The FDA says it will authorize Pfizer's COVID-19 vaccine, paving the way for people in the US to start getting shots as early as Monday

• The Food and Drug Administration said Friday that it would "rapidly work toward" issuing an emergency-use authorization for Pfizer and BioNTech's coronavirus vaccine.
• It would be the first COVID-19 vaccine authorized for use in the US.
• The first shots of Pfizer and BioNTech's vaccine could be given out as early as Monday, according to Health and Human Services Secretary Alex Azar.
• The US hopes to vaccinate 100 million people by March, including 2.9 million doses of Pfizer's shot within days of its authorization.

The US Food and Drug Administration has said it intends to authorize Pfizer and BioNTech's COVID-19 vaccine.

The agency's Commissioner Dr. Stephen Hahn said Friday morning that it would "rapidly work toward finalization and issuance of an emergency-use authorization" for the shot. Pfizer's two-dose vaccine would be the first to be authorized for use in the US.

The FDA said it had notified the drugmakers as well as the Centers for Disease Control and Prevention and the federal vaccine initiative known as Operation Warp Speed so they could "execute their plans for timely vaccine distribution."

The vaccine could be rolled out as early as Monday, Health and Human Services Secretary Alex Azar said on "Good Morning America" on Friday morning.

"We could be seeing people getting vaccinated Monday, Tuesday of next week," Azar said.

—Good Morning America (@GMA) December 11, 2020

The announcement follows an endorsement from an independent expert panel of more than 20 leading scientists and doctors, who reviewed data from a trial of Pfizer and BioNTech's vaccine on Thursday.

Read more: How the pharma giant Pfizer teamed up with a little-known biotech to develop an effective coronavirus vaccine in record time

The US faces challenges in distributing the vaccines

Operation Warp Speed leaders have previously said the plan is to ship 2.9 million doses of the vaccine within days.

The US hopes to vaccinate 100 million people by March: 20 million in December, 30 million in January, and 50 million more in February. Those projections depend on getting vaccines from Pfizer and from Moderna, a biotech company that also has a highly effective shot under FDA review. The shots will be free for people.

Read more: Moderna's ambitions of pumping out up to 1 billion doses of a coronavirus vaccine rest on a former Polaroid factory that's never produced an approved drug

Federal authorities are allocating vaccine doses to states and territories based on their adult populations, and each state will decide how to divvy up its supply. Earlier this month, a CDC advisory group recommended that healthcare workers and nursing-home residents be at the front of the line.

But a key COVID-19 advisor to President-elect Joe Biden has warned Americans to expect "hitches" in the rollout of Pfizer and BioNTech's vaccine.

"The biggest challenge is going to be both production and then getting it into people's arms," Dr. Atul Gawande told CNBC on Thursday.

Health regulators in countries including the UK and Canada have already authorized the use of Pfizer's vaccine. The UK began administering Pfizer and BioNTech's vaccine on Tuesday, making it the first nation in the West to roll out a COVID-19 vaccine.

The FDA's review is taking longer in part to ensure that there was a public discussion of Pfizer's application to help instill confidence in the shot.

Read more: Meet the 19 key scientists, executives, and leaders responsible for pushing coronavirus vaccines across the finish line

Late-stage study showed Pfizer's shot was effective and safe, but questions still remain

Pfizer's vaccine was overwhelmingly protective against COVID-19 in a clinical trial that compared more than 20,000 people who got the shot with a similar group of volunteers who received sham injections. The trial found that the shot was about 95% effective at preventing symptomatic cases of the disease, though the real-world experience could be different.

There were no major safety concerns found in the study. Most people experienced some side effects that were expected, didn't last very long, and typically were mild to moderate in intensity. The most common were pain at the injection site, fatigue, and headaches.

Shortly after the UK started to immunize healthcare workers and older people, authorities said that people with a history of serious allergic reactions to other medicines or food should not get Pfizer's vaccine. The warning followed two reports of adverse reactions in healthcare workers who have a history of severe allergies.

It's unclear if US regulators will issue an equally broad warning. The CDC will be in charge of several tracking systems to monitor the vaccine's safety as it's being rolled out. One of those, a new program called V-SAFE, will keep tabs on immunized people with text messages.

Even with the promising results, there remain crucial questions about the vaccine.

It's unclear how long the shots will protect people from COVID-19. The study also hasn't been going on long enough to determine if there are any long-term safety issues with the vaccine. And it's not clear if immunization can prevent transmission of the virus. That means vaccinated people could still be asymptomatic carriers and contagious.