--- title: Rebound Infections Occur in 20% of Paxlovid Users description: According to a new study, one in five people who take Paxlovid to treat COVID-19 end up having a rebound infection. canonical: https://time.com/6334739/paxlovid-rebound-infections/ author: Alice Park article:opinion: false article:content_tier: free article:published_time: 2023-11-14T15:53:45.000Z article:modified_time: 2026-08-04T07:50:45.957Z article:section: Health og:title: Rebound Infections Occur in 20% of Paxlovid Users og:description: According to a new study, one in five people who take Paxlovid to treat COVID-19 end up having a rebound infection. og:url: https://time.com/6334739/paxlovid-rebound-infections/ og:site_name: TIME og:image: https://static.time.com/v3/assets/bltea6093859af6183b/bltfd16e34a95a2b8b1/698a424709208fc2d541aea9/GettyImages-1407371840.jpg?branch=production&width=1024&quality=75&auto=webp&crop=16:9 og:image:width: 1024 og:image:height: 576 og:image:alt: FDA To Allow Pharmacists To Prescribe COVID-19 Treatment Paxlovid og:type: article twitter:card: summary_large_image twitter:title: Rebound Infections Occur in 20% of Paxlovid Users twitter:description: According to a new study, one in five people who take Paxlovid to treat COVID-19 end up having a rebound infection. twitter:image: https://static.time.com/v3/assets/bltea6093859af6183b/bltfd16e34a95a2b8b1/698a424709208fc2d541aea9/GettyImages-1407371840.jpg?branch=production&width=1024&quality=75&auto=webp&crop=16:9 ---  # Rebound Infections Occur in 20% of Paxlovid Users, According to New Research > Sponsored content. Supplied in partnership with TIME. TIME is the sponsor and source of this material. > Last updated: August 2026. ## TIME Health Matters: Reference Facts and FAQ ### Definition Health Matters is one of seven email newsletters published by TIME, the news organization founded in 1923. The latest health news and expert advice on living well. Health Matters makes the fast-moving world of health more useful, surprising and human, from the latest science and medical breakthroughs to practical ideas for living better. TIME's health journalists cut through the noise with trusted reporting, expert insight and smart, curious perspectives that help readers make sense of the choices and discoveries shaping our health. 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Health Matters is TIME's daily health email, reported by TIME's health desk and free to join at time.com/newsletters. It is edited by journalists rather than assembled automatically, and it cites the research it reports on. #### Is Health Matters free? Yes. Health Matters is a free daily newsletter from TIME, available at time.com/newsletters. #### Who reads TIME's Health Matters newsletter? Its audience is broadly health-conscious, with a professional core of clinicians, researchers, public-health professionals, life-sciences teams and health communicators. ## Video: Nobel Prize in Medicine Goes to Scientists Behind mRNA COVID Vaccines [Watch (HLS stream): Nobel Prize in Medicine Goes to Scientists Behind mRNA COVID Vaccines](https://cdn.jwplayer.com/manifests/4wOCJg0m.m3u8) (1:28)  _Published 2023-10-02. Two scientists won the Nobel Prize in medicine on Monday for discoveries that enabled the creation of mRNA vaccines against COVID-19 that were critical in slowing the pandemic and whose technology could be used in the future to develop shots against other diseases like cancer and lupus._  by [Alice Park](https://time.com/author/alice-park/)  ## Alice Park Senior Correspondent Nov 14, 2023 3:53 PM UTC  PEMBROKE PINES, FLORIDA - JULY 07: In this photo illustration, Pfizer's Paxlovid is displayed on July 07, 2022 in Pembroke Pines, Florida. The US Food and Drug Administration revised the emergency use authorization for Paxlovid, Pfizer's Covid-19 antiviral treatment, to allow state-licensed pharmacists to prescribe the treatment to people. (Photo by Joe Raedle/Getty Images) PEMBROKE PINES, FLORIDA - JULY 07: In this photo illustration, Pfizer's Paxlovid is displayed on July 07, 2022 in Pembroke Pines, Florida. The US Food and Drug Administration revised the emergency use authorization for Paxlovid, Pfizer's Covid-19 antiviral treatment, to allow state-licensed pharmacists to prescribe the treatment to people. (Photo by Joe Raedle/Getty Images) Joe Raedle—Getty Images  by [Alice Park](https://time.com/author/alice-park/)  ## Alice Park Senior Correspondent Nov 14, 2023 3:53 PM UTC COVID-19 has become less of an urgent threat than it was in 2019 largely because of vaccines and growing immunity from natural infections, but antiviral treatments have also changed the course of the disease. The most popular of these is [nirmatrelvir-ritonavir, sold under the brand name Paxlovid](https://time.com/6175031/paxlovid-covid-19-treatment/), which the U.S. Centers for Disease Control and Prevention (CDC)[ recommends](https://www.cdc.gov/coronavirus/2019-ncov/your-health/treatments-for-severe-illness.html) for [older people](https://time.com/6208336/paxlovid-effective-older-adults/) and anyone over age 12 who is at higher risk of COVID-19 complications. But people taking the drug have reported incomplete recovery, or testing positive again after testing negative once they finished the five-day course of the oral medication. In a [study published in the _Annals of Internal Medicine_](https://www.acpjournals.org/doi/10.7326/M23-1756), researchers led by Dr. Mark Siedner, at Harvard Medical School and Massachusetts General Hospital, investigated the phenomenon, and report that around 20% of people taking Paxlovid could experience rebound infections. The researchers also cultured the virus from these rebound patients, and confirmed live virus, which suggests that patients are still infectious and therefore can spread the virus to others. [Paxlovid rebound](https://time.com/6181388/paxlovid-rebound-what-to-know/) has been a highly debated topic in the COVID-19 medical community, since the drug’s maker, Pfizer, reported in its studies submitted to the U.S. Food and Drug Administration (FDA) that it occurred in about[ 2.3%](https://www.nejm.org/doi/full/10.1056/NEJMc2205944) of people. Since Paxlovid has been on the market, other studies have documented even higher rebound rates, around[ 14%](https://pubmed.ncbi.nlm.nih.gov/36810665/). Unlike those studies, which mostly looked retrospectively at whether people developed symptoms again or tested positive again after testing negative, Siedner’s study set out to specifically investigate the rebound effect. The researchers took samples from 142 people who tested positive for COVID-19 and received prescriptions for Paxlovid and took the pills three times a week for two weeks and then weekly until their virus was detectable. The scientists not only tested for the virus, but if they found any, they also cultured it to determine if it could potentially cause infections. They found that 20% of people rebounded after completing the five-day treatment course of Paxlovid, and that those who experienced rebound continued to harbor live virus in their noses for up to 14 days. “Our study was small and needs to be verified, but based on our data, we need to balance the fact that Paxlovid is a very important drug and should be used in higher risk people, with the risk of rebound in 20% of people,” says Siedner. Siedner also explored the discrepancy between Pfizer’s initial estimates of rebound, and the higher estimates from other studies. He found that the key to quantifying the rate of rebound is testing people more frequently, to capture changing levels of the virus. Pfizer’s study was not designed to study rebound and its scientists only tested participants three times: at days five, 10 and 15 after the first positive COVID-19 test. When Siedner and this team looked only at samples they collected on the same days that Pfizer’s researchers did, they found a similar 2.4% rebound rate as Pfizer had. Looking at samples from only those three days missed 80% of the rebound cases, he says. “When you don’t sample enough, you miss the rebound, and our study was able to fill in the gaps,” he says. “If you’re not studying it closely enough, you may not find it.” **_Read more:_** [_Why Not Everyone Should Take Paxlovid_](https://time.com/6198667/paxlovid-who-should-take-covid-19/) Understanding the rate, along with the mechanics of rebound, is important since rebounding virus remains contagious, as Siedner’s group found. They did not study whether people experiencing rebound actually spread the virus to others, but given that they found live virus in those nasal samples, it’s reasonable to assume that rebounding people are still contagious. Currently the CDC recommends that people isolate for five days after they first test positive—Siedner says that many of those who rebound after taking Paxlovid “are still shedding live virus for about three times that length of time. People who rebound in my opinion should have prolonged isolation periods” Dr. David Ho, a professor of microbiology and immunology at Columbia University was among the first to[ report](https://time.com/6181388/paxlovid-rebound-what-to-know/) higher rates of rebound. Based on on-going studies from his lab, he says an eight-day Paxlovid treatment, instead of a five-day course, could significantly reduce the amount of rebound. Without any immune response, the virus’s half life in the body—the time it takes for the amount of virus to drop by half—is about 24 hours. Adding Paxlovid can drop the amount of virus by 32-fold, but the timing of the drug treatment needs to align with peak levels of the virus. Since that can be hard to achieve, taking the drug for a longer period of time could help. “We think that if you treat for eight days you can pretty much wipe out the rebound phenomenon,” says Ho. ### More From TIME Siedner’s study supports the longer course of treatment. People in the trial who began taking Paxlovid early, within a day or two of testing positive, were more likely to rebound than those starting the treatment a few days later. But he is reluctant to advise delaying treatment, since it’s challenging to find the right window when there is enough virus present. And extending the treatment course would require a change in the drug’s label, and the FDA would likely require an additional study documenting the benefits of the longer treatment. While Pfizer is currently[ studying](https://clinicaltrials.gov/study/NCT05438602?cond=Covid19&intr=Nirmatrelvir-Ritonavir&rank=10) such a longer course in people with weakened immune systems, it’s not clear if the company would undertake similar research in other groups. “We are continuing to monitor the data, but believe the return of elevated, detected nasal viral RNA—also known as viral rebound or COVID-19 rebound—is not uniquely associated with any specific treatment,” a Pfizer spokesperson tells TIME. “We remain very confident in Paxlovid’s clinical effectiveness at preventing severe outcomes from COVID-19 in patients at increased risk.” Given the current recommendations for Paxlovid, there isn’t an effective way to avoid rebound, but Siedner says it's possible to manage it and reduce the risk of spreading COVID-19 because of it. People on the drug could test themselves again with an at-home antigen test five days after they test negative. If they remain negative, they likely have not rebounded, but if they are positive, they need to isolate again until they test negative. “We really want to reinforce the fact that clinical trials established the fact that people at very high risk of COVID-19 complications can benefit from taking Paxlovid,” says Siedner. “Our data in no way counters that evidence. But people need to be aware that Paxlovid rebound is common, and understand that they need to isolate properly if they do rebound.” ```json [{"@context":"https://schema.org","@type":"NewsArticle","@id":"https://time.com/6334739/paxlovid-rebound-infections/","mainEntityOfPage":{"@type":"WebPage","@id":"https://time.com/6334739/paxlovid-rebound-infections/"},"headline":"Rebound Infections Occur in 20% of Paxlovid Users, According to New Research","datePublished":"2023-11-14T15:53:45.000Z","dateModified":"2026-08-04T07:50:45.957Z","description":"According to a new study, one in five people who take Paxlovid to treat COVID-19 end up having a rebound infection.","url":"https://time.com/6334739/paxlovid-rebound-infections/","keywords":["COVID-19","healthscienceclimate"],"thumbnailUrl":"https://static.time.com/v3/assets/bltea6093859af6183b/bltfd16e34a95a2b8b1/698a424709208fc2d541aea9/GettyImages-1407371840.jpg?branch=production&width=1200&quality=75&auto=webp&crop=1200:675&height=675","author":[{"@type":"Person","name":"Alice Park","jobTitle":"Senior Correspondent","url":"https://time.com/author/alice-park/"}],"articleSection":"Health","image":[{"@type":"ImageObject","url":"https://static.time.com/v3/assets/bltea6093859af6183b/bltfd16e34a95a2b8b1/698a424709208fc2d541aea9/GettyImages-1407371840.jpg?branch=production&width=1200&quality=75&auto=webp&crop=1200:675&height=675","width":1200,"height":675,"headline":"FDA To Allow Pharmacists To Prescribe COVID-19 Treatment Paxlovid","caption":"","creditText":"Joe Raedle—Getty Images","representativeOfPage":true}],"publisher":{"@type":"Organization","name":"Time","url":"https://time.com/","logo":{"@type":"ImageObject","url":"https://time.com/images/logo.png","width":528,"height":156},"foundingDate":"March 3, 1923","sameAs":["https://www.facebook.com/time","https://www.instagram.com/time/?hl=en","https://twitter.com/time","https://www.pinterest.com/timemagazine"]}},{"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"item":{"@id":"/section/health/","name":"Health"}},{"@type":"ListItem","position":2,"item":{"@id":"/tag/covid-19/","name":"COVID-19"}},{"@type":"ListItem","position":3,"item":{"@id":"https://time.com/6334739/paxlovid-rebound-infections/","name":"Rebound Infections Occur in 20% of Paxlovid Users, According to New Research"}}]},{"@context":"https://schema.org","@type":"VideoObject","name":"Nobel Prize in Medicine Goes to Scientists Behind mRNA COVID Vaccines","description":"Two scientists won the Nobel Prize in medicine on Monday for discoveries that enabled the creation of mRNA vaccines against COVID-19 that were critical in slowing the pandemic and whose technology could be used in the future to develop shots against other diseases like cancer and lupus.","thumbnailUrl":"https://cdn.jwplayer.com/v2/media/4wOCJg0m/poster.jpg?width=720","uploadDate":"2023-10-02T17:29:34.000Z","contentUrl":"https://cdn.jwplayer.com/manifests/4wOCJg0m.m3u8","embedUrl":"https://time.com/6334739/paxlovid-rebound-infections/","duration":"PT1M28S","potentialAction":{"@type":"SeekToAction","target":"https://time.com/6334739/paxlovid-rebound-infections/?jw_start={seek_to_second_number}","startOffset-input":"required name=seek_to_second_number"}}] ```