What is Paxlovide rebound?

President Joe Biden tested positive for Covid-19 for the second time on Saturday, nine days after his original diagnosis and several days after testing negative.

The president’s physician, Dr. Kevin O’Connor, said the latest infection was due to “rebound COVID-positiveness” as a result of treatment from the drug Paxlovid.

So what is Pavloxid rebound?

A series of studies have shown that people with Covid-19 can remain contagious long into their second week with the virus after they first experience symptoms.

Harvard TH Chan School of Public Health specialist in infectious diseases, Yonatan Grad told Nature that ten days is a good rule of thumb for when people should no longer be contagious.

But he added that a small number could be contagious beyond ten days, with some of those cases linked to the use of the antiviral drug Paxlovid.

“There is a rebound phenomenon where people will see that their symptoms seem to go away and they may even test negative on a rapid test, but a few days later the symptoms and the virus come back,” he told Nature.

In a July 30 memo, the president’s physician, Dr O’Connor, wrote that “as described last week, recognizing the potential for so-called “rebound” COVID positivity observed in a small percentage of patients treated with PAXLOVID, the president increased his testing cadence, both to protect people around him and to ensure early detection of any return of viral replication”.

“After testing negative on Tuesday night, Wednesday morning, Thursday morning and Friday morning, the President tested positive late Saturday morning by antigen testing. This actually represents ‘rebound’ positivity,” Dr O’Connor added.

“People, today I tested positive for COVID again,” Biden tweeted at 2:45 p.m. Saturday. “This happens to a small minority of people. I have no symptoms, but I’m going to isolate myself for the safety of everyone around me. I’m still at work, and will be back on the road soon.”

“The president has not experienced any re-emergence of symptoms and continues to feel quite well,” Dr O’Connor added. “As this is the case, there is no reason to resume treatment at this time, but we will of course continue careful observation.”

Infection specialist at Massachusetts General Hospital, Amy Barczak, told Nature that “it is very unusual for people to remain contagious after ten days”.

Mr Biden first tested positive on July 21 – nine days ago.

Dr Barczak told Nature that “antivirals change the dynamics of the symptoms, change the dynamics of the immune response, and change the dynamics of how you shed” the virus.

“I think this is very important, because people are out in the world thinking they’re not contagious after ten days. But if they have Paxlovid rebound, they might be,” she added.

In a May 24 health advisory from the Centers for Disease Control and Prevention, the agency said that “Paxlovid continues to be recommended for the early treatment of mild to moderate Covid-19 among people at high risk of progression to severe disease” and that “the treatment helps to prevent hospitalization and death”.

“Covid-19 rebound has been reported to occur between two and eight days after initial recovery and is characterized by the recurrence of Covid-19 symptoms or a new positive viral test after testing negative”, the agency added. “A brief return of symptoms may be part of the natural history of … the virus that causes COVID-19 … in some people, regardless of treatment with Paxlovid and regardless of vaccination status.”

The agency also said that “limited information currently available from case reports suggests that people treated with Paxlovid who experience Covid-19 rebound have had mild disease; there are no reports of serious illness”, adding that “there is currently no evidence that further treatment with Paxlovid or other … therapies is necessary in cases where Covid-19 rebound is suspected”.

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