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The Food and Drug Administration on Wednesday authorized Paxlovid, a pill developed and made by Pfizer, as a treatment for Covid-19, a significant step in the battle against the SARS-CoV-2 virus.

The drug was authorized for use in people as young as 12 so long as they weigh at least 88 pounds.

The authorization of an oral antiviral to beat back Covid has been eagerly anticipated because such a medicine could reach large numbers of people infected with the virus and prevent them from becoming seriously ill or hospitalized. Existing medicines, such as monoclonal antibodies, must be given intravenously or as injections.

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Still, initial supplies of Paxlovid will be limited. Pfizer has said it expects to produce more than 180,000 courses of the treatment this year. The company said Wednesday it now expects to provide 120 million courses by the end of 2022, up from 80 million previously, thanks in part to new contract manufacturers. Pfizer has contracted with the U.S. government to provide 10 million courses by the end of 2022 at a cost of $5.29 billion.

“Today’s authorization introduces the first treatment for COVID-19 that is in the form of a pill that is taken orally — a major step forward in the fight against this global pandemic,” Patrizia Cavazzoni, director of the FDA’s Center for Drug Evaluation and Research, said in a statement. “This authorization provides a new tool to combat COVID-19 at a crucial time in the pandemic as new variants emerge and promises to make antiviral treatment more accessible to patients who are at high risk for progression to severe COVID-19.” 

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The FDA emphasized that Paxlovid is not authorized to be given to prevent infection or right after an exposure, nor is it for those who already require hospitalization. It is also, the agency said, not a substitute for vaccination or a booster shot.

Earlier this month, Pfizer released results of a study of its treatment in 2,246 Covid patients who were at high risk of developing more severe disease because of risk factors such as being over 65, overweight, or having cardiovascular disease. Patients were randomly assigned to receive a five-day course of either Paxlovid and a second drug, ritonavir, or a placebo. Among those who received Paxlovid within five days of developing symptoms, eight, or 0.8% of the total, were hospitalized. In the placebo group, 66, or 6.3%, were hospitalized. That meant overall Paxlovid reduced the risk of hospitalization by 89%. No patients who received Paxlovid died, compared to 12 who received placebo.

Pfizer previously said that potential side effects occurred at similar rates whether patients received Paxlovid or placebo. There were fewer serious adverse events (1.6% versus 6.6%) among those who received Paxlovid, and patients were more likely to stop taking placebo than Paxlovid.

The results from that study, and a second study in lower-risk patients, have not yet been published in medical journals.

Developing new medicines can take decades, and the process is replete with failure. A 2016 report from the Biotechnology Industry Organization estimated that only 1 in 5 infectious disease drugs that start clinical trials reach patients. Yet Paxlovid, a drug specifically targeting the SARS-CoV-2 virus, was developed in less than two years.

The effort began in March 2020, as Pfizer’s CEO, Albert Bourla, made a public declaration that the company was “committed to doing all we can to respond to the Covid-19 pandemic.”

The team assigned to the effort included Annaliesa Anderson, chief scientist in Pfizer’s vaccine development group, and Charlotte Allerton, its head of medicine design. In four months, they had hundreds of potential medicines. By March 2021, Paxlovid was ready to begin testing in humans, and a few months later, efficacy testing began.

This breakthrough therapy, which has been shown to significantly reduce hospitalizations and deaths and can be taken at home, will change the way we treat Covid-19, and hopefully help reduce some of the significant pressures facing our health care and hospital systems,” Bourla said in a statement Wednesday.

Anderson told STAT that she had been thrilled when vaccines were proven effective at preventing Covid-19. While she knew that there would be a need for antiviral medicines, she hoped it would not be too great. 

“We all spent a lot of time developing this antiviral,” Anderson said, “and we hoped we wouldn’t need it.”

Paxlovid is a protease inhibitor, meaning that it works by blocking one the main enzymes (a protease called mPRO) that the virus uses to replicate its genetic material, called RNA. Other drugs that inhibit other proteases are used to treat HIV. Like some HIV drugs, Paxlovid must be combined with another drug, in this case ritonavir, which keeps the body from breaking down protease inhibitor drugs.

Patients take two Paxlovid pills and one ritonavir pill twice a day for five days, for a total of 30 pills in a treatment course.

The FDA is also expected to reach a decision soon on whether to authorize another Covid-19 pill, molnupiravir, invented at Emory University and developed by Merck and the biotechnology firm Ridgeback Biotherapeutics. The efficacy of that pill at reducing hospitalization, initially estimated at 50%, dropped to 30% after its study was completed. There are also concerns that it might affect bone growth in children or a developing fetus. An expert panel narrowly recommended the pill be authorized in November; Merck’s application for authorization was submitted before Pfizer’s.

The U.S. government is likely to have millions of doses of molnupiravir by early next year, if it is authorized as expected. By contrast, it will initially have only a few hundred thousand courses of Paxlovid, due to manufacturing constraints.

Possible side effects of Paxlovid include impaired sense of taste, diarrhea, high blood pressure, and muscle aches. It can interact with other drugs, and the FDA said care should be taken when giving ritonavir to patients with liver disease.

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