The Food and Drug Administration has issued an emergency use authorization for Pfizer-BioNTech's coronavirus vaccine, the agency announced on Friday night.

Why it matters: It's a major milestone in the U.S. fight against COVID-19, clearing the way for the initial rollout of a vaccine that has been found to be 95% effective with no serious side effects.

What to watch: A Department of Health and Human Services official told Axios that Operation Warp Speed is working with governors to ensure that vaccine distribution begins "within 24 hours" of the FDA's authorization.

• 636 locations equipped with ultra-cold storage capacity across all 50 states will receive about 2.9 million doses in the initial Pfizer shipment, according to Operation Warp Speed officials.
• An equal number will be held for the second dose of the vaccine, which is meant to be administered 21 days later.
• Health care workers and nursing home residents are expected to be the first to receive the vaccine.

Worth noting: An EUA is not a full approval. Rolling out the vaccine is contingent upon companies generating more data needed to support full approval, including ongoing placebo-controlled trials to understand the long-term effects.

The big picture: The U.K. became the first country to grant emergency approval to the Pfizer vaccine on Dec. 2, followed by Bahrain and Canada. The FDA will meet later this month consider granting an EUA for Moderna's vaccine, which has been found to be 94.5% effective.

By the numbers: The U.S. is the country with the most confirmed total cases, reporting more than 15 million cases and over 290,000 deaths as of Friday, according to data from Johns Hopkins University.

Go deeper: Pfizer board member confirms U.S. government turned down offer for more vaccine doses