Biden and Fauci Paxlovid rebound
US President Joe Biden (left) recently confirmed he tested COVID-positive again, which a White House doctor said was a result of "Paxlovid rebound." Dr Anthony Fauci, America's top infectious disease expert, also confirmed he was also reinfected with COVID, also after taking a full course of Paxlovid. Image Credit: File photo | Agencies


  • Paxlovid, a COVID-19 pill, is approved under emergency-use authorisation, and does not replace the prophylactic effect of an approved vaccine.
  • A growing body of patients data show some users can experience a “rebound” after completion of a round of Paxlovid treatment.
  • The rebound is a situation where patients who took it will test negative — and then suddenly begin testing positive again.

Paxlovid, an antiviral pill developed by Pfizer, has become the drug of choice by many doctors against COVID-19. It is convenient to use (at-home) and showed impressive results in heading off severe disease in human trials.

Paxlovid is an antiviral pill developed by Pfizer. It is the first pill approved by the US Food and Drug Administration for COVID-19, with emergency-use authorisation (EUA) granted in December 2021.

The World Health Organisation "strongly recommends" it. The Europeans approved the pill for COVID in April 2022.

201111 Biden
US President-elect Joe Biden adjusts his face mask during a brief news conference. Biden recently confirmed he was tested COVID-positive again, which a White House doctor said was a result of "Paxlovid rebound."

The pill was authorised for adults at high risk of severe COVID-19 based on a research data on 1,000 adults who initially received the medication in a clinical trial. In May, the White House said Paxlovid helps curb hospitalisations and deaths among at-risk people by almost 90 per cent.

Experts, however, say there is still much to be learned about the drug.

Here's an update on what is known as "Paxlovid rebound":

What is Paxlovid rebound?

It refers to what a certain percentage of users experience, testing positive against after already testing negative following an initial lab-confirmed COVID infection, after completing of a round of Paxlovid treatment.

It is called a "rebound" as it refers to a situation where patients who took it will test negative — and then suddenly begin testing positive again.

Who are some of famous people who have had Paxlovid rebound?

US President Joe Biden has tested positive for COVID-19 — again — just over three days after he was cleared to move out of isolation.

Another is Dr. Anthony Fauci, America's top infectious disease expert and chief medical adviser to President Joe Biden. Fauci took Paxlovid after testing positive for COVID. After completing a full course of treatment, Fauci told US media in June he had a recurrence of symptoms, i.e. he tested positive again.

Following his completion of the treatment, Fauci said the symptoms that he had were more severe than those he had at first from the coronavirus.

What happened to Biden?

White House physician Dr. Kevin O’Connor said in a letter that Biden initially tested positive on July 21, “has experienced no re-emergence of symptoms and continues to feel quite well.” Dr. O’Connor said there is no need for the president to begin another round of treatment.

Rather, the US leader will return to isolation while waiting to again test negative for the virus.

Biden said on Twitter: "I'm feeling fine, everything's good," did not have any symptoms and was continuing to work, albeit with quarantine precautions.

What happened to Dr. Fauci?

“After I finished the 5 days of Paxlovid," Fauci told a health forum, "I reverted to negative on an antigen test for 3 days in a row. And then on the fourth day, just to be absolutely certain, I tested myself again. I reverted back to positive.” 

The positive test represents a “Paxlovid rebound” case, a health condition that occurs with some patients: They take the Paxlovid and anywhere from 10 to 14 days from the initial onset of symptoms, they get another recurrence in symptoms.

How common is ‘Paxlovid rebound’?

Data from Pfizer now shows the rebound is happening in about 10% of patients, which is relatively small.

10 %

percentage of of patients in which Paxlovid rebound has been noted, based on drugmaker Pfizer's data.

The US government has spent more than $10 billion to purchase enough Paxlovid pills to treat 20 million people.

Is Paxlovid approved to prevent a person from getting COVID?

No. Paxlovid is under an EUA protocol to treat people with COVID. It is NOT approved as a preventive or prophylactice treatment, to help a person ward off COVID, or in place of getting a vaccine.

The pill remains in high demand despite concerns about the "Paxlovid rebound".

Why do some patients seem to relapse?

In May 2022, Doctors have started reporting rare cases of patients whose symptoms return several days after completing Paxlovid's five-day regimen of pills.

That's prompted questions about whether those patients are still contagious and should receive a second course of Paxlovid.

STOCK Paxlovid
FILE PHOTO: Coronavirus disease (COVID-19) treatment pill Paxlovid is seen in a box, at Misericordia hospital in Grosseto, Italy. Image Credit: Reuters

The Food and Drug Administration, also in May, advised against a second round because there's little risk of severe disease or hospitalisation among patients who relapse.

In April, Dr. Michael Charness reported on a 71-year-old vaccinated patient who saw his symptoms subside but then return 9 days into his illness. Dr. Charness says Paxlovid remains a highly effective drug, especially against Delta against which the EUA was granted. But he says he's not sure if it might be less potent against the current Omicron variant.

How much is Paxlovid? 

The $500 drug treatment was tested and EUA-approved based on its performance against the delta version of the coronavirus.

$ 500

cost of a full course (5 days) of Paxlovid

Could some people just be susceptible to a "relapse"?

Experts hypothesise that the ability to clear the virus after it's suppressed may be different from Omicron to Delta, especially for vaccinated people.

Both the FDA and Pfizer point out that 1% to 2% of people in Pfizer's original study saw their virus levels rebound after 10 days. The rate was about the same among people taking the drug or dummy pills, "so it is unclear at this point that this is related to drug treatment," the FDA stated.

Some experts point to another possibility: The Paxlovid dose isn't strong enough to fully suppress the virus. Andy Pekosz of Johns Hopkins University worries that could spur mutations that are resistant to the drug.

"We should really make sure we're dosing Paxlovid appropriately because I would hate to lose it right now,'' said Pekosz, a virologist. "This is one of the essential tools we have to help us turn the corner on the pandemic.''

How well does Paxlovid work in vaccinated people?

Pfizer tested Paxlovid in the highest-risk patients: unvaccinated adults with no prior COVID-19 infection and other health problems, such as heart disease and diabetes. The drug reduced their risk of hospitalisation and death from 7% to 1%.

What's the way forward?

There's an ongoing large study that includes high-risk vaccinated people who took Paxlovid. No results have been published yet, the study is expected to be completed before the end of the year.

Pfizer said they are also studying several other potential benefits of early use, including whether Paxlovid reduces the length and severity of COVID-19 among households.

Can Paxlovid be used to help prevent COVID-19 infections?

Pfizer recently reported that proactively giving Paxlovid to family members of people infected with COVID-19 didn't significantly reduce their chances of catching it.