WASHINGTON — The US has authorized the emergency use of the COVID-19 vaccine produced by pharmaceutical company Pfizer and German biotechnology company BioNTech, paving the way for a mass immunization program in the country.

The US Food and Drug Administration gave the coronavirus vaccine a green light on Friday, after a government expert advisory panel concluded the vaccine appeared safe and effective.

"The FDA’s authorization for emergency use of the first COVID-19 vaccine is a significant milestone in battling this devastating pandemic that has affected so many families in the United States and around the world,” said agency Commissioner Dr. Stephen M. Hahn.

Hahn said the action followed "an open and transparent review process" that included input from multiple experts and scientists to determine the vaccine met the agency's standards.

"The tireless work to develop a new vaccine to prevent this novel, serious, and life-threatening disease in an expedited timeframe after its emergence is a true testament to scientific innovation and public-private collaboration worldwide."

US President Donald Trump called the vaccine news a "medical miracle" and one of the "greatest scientific achievements in history" in a video posted to his Twitter account. He said that the first vaccine would be administered within 24 hours.

Before vaccinations can begin, a US Centers for Disease Control and Prevention immunization advisory committee still must vote to recommend the vaccine, and the CDC must accept that recommendation.

The Advisory Committee on Immunization Practices was scheduled to meet Saturday morning. About 20 million people could be vaccinated in the next few weeks — an important step toward ending the pandemic that has killed nearly 300,000 people in the United States.

But until many more are vaccinated, mitigation efforts such as wearing masks and keeping a safe distance need to continue to stop the virus from spreading.

Pfizer and BioNTech's coronavirus vaccine, called BNT162b2, is an mRNA vaccine given as two shots administered 21 days apart. One dose is to prime the body, and then a few weeks later, the second dose is to boost the response.

As vaccine doses get distributed across the country, state and local officials will oversee the distribution of the vaccine in their states as well as the logistics — and health care workers and residents of long-term care facilities are prioritized to receive the vaccine first.

At this point, the number of doses each state is getting in its first shipment is fluid and, in many cases, the amount is less than what states were initially told to expect, leading states to change distribution plans. In most cases, the first shipments of the vaccine will be transported from Pfizer's facility in Kalamazoo, Michigan, to hospital systems and pharmacies.

The distribution from vaccine production company to individual hospitals and pharmacies in each state as directed by the federal government, is different for almost every state. State health department officials are coordinating with federal officials in Operation Warp Speed.

However, there is no clear road map that explains how this process is working simultaneously in all 50 states. Instead, states have developed individualized vaccine distribution plans that vary slightly for each individual state's demographic and geographic needs.

CDC officials have been in close contact with state leaders about how their distribution plans will work and challenges that could crop up state to state.

The United Kingdom was the first country to authorise the Pfizer/BioNTech vaccine and began vaccinating elderly people who are at risk for developing a more severe form of the virus earlier this week.

The European Medicines Agency is currently reviewing the vaccine's safety and efficacy and is set to give a decision before Dec. 29. — Agencies