The Food and Drug Administration authorized Moderna’s Covid-19 vaccine for emergency use Friday, adding a second vaccine to the country’s arsenal to fight the pandemic.

The United States is the first country to greenlight use of Moderna’s vaccine, which was developed in part with the National Institutes of Health.

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“This is another crucial step in the fight against the global pandemic that is causing vast numbers of hospitalizations and deaths in the United States every day,” FDA commissioner Dr. Stephen Hahn said during a media briefing late Friday.

Dec. 19, 202002:24

Indeed, the news was met with optimism. “I’ve never been more hopeful that we will eventually turn the corner on this pandemic,” said Dr. Richard Besser, former acting director of the Centers for Disease Control and Prevention and current president of the Robert Wood Johnson Foundation. “I encourage all Americans to get vaccinated when their groups are called, and I look forward to doing so when it is my turn.”

Nearly 6 million doses of the Moderna vaccine have been earmarked for distribution and are expected to be shipped to 3,285 locations across the country, according to officials with Operation Warp Speed, which partially funded the Moderna work.

In a statement issued Friday, Moderna said that about 20 million doses of its vaccine will be delivered to the U.S. government by the end of this year. “The Company expects to have between 100 million and 125 million doses available globally in the first quarter of 2021, with 85-100 million of those available in the U.S.,” the statement read.

Moderna’s vaccine has been shown to be safe and highly effective at preventing symptomatic illness, according to documents released Tuesday. The data also suggest that the vaccine may reduce the virus’s spread by preventing infections, though more data are needed to fully understand that finding.

Dr. Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, called the finding “intriguing” during the Friday briefing, adding that the NIH is considering studying this further.

The FDA authorization comes the day after an advisory committee to the agency recommended the vaccine in a near unanimous vote. Of 21 voting members, 20 voted in favor of authorization.

One person, Dr. Michael Kurilla, an infectious disease expert with the National Institutes of Health, abstained from voting because he said he felt the question on which the committee voted, “Based on the totality of scientific evidence available, do the benefits of the Moderna Covid-19 vaccine outweigh its risks for use in individuals 18 years of age and older?” was too broad, and should have been targeted toward people at highest risk for severe Covid-19.

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Last week, the FDA authorized Pfizer-BioNTech’s Covid-19 vaccine for those ages 16 and older. Nearly 3 million doses of that vaccine were on target for delivery this week to health care workers and people in long-term care facilities. A second batch of doses has been reserved for the second shot, administered 21 days after the first.

Moderna’s vaccine is also given in two doses, but 28 days apart. Clinical trials showed the vaccine was 94 percent effective in preventing symptomatic illness within two weeks after the second dose. Pfizer’s has been shown to be similarly effective.

Unlike the Pfizer vaccine, the Moderna vaccine does not need to be stored at ultracold temperatures. Rather, the vaccine can be stored at 36 to 46 degrees Fahrenheit — the temperature of a standard refrigerator — for up to 30 days, the company said in a statement Wednesday.

One potentially thorny issue is the risk of severe allergic reactions. The FDA’s Marks said Friday night that the agency was looking into “roughly five” allergic reactions linked to the Pfizer vaccine. One of those reactions was reported in a health care worker in Alaska who needed to be hospitalized.

While no such reactions were reported during Moderna’s clinical trials, the FDA is advising people who have had severe reactions to vaccines in the past to forgo the vaccine at this time, until more information is available.

One potential — but as yet unproven — culprit for the allergic reactions is polyethylene glycol, which is present in both the Pfizer and Moderna vaccines, Marks said, adding any ideas about what could be leading to allergic reactions remain speculative.

No long-term problems have been reported.

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