The U.S. Food and Drug Administration (FDA) has formally authorized Moderna's COVID-19 vaccine for emergency use on Friday.

Moderna is now the second company approved to distribute vaccines in the country to combat the coronavirus pandemic. The approval came just one week after Pfizer and BioNTech's COVID-19 vaccine received a federal green light.

With the FDA's approval, the government plans to distribute 5.9 million doses of Moderna's vaccine initially, CNBC reported. Around 64 states and major cities across the country are set to receive shipment next week.

Dr. Anthony Fauci, the nation's top infectious disease expert, said they would likely see more "shots in the arm by the very early part of next week."

Fauci, who is also the director of the National Institute of Allergy and Infectious Diseases, shared that he would also get a shot very soon. He said he hopes to receive a vaccine a few days ahead of his 80th birthday and Christmas. Fauci turns 80 on Dec. 24.

President Donald Trump has commented on the Moderna vaccine's approval.

"Congratulations, the Moderna vaccine is now available!" Trump tweeted.

Gen. Gustave Perna, part of the Operation Warp Speed vaccine project, said the government also intends to send out another two million doses of Pfizer's vaccine after 2.9 million doses were cleared for shipment this week.

Both COVID-19 vaccines require two doses, with three-to-four-weeks apart. Moderna has gotten its first-ever FDA authorized product with its COVID-19 vaccine.

FDA Commissioner Dr. Stephen Hahn said the agency had taken another crucial step to fight the global pandemic, causing vast numbers of hospitalizations and deaths in the United States every day.

Related story: Moderna COVID-19 Vaccine Gets Key Endorsement From FDA Advisory Panel

Dr. Paul Offit, a panel member and vaccine researcher at the Children's Hospital of Philadelphia, said one of the questions being asked to them is whether they do have enough evidence to say that the vaccine's benefits outweigh the risks.

Offit said the answer is clearly yes, adding that the question is when does one know enough, as per an NPR report.  

Challenges Evaluating the Vaccines

Offit also cited challenges faced by the federal regulators who have been responsible for evaluating these vaccines.

To fast-track the process of approving these vaccines, the FDA and other regulators worldwide have loosened their usual stringent approval procedures.

The health regulators have also adopted a more urgent procedure known as emergency use authorization. Under its federal supply contract, Moderna is set to deliver 20 million doses before the year ends.

Meanwhile, the Moderna vaccine will be given only to adults. Pregnant women and women who are nursing were not included in the study of more than 30,000 participants. People with severe health conditions were also not included in the study group.

Some people reported side effects such as arm pain, fatigue, muscle aches, but with no recorded serious health emergency.

Recipients of the vaccine tend to experience temporary flu-like side effects. It includes fever, fatigue, and aches. This usually happens after the second shot as the vaccine ramps up the recipient's immune system, according to the Atlanta Journal Constitution report. 

Dr. James Hildreth, president and CEO of Meharry Medical College in Nashville, Tennessee, said that having two vaccines available this month is a remarkable achievement. 

Related story: FDA Scientists Endorse Moderna's COVID-19 Vaccine for Emergency Use