• An influential FDA advisory panel overwhelmingly voted to recommend the approval of Moderna's coronavirus vaccine for emergency use.
• The FDA's Vaccines and Related Biological Products Advisory 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.

An influential Food and Drug Administration advisory panel on Thursday overwhelmingly backed Moderna's coronavirus vaccine, a key step paving the way to distribute the second Covid-19 vaccine in the United States next week.

The nonbinding decision, which was adopted 20-0 with one abstention, by the FDA's Vaccines and Related Biological Products Advisory Committee came exactly a week after the outside group of vaccine and infectious disease experts voted to recommend Pfizer and BioNTech's vaccine for an emergency use authorization, or EUA. The agency granted Pfizer's EUA the next day, and the first inoculations in the U.S. were given Monday.

The FDA advisory 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 U.S. plans to ship just under 6 million doses next week, pending the agency's OK, Gen. Gustave Perna, who oversees logistics for the Operation Warp Speed vaccine project, told reporters Monday.

Prior to the vote, some members of the committee stressed that their endorsement for Moderna's vaccine was not for a full FDA approval, reiterating that the agency will still need to review more data on safety and effectiveness.

"I don't want people to interpret this the same way they would a licensed vaccine," said Dr. Cody Meissner, a voting member of the committee and an infectious diseases expert at Tufts University School of Medicine. The endorsement is "based on the available evidence, but that's limited."

An EUA means the FDA will allow some people to receive the vaccine as the agency continues to evaluate data. It isn't the same as full approval, which requires more data and can typically take several months longer. Moderna has submitted only two months of follow-up safety data. The agency usually requires six months for full approval. The committee recommended emergency authorization of the vaccine for people who are 18 years old and older.

Member Dr. Michael Kurilla, an infectious diseases expert with the National Institutes of Health, was the only member to not vote yes. He said he decided to abstain because he was "uncomfortable" endorsing the vaccine for everyone age 18 and older.

"In the midst of a pandemic and with limited vaccine supply, a blanket statement for individuals for 18 years and older is just too broad," said Kurilla. "I'm not convinced for all of those age groups the benefits do actually outweigh the risk."

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. The Centers for Disease Control and Prevention has provided states with an outline that recommends prioritizing health-care workers and nursing homes, but states can distribute the vaccine as they see fit.

Moderna'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 more than 94% effective in preventing Covid, is safe and appears to fend off severe disease. To achieve maximum effectiveness, the vaccine requires two doses taken four 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.

FDA staff endorsed Moderna's vaccine on Tuesday, saying in documents that the clinical trial results and safety data were "consistent with the recommendations set forth in FDA's Guidance on Emergency Use Authorization for Vaccines to Prevent COVID-19."

The FDA said 13 deaths in the trial were reported as of Dec. 3, with six in the vaccine group. Two deaths in the vaccine group were people over 75 with preexisting conditions, the agency said. One case was a 72-year-old vaccine recipient with Crohn's disease and short bowel syndrome who was hospitalized for thrombocytopenia and acute kidney failure.

Fatigue, headaches and muscle pain are the most common side effects from Moderna's vaccine, along with some rare symptoms such as intractable nausea or vomiting and facial swelling that are likely triggered by the shots, according to the FDA. Some side effects were hard to shake, though most resolved within a week, the FDA said. 

During the meeting Thursday, medical experts also asked the agency about allergic reactions after two cases occurred in Alaskan health-care workers who took Pfizer's vaccine. Doran Fink, deputy director of FDA's division of vaccines and related products applications, said the agency would continue to investigate the cases.

"As we continue to investigate and evaluate the data, we will consider whether additional recommendations need to be made," he said. "At this point, we don't have enough data to make a definitive recommendation one way or the other."

There is little data regarding the safety of the vaccine in pregnant women and children. Thirteen pregnancies were reported in Moderna's trial through Dec. 2, with six in the vaccine group, the FDA has said. The company is tracking all women who got pregnant after receiving the vaccine or who were pregnant when they were immunized but didn't know, according to the agency.

Dr. David Martin, vice president for drug safety at Moderna, told the committee Thursday that the company will establish a "pregnancy registry" to track how its vaccine performs in pregnant women.

"With respect to safety in the event of vaccine exposure during pregnancy, a developmental and reproductive study was completed in December 2020, with no adverse findings," Martin told the committee. "Given the limited human exposure to date in the phase three trial, we will establish a pregnancy registry that includes a cohort recruited from the general population."

While it is not necessarily a side effect, the FDA said it recommends monitoring people who get Pfizer's or Moderna's vaccine shots for possible cases of Bell's palsy, a condition that causes sudden freezing or weakness in facial muscles. The agency also noted a higher prevalence of lymphadenopathy, a disease that can produce swollen or enlarged lymph nodes, in Pfizer's and Moderna's trials in the vaccine group compared with the placebo groups.

Moderna has said its vaccine remains stable at 36 to 46 degrees Fahrenheit, the temperature of a standard home or medical refrigerator, for up to 30 days. It can be stored for up to six months at minus 4 degrees Fahrenheit. By comparison, Pfizer's vaccine requires a storage temperature of minus 94 degrees Fahrenheit.

The federal government announced last week that it will purchase an additional 100 million doses of Moderna's vaccine. The U.S. entered into an agreement with Moderna in August to acquire 100 million doses for about $1.5 billion. Moderna said that month it was charging $32 to $37 per dose for its vaccine for some customers, under cheaper "pandemic pricing." The company said it was in discussion for larger volume agreements that will have a lower price.

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

Correction: This story has been updated to reflect that Dr. Michael Kurilla, an infectious diseases expert with the National Institutes of Health, was the only member to abstain from voting.