The U.S. Food and Drug Administration on Thursday granted Pfizer's Paxlovid full approval for the treatment of COVID-19.

Paxlovid becomes the first oral antiviral drug -- and fourth drug overall -- to receive approval from the FDA for the treatment of mild to moderate COVID-19 in adults.

"While the pandemic has been challenging for all of us, we have made great progress mitigating the impact of COVID-19 on our lives," Patrizia Cavazzoni, director of the FDA's Center for Drug Evaluation and Research, said in a statement.

"Today's approval demonstrates that Paxlovid has met the agency's rigorous standards for safety and effectiveness and that it remains an important treatment option for people at high risk for progression to severe COVID-19, including those with prior immunity."

The FDA advisory panel had been considering the approval of Paxlovid since March for full approval after the antiviral pill had been proven safe and effective for treating adults at risk for severe complications of COVID-19 in previous testing.

Paxlovid had been under emergency use authorization. The Department of Health and Human Services said it will continue to be made available for adults, as well as treatment of eligible children ages 12-18 who are not covered by today's approval.

A new drug approval requires, among other things, substantial evidence of effectiveness and a demonstration of safety for the drug's intended uses. The FDA also conducts a benefit-risk assessment based on rigorous scientific standards to ensure that the product's benefits outweigh its risks for the intended population.

In March, concerns arose about rebound cases after some patients, including President Joe Biden and his former medical adviser Anthony Fauci, reported testing positive or experiencing symptoms after completing treatment with the drug.

The FDA found that the rebound rates for patients who took Paxlovid were 10% to 16%, which was about the same for those who took a placebo.

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