WASHINGTON (BLOOMBERG) - The US stockpile of the controversial AstraZeneca coronavirus vaccine has grown to more than 20 million doses, according to people familiar with the matter, even as the shot looks increasingly unlikely to factor into President Joe Biden's domestic vaccination campaign.

AstraZeneca has yet to request Food and Drug Administration authorisation for the two-dose vaccine, and the company faces safety questions abroad and scrutiny from US regulators who've already rebuked it for missteps during clinical trials and partial data releases.

Three other vaccines already authorised in the US are going into Americans' arms at a rate of about 3 million doses per day, with hundreds of millions of additional doses set to be delivered by August.

That raises the question for Biden: What to do with AstraZeneca's vaccine? The company has more than 20 million doses already on hand, part of a total of between 80 and 90 million in some stage of production for the US order, the people familiar with the matter said. American allies have already sought doses from the US AstraZeneca stockpile, and the cheaper vaccine could inoculate people in scores of low-income countries that can't afford inoculations from Pfizer and Moderna.

"Give them all away. By the time we even think about authorising it, we are going to be in a glut situation domestically," said Zeke Emanuel, a medical doctor and University of Pennsylvania vice provost who served as a senior health policy adviser in the Obama administration and on Biden's Covid transition advisory board.

"We're never going to use them," he said.

Celine Gounder, a physician who also served on a Covid advisory board for the Biden transition, agreed the doses should be donated after the company secures authorisation from the FDA.

"We have enough - we don't even need Johnson & Johnson," the third authorised US manufacturer, she said.

"I would like to see the FDA continue its process, issue the emergency use authorisation assuming it passes snuff, and then donate that," she added.

FDA authorisation is "really important," she said, "because of all the different questions with the AstraZeneca vaccine."

Clotting 'association'

Earlier this week, the European Union drug regulator said it had established a "strong association" between AstraZeneca's shots and rare blood clots, particularly in younger patients. UK regulators said that people under 30 should be offered an alternative vaccine, if one is available.

That followed an unusual rebuke from American regulators in March, who accused the company of releasing "potentially misleading" data from a large US clinical trial. AstraZeneca revised estimates of the vaccine's efficacy slightly downward.

AstraZeneca is now searching for new US manufacturing of the vaccine's active ingredient, after it agreed to vacate a troubled Emergent Biosolutions plant in Baltimore that had confused production of its shot with Johnson & Johnson's. The Biden administration brokered J&J's takeover of the plant.

The US is on pace to have enough vaccine for its entire adult population by the end of next month, with another 200 million doses arriving from Moderna and Pfizer by late July. White House officials have said they want a surplus of doses in part to vaccinate children, once a dose is approved for people under 16.

Given its issues with younger adults, it's unclear if the AstraZeneca shot would ever be approved for use by American youngsters.

The administration isn't making any decisions either way about AstraZeneca until the FDA completes its review, said an official familiar with the matter, who, like the other officials, was granted anonymity to discuss the issue.

The US government last year spent US$1.2 billion (S$1.6 billion) to accelerate research, development, production and delivery of 300 million doses of AstraZeneca's vaccine. At the time, the Department of Health and Human Services under the Trump administration said it expected the first doses to be delivered as early as last October.

Biden didn't change that order, so AstraZeneca began producing shots in the US in anticipation of eventual FDA approval. Last month, the President agreed to send 4.2 million doses of AstraZeneca's vaccine to Mexico and Canada, an arrangement the administration termed a "loan."

When those doses are included, the running US AstraZeneca total is nearly 30 million so far, the people said.

The company expects to have 50 million doses ready by the end of this month, one of the officials said.

AstraZeneca declined to directly comment on the US stockpile, but said in a statement that it "expects to have up to approximately 50 million doses available to the US government at the time of Emergency Use Authorisation, and millions of additional doses thereafter."

There is no evidence at this point that any AstraZeneca batches were affected by problems at the Emergent plant in Baltimore, a company spokesperson said. Canadian and Mexican officials, who are starved for vaccine supply, haven't expressed concern. On the contrary, Mexico has asked the US for another shipment.

Mexican President Andres Manuel Lopez Obrador said Thursday he would consider receiving the AstraZeneca shot himself.