When will AstraZeneca's COVID-19 vaccine be available in the US?

• In the UK, vaccines Pfizer, Moderna, and AstraZeneca are being used to fight the pandemic. AstraZeneca's hasn't been approved for use yet in the US.
• AstraZeneca may not apply for a US FDA Emergency Use Authorization until the spring. The data from their UK trial was "odd" and had one "pretty serious error" in it, a US vaccine expert said.
• AstraZeneca is now conducting a larger trial of nearly 30,000 people in the US.

The UK now has three different COVID-19 vaccines in use to fight the pandemic, while the US has just two.

AstraZeneca's two-shot vaccine is still missing from the US vaccine arsenal. Developed in partnership with the University of Oxford, it's been authorized for use across the UK since December 30, and it's also been cleared to go into arms in India, Mexico, Argentina, and several more countries.

But in the US, it's unlikely the same two-shot course will be available any day soon.

The difference comes down to how the US Food and Drug Administration independently scrutinizes vaccines, doing their own scientific review of a company's safety and efficacy data, before any new shot is allowed to be put on the market.

In the US, there's even an independent advisory committee, which reviews data sets from both the FDA and drug companies, before the agency makes its final decision on whether to greenlight a new shot.

In the UK, regulators rely more heavily on a company's data to make conclusions about whether their new vaccine is safe and effective.

"No stone is left unturned when it comes to our assessments," the UK's Medicines and Healthcare products Regulatory Agency Chief Executive Dr. June Raine said in a statement when AstraZeneca's new vaccine was approved there last month.

"We are delighted to announce the good news that the Oxford University/AstraZeneca vaccine for COVID-19 is now approved for supply, following a robust and thorough assessment of all the available data."

The FDA is waiting for AstraZeneca to submit its vaccine paperwork, which may not be ready until the spring

That data Raine was referring to came from a vaccine research trial which included more than 5,800 volunteers around the UK, South Africa, and Brazil. The results suggested that AstraZeneca's vaccine was 62% effective at preventing symptomatic COVID-19 infections when taken as two full-strength shots. That's a much lower potency than both Pfizer and Moderna's shots, which were each more than 90% effective in trials.

The AstraZeneca trial also included at least one big mistake. A subset of trial participants under 55 years old were accidentally administered a half-dose first shot, followed by a full-strength second jab.

"That's a pretty serious error," Dr. Cody Meissner, chief of pediatric infectious disease at Tufts Medical Center, and one of the vaccine experts on the FDA's advisory committee, told Insider of the mishap.

Those patients who had a less potent initial shot, followed by a full-strength booster actually appeared more protected from coronavirus infections, with efficacy surging to 90% in the subgroup. Experts were puzzled by that.

"I won't go to the point of saying that it's not biologically plausible, but it's a little bit odd," Meissner said.

"It generates a little bit of pause, and makes one eager to actually look at the data and see what they found and to understand it a little bit more carefully."

But the FDA hasn't been offered that data to review for an emergency use authorization (EUA), which would be required for Americans to start getting vaccinated with the shot. Instead, AstraZeneca is going to wait for the results of a larger, US-based trial, where no half-doses are planned.

Read More: What's coming next for COVID-19 vaccines? Here's the latest on 11 leading programs.

AstraZeneca told Insider in an email that "recruitment is almost complete" for its 30,000 person US vaccine trial. Trial participants are to be given two full strength vaccine doses, 28 days apart.

Then, researchers will wait to see who gets infected and whether people who got the vaccine are well-protected. Once 75 people in the study have gotten sick with the coronavirus (either in the vaccinated group, or in untreated controls) AstraZeneca will release preliminary results. But the company will still have to wait until at least two months of safety data are available to submit for an EUA.

Moncef Slaoui, the lead scientist at Operation Warp Speed, the federal government's COVID-19 vaccine drive, estimated that data could be ready for an FDA review by April.

By then, it's possible that a third COVID-19 vaccine, from Johnson & Johnson, may already be cleared for use in the US.

Meissner says even if AstraZeneca's shot does end up being 62% effective, a far less impressive level of protection than Pfizer or Moderna's jabs, it would still be a good tool to prevent COVID-19 infections and deaths in the US.

"We need more vaccines," Meissner said. "We need to get this pandemic under control just as quickly as possible, because remember more people are dying from COVID-19 on a daily basis than are dying from cancer or heart disease. This is the number one killer of Americans."