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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.
  • Studies suggest that the “rebound” of symptoms occurs in only a small number of patients.
  • 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.

Research suggests that this “rebound” occurs in a very small number of patients, with most people having mild or moderate symptoms upon their return.

More research is needed to confirm these results, but here’s what we know so far about this return of COVID-19 symptoms after Paxlovid treatment.

The Centers for Disease Control and Prevention (CDC) 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.

With this medication available for some time now, researchers are learning how often this type of rebound occurs.

In one study, researchers examined the medical records of over 5,200 people who received a Paxlovid prescription in California between December 2021 and May 2022 to see how many might have had a return of symptoms.

In the five to 15 days after the prescription was filled, less than 1 percent of those people were seen at the emergency department or hospitalized with a positive COVID-19 test result.

About half of those with recurrent symptoms were over 65 years of age, and over three-quarters had an underlying medical condition.

This study, though, only captures people whose rebound symptoms were severe enough that they needed medical attention at a hospital. Some of these people may have also shown up at the hospital for a non-COVID reason and tested positive.

Additional people in this study may have had a rebound, but their symptoms were so mild that they didn’t seek medical care.

The study was published June 21 in the CDC’s journal Morbidity and Mortality Weekly Report (MMWR).

Another study, published June 14 in Clinical Infectious Diseases, found that of nearly 500 high-risk patients treated with Paxlovid, only 0.8 percent experienced rebound symptoms, which were generally mild.

This research suggests that the rebound of symptoms after Paxlovid treatment is, in fact, uncommon.

It also fits with data from 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 May.

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 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 planning studies on this.

One study by researchers at the University of California San Diego School of Medicine suggests that some people may experience rebound symptoms due to not enough of the drug reaching their infected cells to stop the virus from replicating.

This could be due to the drug being broken down too quickly in those people or the drug needing to be given over a longer period, the researchers suggested.

This study was published June 20 in Clinical Infectious Diseases.

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.

Preliminary data released by Pfizer on June 14 suggests that the medication may not meaningfully benefit people without underlying medical conditions.

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.