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What is Paxlovid and can it cause a viral rebound?

by Health Desk | Published on June 1, 2022 – Updated on June 1, 2022 | Explainer

Paxlovid is an antiviral drug used to treat mild and moderate cases of COVID-19 in people who are at highest risk for hospital admission and severe symptoms, but who are not already hospitalized. Examples of people who might qualify for Paxlovid include older individuals, people who are immunocompromised, and people who are unvaccinated. Recent studies have shown that the medication reduces the risk of hospitalization by 85%.

Paxlovid is actually a combination of two different drugs – nirmatrelvir and ritonavir – packaged together. These medications work together to treat adults with active COVID-19 infections but who do not require supplemental oxygen.

Paxlovid works by making it difficult for COVID-19 to reproduce in the body. It does this by blocking a specific enzyme that SARS-CoV-2 (the virus that causes COVID-19) needs to replicate itself. Without the ability to replicate itself, COVID-19 is much less likely to continue developing, spreading, and causing severe symptoms.

Paxlovid is a prescription medication, so in order to get it a patient has to meet with a medical provider and have it prescribed. Once someone has been prescribed Paxlovid, the standard dose is two tablets (one of nirmatrelvir and one of ritonavir), taken by mouth twice a day for five days. Paxlovid should be taken as soon as possible following a confirmed COVID-19 diagnosis and within five days of the start of symptoms.

Since Paxlovid cannot be used with certain medications, people with COVID-19 infections who are considering Paxlovid should tell a medical provider about all drugs and supplements they take before being prescribed the treatment.

The most frequently reported side effects with Paxlovid are changes in taste, diarrhea, headache, and vomiting. However, side effects likely only affect less than 1 in 10 people.

In addition, some people who have taken Paxlovid have reported what is known as a "viral rebound" after they finish their five-day medication regimen. This type of rebound in which COVID-19 symptoms come back up has been reported between 2 and 8 days after initial recovery and can also include a newly positive viral test after testing negative.

At this point in time, scientists are unsure what this viral rebound means but it may be part of the natural history of the virus in some people regardless of their vaccination status or having taken Paxlovid.

People who have reported a rebound in symptoms have only reported mild issues in the United States and there is no information to suggest more treatment is needed in rebound cases, including another course of Paxlovid.

Context and background

Paxlovid has been authorized for use in COVID-19 patients in many nations, but should only be prescribed to people who fit into one of the following categories:

  • People with a positive COVID-19 test results
  • People who are not hospitalized or have severe or critical cases when treatment begins
  • People whose symptoms started within 5 days
  • People who have mild to moderate illness
  • People who are high risk by having one ore more risk factors for progression to severe disease
  • People who do not have severe renal (kidney) or hepatic (liver) impairment
  • People who are not taking a medication that can interfere with the medication

When used in appropriate patients, Paxlovid has been shown to have a dramatic impact in terms of preventing COVID-19 from advancing to severe disease that could hospitalize or kill high-risk patients.


  1. WHO recommends highly successful COVID-19 therapy and calls for wide geographical distribution and transparency from originator (World Health Organization)
  2. Paxlovid (European Medicines Agency)
  3. COVID-19 Rebound After Paxlovid Treatment (United States Centers for Disease Control and Prevention)

Used with Permission from Health Desk, a public health hub that explains emerging COVID-19 science.

This article was written and edited by the Tayo editorial desk and has been reviewed by an independent panel of subject matter experts.

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