerged winner.

KKR also went after Venkatesh Iyer before pulling out of the race against Royal Challengers Bengaluru, who fetched the services of the India all-rounder for Rs 7 crore.

As far as Green is concerned, his salary for the season would still be Rs 18 crore (USD 1.9 million) as the rest of the amount will go towards the BCCI's player development programme as per the rules of the auction for foreign players.

Green, who previously turned up for Mumbai Indians and Royal Challengers Bengaluru, has so far played 29 matches in the IPL to aggregate 707 runs and take 16 wickets.

Shaw, however, went unsold despite his fine run of form in the domestic circuit lately, and so was the case with Sarfaraz, who smashed a 22-ball 73 in a Syed Mushtaq Ali Trophy match for Mumbai on Tuesday.

Big-hitting South African batter David Miller was bought by Delhi Capitals for his base price of Rs 2 crore, but New Zealand swashbuckler Devon Conway, whose base price was also Rs 2 crore, went unsold in the auction.

Seasoned South African opener Quinton de Kock returned to his old base Mumbai Indians for a base price of Rs 1 crore.

A total of 359 players -- 246 Indians and 113 overseas players -- are part of the mini auction pool with the 10 franchises bidding to fill up a maximum of 77 slots, including 31 reserved for foreign players.