Why Pfizer's Covid vaccine has been authorized in the UK but not the U.S. yet

The U.K. this week became the first country to authorize Pfizer's coronavirus vaccine for emergency use, likely adding pressure on the U.S. Food and Drug Administration to quickly do the same.

To be sure, the U.S. and the U.K. review vaccines differently. Pfizer had been submitting data on its vaccine with BioNtech on a "rolling basis" to the U.K., meaning regulators there were able to review the data in real time and do so until there was enough evidence to support a formal authorization. A rolling review is a tool that regulators use to speed up the assessment of promising drugs or vaccines.

In the U.S., the FDA will go through every aspect of the data submitted in the application, including reviewing all safety information "to make sure there are no cracks" and everything is "solid," said Dr. Paul Offit, a voting member of the Vaccines and Related Biological Products Advisory Committee, which is scheduled to review Pfizer's vaccine on Dec. 10.

"I don't know what data the U.K. was working with. I know when data is submitted to the FDA, it's voluminous," said Offit, also director of the Vaccine Education Center at Children's Hospital of Philadelphia. "When you talk about a 44,000-person trial, that's a lot of clinical data."

He said when Merck submitted its data from its 70,000-person rotavirus vaccine trial, the pages that were generated could have exceeded the height of the Sears Tower. That building, now known as Willis Tower, is a 1,450-foot skyscraper in Chicago. "The point being: It's a lot of data," Offit said.

Pfizer submitted its Covid vaccine data to the FDA on Nov. 20. The FDA has indicated it would authorize a vaccine that's safe and at least 50% effective. The flu vaccine, by comparison, generally reduces people's risk of getting influenza by 40% to 60% compared with people who aren't inoculated, according to the Centers for Disease Control and Prevention. 

The agency's review of the vaccine is expected to take a few weeks, and public health officials expect that the first doses of the vaccine could be distributed in as little as two weeks.

Offit said he doesn't think a U.S. authorization about a week after the U.K. makes a difference when you're talking about vaccinating hundreds of millions of people. Immunizing everyone who wants a vaccine is likely going to take more than a year, he added.

Additionally, the U.K. has been sharply criticized for approving Pfizer's vaccine so quickly. Dr. Anthony Fauci, the nation's leading infectious disease expert, told Fox News that the U.K. did not review Pfizer's data "carefully."

"We have the gold standard of a regulatory approach with the FDA. The U.K. did not do it as carefully and they got a couple of days ahead," he said. "I don't think that makes much difference. We'll be there. We'll be there very soon."

Drugmakers and states are gearing up to distribute a vaccine starting in mid-December. The Federal Aviation Administration said it supported the "first mass air shipment" of vaccines last week. United Airlines carried Pfizer's Covid-19 vaccine from Brussels to Chicago O'Hare International Airport on Friday, people familiar with the matter told CNBC.

President Donald Trump has previously accused the FDA, without proof, of working to slow the Covid-19 vaccine development process. Earlier this week, White House chief of staff Mark Meadows summoned FDA Commissioner Dr. Stephen Hahn to discuss why the agency hasn't moved faster to authorize Pfizer's vaccine, a person familiar with the matter confirmed to NBC News.

Axios first reported the meeting, along with a preemptive statement from Hahn, who told the outlet: "Let me be clear — our career scientists have to make the decision and they will take the time that's needed to make the right call on this important decision."

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