• The full FDA could grant emergency use authorization of Pfizer's vaccine as early as Friday.
• The Vaccines and Related Biological Products Advisory Committee plays a key role in approving flu and other vaccines in the U.S.
• The FDA doesn't have to follow the advisory committee's recommendation, but the agency often does.

A key Food and Drug Administration advisory panel voted 17 to 4 with one member abstaining on Thursday to recommend the approval of Pfizer and BioNTech's coronavirus vaccine for emergency use, the last step before the FDA gives the final OK to broadly distribute the first doses throughout the United States.

If the FDA accepts the nonbinding recommendation from the Vaccines and Related Biological Products Advisory Committee — which is expected — it would mark a pivotal moment in the Covid-19 pandemic, which has infected more than 15.4 million people and killed roughly 290,000 in the U.S. in less than a year.

The committee plays a key role in approving flu and other vaccines in the U.S., verifying the shots are safe for public use. While the FDA doesn't have to follow the advisory committee's recommendation, it often does.

The FDA could grant emergency use authorization of Pfizer's vaccine as early as Friday, James Hildreth, a member of the committee, told NBC's "Weekend Today" on Saturday. An emergency use authorization, or EUA, generally allows a drug or vaccine to be administered to a limited population or setting, such as to hospitalized patients, as the agency continues to evaluate safety data.

It's unclear whether the FDA will authorize Pfizer and BioNTech's vaccine for use in certain groups. Some people, including pregnant women and young children, will likely have to wait to get the vaccine in the U.S. until Pfizer can finish trials on those specific groups. The FDA said Tuesday that there is currently insufficient data to make conclusions about the safety of the vaccine in children under age 16, pregnant women and people with compromised immune systems. Regulators in Canada, the U.K. and Bahrain have all cleared the vaccine for use by most adults.

The committee recommended emergency authorization of the vaccine for people who are 16 years old and older. Prior to the vote, some experts in the meeting argued to limit its recommendation to people who are at least 18, saying the safety data on 16- and 17-year olds was "thin."

"I would like to just join that lineup of people who are advocating for that. .. I think the data is very thin and it's inadequate to really say we have safety given the low incidents of disease," Dr. Mark Sawyer, a voting member of the panel and infectious disease expert at the University of California San Diego School of Medicine, said at the meeting.

An EUA isn't the same as a full approval, which can typically take months. Pfizer has submitted only two months of follow-up safety data, but the agency usually requires six months for full approval.

In a statement after the vote, Pfizer CEO Albert Bourla said the company was "pleased" with the vote, "and if the FDA issues an authorization, stand at the ready to bring this vaccine to people in the U.S. in an effort to help combat this devastating pandemic."

U.S. officials say they are prepared to distribute the vaccine within 24 hours of authorization. Initial doses will be limited as manufacturing ramps up, with officials predicting it will take months to immunize everyone in the U.S. who wants to be vaccinated. Pfizer has said it plans to ship 50 million vaccine doses this year, enough to inoculate 25 million people. The vaccine is expected to be distributed in phases, with the most critical U.S. workers and vulnerable people getting it first. The Centers for Disease Control and Prevention has provided states with an outline that recommends prioritizing health-care workers and nursing homes first, but states can distribute the vaccine as they see fit.

U.S. officials plan to continue watching for any adverse reactions to Pfizer's vaccine in the health-care workers and nursing home residents who receive it, Dr. Nancy Messonnier, director of the CDC's National Center for Immunization and Respiratory Disease, said during the agency's meeting Thursday. Officials will use a text-messaging system, called v-safe, which is intended to provide early indications about possible adverse reactions from the vaccines.

The U.K., which authorized Pfizer's vaccine for emergency use last week, began mass inoculations on Tuesday. The vaccine there is being given to front-line health workers, nursing home workers and those over age 80 before being given more widely among the U.K. population.

Pfizer's vaccine uses messenger RNA, or mRNA, technology. It's a new approach to vaccines that uses genetic material to provoke an immune response. Late-stage clinical trial data shows the vaccine is 95% effective in preventing Covid, is safe and appears to fend off severe disease. To achieve maximum effectiveness, the vaccine requires two doses taken about three weeks apart.

The FDA has indicated it would authorize a Covid-19 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 CDC.

Two days before the meeting, the FDA published documents on its website that said trial data submitted by Pfizer was consistent with recommendations put forth by the agency for emergency use. It also confirmed that the vaccine was highly effective and did not raise any major safety concerns, adding that more safety data is still needed on the vaccine in children, pregnant women and people with compromised immune systems.

During the meeting, FDA vaccine reviewer Dr. Susan Wollersheim noted the trial data showed a "numerical imbalance" in cases of Bell's palsy, a condition that causes a temporary weakness or paralysis of the muscles in the face. She said there were four cases in the vaccine group and none in the placebo group. While the frequency of cases was not above that of the general population, the FDA recommended further surveillance if the vaccine is authorized for more broad use. 

Pfizer's vaccine requires a storage temperature of minus 94 degrees Fahrenheit, presenting potential logistical challenges for rural areas and inner cities that may not have good health-care infrastructure. By comparison, Moderna has said its vaccine can be stored for up to six months at minus 4 degrees Fahrenheit.

President Donald Trump's coronavirus vaccine czar, Dr. Moncef Slaoui, has said the U.S. should be able to distribute enough coronavirus vaccine doses to immunize 100 million Americans, nearly a third of the U.S. population, by the end of February. He has said the entire U.S. population could be vaccinated against Covid-19 by June.

Pfizer said last week its full-scale production lines in the U.S. and Europe are now complete and it is "confident" it will be able to supply the targeted doses.

— CNBC's Noah Higgins-Dunn and Will Feuer contributed to this report.