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  • Some people who have taken Pfizer Inc.’s COVID-19 antiviral report that their symptoms return after completing the 5-day treatment.
  • In Pfizer’s clinical trial, 1 to 2 percent of people treated with the antiviral still tested positive for COVID after finishing the treatment.
  • Experts say that while these cases need to be studied, Paxlovid is still a key treatment for COVID-19.

Some patients who have taken Pfizer Inc.’s oral antiviral Paxlovid are reporting that their COVID-19 symptoms returned after initially improving when they completed treatment.

Here’s what we know so far about this rebounding of symptoms.

The Centers for Disease Control and Prevention said in a statement on May 24 that some people treated with Paxlovid experience “COVID-19 rebound” between two and 8 days after their initial recovery.

These people experience a return of their symptoms and/or a new positive COVID test after having tested negative.

This has occurred in unvaccinated people, as well as those who are vaccinated and boosted.

It’s not clear how common this is.

So far, only one case has shown up in the medical literature as a pre-print.

In this report, the patient’s symptoms cleared up and then returned about a week after treatment. This coincided with an increase in the amount of virus in his body or viral load.

Other people have posted about their rebounding symptoms on social media or reported them to the Food and Drug Administration.

Currently, this type of rebounding appears to be rare.

In Pfizer’s clinical trial, 1 to 2 percent of people treated with the antiviral had a positive COVID-19 test — or an increase in the amount of virus detected — after finishing the treatment.

However, this type of rebound also occurred in people who received the inactive placebo, so it’s not clear if it is related to the drug, said the FDA.

In addition, the agency said that people in the trial whose symptoms recurred did not have a higher risk of hospitalization or death. Nor were there signs that the coronavirus had developed resistance to the drug.

It’s not clear why some people see a recurrence of their symptoms.

It may be “part of the the natural history” of the coronavirus that causes COVID-19, the CDC said, independent of whether someone was treated with Paxlovid or was vaccinated or boosted.

U.S. government researchers are already planning studies on this.

Experts say that while these cases of rebound need to be studied, this should not be seen as a failure of Paxlovid.

Case reports suggest that people who have COVID-19 rebound experience mild illness, with no reports of severe illness, the CDC said. People’s symptoms improved in an average of three days without additional treatment.

In the Pfizer clinical trial, the antiviral reduced the risk of COVID-19-related hospitalization and death by almost 90 percent among non-hospitalized patients at risk of severe illness.

Dr. Jeffrey Klausner, a clinical professor in the Keck School of Medicine of the University of Southern California, said Paxlovid is a “life-saver” — decreasing the amount of virus present in the body, reducing symptoms, and preventing illness from getting worse.

“The medication works exceptionally well at preventing people at risk — [such as] elderly, obese or those with other medical conditions like diabetes or high blood pressure — from ending up the hospital,” said Klausner.

“That’s what is important — preventing people from going to the hospital,” he added.

While some scientists have suggested that a 10-day course of Paxlovid may be needed, the CDC said there is no evidence that people require additional treatment with Paxlovid or another antiviral COVID treatment.

Dr. John Mourani, medical director of infectious disease at Pomona Valley Hospital Medical Center, said if symptoms recur, “the first thing patients should do is contact their primary physician for a COVID antigen check.”

They can also use an at-home test kit. Some doctors recommend having these on hand if you are taking Paxlovid.

Scientists don’t know whether all people whose symptoms recur can spread the virus to others, but they recommend taking steps to protect others from infection.

“If symptoms return after treatment, there is a possibility that someone may still be contagious,” said Klausner. “People should continue isolating and wearing a mask until their symptoms are gone, or until they test negative on a rapid test.”

The CDC recommends that people with COVID rebound treat it as an initial infection and follow the agency’s guidance on isolation.

This includes restarting isolation and isolating for at least five days, plus wearing a mask around others for 10 days after the start of rebound symptoms.

In the United States, Paxlovid is authorized by the FDA for use in people 12 years and older who have tested positive for coronavirus infection and are at high risk for severe illness.

People at higher risk include those with risk factors such as chronic lung or kidney disease, diabetes, obesity, cancer, or any condition that weakens their immune system.

Both vaccinated, and unvaccinated people are eligible to receive Paxlovid. However, vaccination provides an added layer of protection.

“Overall, having the vaccine and therapeutic options in combination are great tool to protect from severe COVID,” said Mourani.

A prescription is required for the antiviral pills, and treatment needs to be started within five days of the symptoms starting.

To get a prescription, you will need to show your positive test result to your health provider and review your risk factors. Some telehealth providers also offer virtual visits to assess your risk and prescribe Paxlovid if it is appropriate.

You can also visit one of the test to treat locations supported by the federal government. These sites offer testing and have Paxlovid on-hand.

In late April, the White House made a new push to get Paxlovid to Americans who could benefit.

Despite this, Klausner is concerned that those most at risk are still not aware of this treatment and are not getting treated.

“We have to do a much better job promoting the medication to at-risk people and making it easier to get,” he said.