What to know about Paxlovid, the COVID-19 therapy that Biden is taking

When White House officials announced President Joe Biden tested positive for COVID-19 Thursday, they revealed he was prescribed Paxlovid.

Here's what to know about the COVID-19 treatment.

Paxlovid is an at-home antiviral therapy developed by Pfizer, which was authorized under emergency use by the U.S. Food and Drug Administration for those aged 12 and older at higher risk for severe illness from COVID-19 in December 2021.

High-risk patients included those with underlying medical conditions, and those who are older or unvaccinated.

Clinical trial data showed Paxlovid reduced the risk of hospitalization and death for high-risk patients with mild-to-moderate symptoms by nearly 90%.

Paxlovid was hailed a game-changer because it was the first COVID-19 treatment that did not require an infusion, making it more convenient to give to patients.

The treatment is made up of two medications: ritonavir, commonly used to treat HIV and AIDS but which helps boost levels of other antiviral medications, and nirmatrelvir, an antiviral that works to inhibit an enzyme the virus uses to make copies of itself. Together, these drugs work to prevent the spread of the virus throughout the body.

Patients take three pills -- two nirmatrelvir pills and one ritonavir pill -- twice daily over the course of five days and it requires a prescription.

Side effects are rare but include an altered sense of taste, nausea, diarrhea, muscle aches and abdominal pain.

Doctors have said that Paxlovid is most effective when given as soon after a diagnosis of COVID-19 as possible. Taking it later during the course of the illness may result in the drug not being as effective.

Paxlovid is not meant to be taken as a prophylactic after exposure to COVID-19 or if a patient is already hospitalized.

In May, the Centers for Disease Control and Prevention issued an advisory, warning doctors to be on the lookout for a rare, but increasingly reported phenomenon known as "COVID-19 rebound."

COVID-19 rebound has typically been reported to occur among patients who took Paxlovid between two and eight days after recovery. Patients either experience a recurrence of COVID-19 symptoms or test positive after having tested negative.

"Limited information currently available from case reports suggests that persons treated with Paxlovid who experience COVID-19 rebound have had mild illness; there are no reports of severe disease," the CDC wrote.