Up until now, the mRNA vaccine, which will be marketed as Comirnaty, was on the U.S. market under an emergency use authorization that was granted by the FDA in December. Since then, more than 204 million of the Pfizer shots have been administered, according to data compiled by the Centers for Disease Control and Prevention.

Federal health officials had been under mounting pressure from the scientific community and advocacy groups to fully approve Pfizer and BioNTech's vaccine ever since the drugmakers submitted their application to the agency in early May. The companies submitted a Biologics License Application, which secures full approval, to the FDA on May 7 for patients age 16 and up.

FDA scientists evaluated "hundreds of thousands of pages" of vaccine data from 40,000 trial participants, according to the U.S. agency. The vaccine was found to be 91% effective in preventing Covid – slightly lower than the 95% efficacy rate trial data showed when the shot was authorized late last year and before the delta variant took hold in the U.S. 

Pfizer's vaccine met the agency's "high standards for safety, effectiveness, and manufacturing quality," acting FDA Commissioner Janet Woodcock said in a statement. "While millions of people have already safely received COVID-19 vaccines, we recognize that for some, the FDA approval of a vaccine may now instill additional confidence to get vaccinated."

The FDA held a media briefing Monday morning to discuss the approval.

Although more than 60% of the total U.S. population has had at least one dose of a Covid vaccine, many Americans say they are still hesitant about getting vaccinated, even as the highly contagious delta variant spreads. Full approval may convince some Americans that the shots are safe, doctors and epidemiologists say. In fact, a survey from the Kaiser Family Foundation found 3 in 10 unvaccinated adults said they would be more likely to get vaccinated if one of the vaccines receives full approval.