Coronavirus: What is a ‘rebound’ COVID-19 case and what causes it?

The White House announced Saturday that President Joe Biden had experienced a “rebound” COVID-19 infection following a five-day course of treatment with the antiviral drug Paxlovid.

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While it is rare for someone who has tested positive, been given antiviral drugs and tested negative to then test positive again, some doctors are seeing such cases in those who have taken Paxlovid.

After first testing positive for the coronavirus on July 21, Biden began a treatment of Paxlovid and then tested negative on Tuesday evening. The White House announced that Biden had tested positive again on Saturday.

The president’s physician, Dr. Kevin C. O’Connor, released a statement on Sunday saying that Biden “continues to feel well,” but had tested positive again on Sunday. O’Connor said Biden will continue conducting business while remaining under “strict isolation.”

What is a “rebound” infection and is it more likely with Paxlovid? Here’s what we know currently.

What is Paxlovid?

Paxlovid is an antiviral drug treatment comprising two drugs — nirmatrelvir and ritonavir — that work together to suppress SARS-CoV-2 by blocking an enzyme that allows the virus to replicate in the body.

The drug needs to be started within five days of the appearance of symptoms. It is taken twice daily for five consecutive days.

Paxlovid has been shown in clinical trials to reduce the risk of hospitalization and death from COVID-19 by 89%. It was made available under an emergency use authorization from the U.S. Food and Drug Administration in December 2021.

What is a rebound infection?

According to the Centers for Disease Control and Prevention, a COVID-19 rebound case happens when a person has “a recurrence of symptoms or a new positive viral test after having tested negative.”

A rebound infection can happen in people who have been vaccinated and/or boosted, as Biden was.

How quickly can a person get a rebound case?

Rebound cases have been reported in patients between two and eight days after their initial recovery.

How common are rebound cases?

Rebound cases are “not rare, but uncommon,” Catherine Bennett, a professor of epidemiology at Deakin University in Australia told The Washington Post. According to Bennett, rebound cases happen in about 10% of people who’ve been treated with Paxlovid.

Why would Paxlovid cause rebound cases?

Researchers at the University of California San Diego School of Medicine say some of those given Paxlovid have a chance at a rebound case because they do not get enough of the drug to fight off the virus completely.

A study published last month in Clinical Infectious Diseases showed that higher dosages of the drug or treatment with it for a longer period of time could be needed.

Is a rebound case a new infection of COVID-19 or the original infection?

“Based on information from the case reports, COVID-19 rebound did not represent reinfection with SARS-CoV-2 or the development of resistance to Paxlovid,” CDC guidance said.

“Our main concern was that the coronavirus might be developing resistance to Paxlovid, so to find that was not the case was a huge relief,” said Dr. Aaron Carlin, assistant professor at UC San Diego School of Medicine.

What should you do if you have a rebound case of COVID-19?

If someone has a rebound case of COVID-19, they should continue to wear a mask for 10 days after the rebound, according to CDC guidance.

“There is currently no evidence that additional treatment is needed with Paxlovid or other anti-SARS-CoV-2 therapies in cases where COVID-19 rebound is suspected,” the CDC guidance reads.