A small number of people who have COVID-19 experience a return of COVID-19 symptoms after initially feeling better.

This clinical trial is evaluating if an additional 5-day course of PAXLOVIDTM may provide benefit for adults and children at least 12 years of age with rebound of COVID-19 symptoms.

The development of safe and effective treatments is invaluable in the fight against COVID-19.

If your COVID-19 symptoms return after completing an initial 5-day course of PAXLOVID treatment, please consider this clinical trial. By joining this study, you will help Pfizer understand why some people experience a rebound of COVID-19 symptoms and may need an additional 5-day course of PAXLOVID.

Card Summary Component

Who may participate

Joining a clinical trial is an important decision. Thank you for your interest.


This study may be an option for people who:

  • Are 12 years of age or older and weigh at least 40 kg (88 lbs.)
  • Have completed an initial 5-day course of PAXLOVIDTM (nirmatrelvir/ritonavir) and experienced initial improvement in COVID-19 signs/symptoms followed by a worsening (rebound) of COVID-19 signs/symptoms
  • Are willing to take a COVID-19 test at a study site within 48 hours of first symptom rebound  
  • Possibly have an increased risk of developing severe illness from COVID-19 (for example but not limited to: high blood pressure, obesity, diabetes, heart disease)

There are other requirements to participate in this study. A study team member will help determine if this study is right for you.



The disease, disorder, syndrome, illness, or injury that is being studied. On, conditions may also include other health-related issues, such as lifespan, quality of life, and health risks.


12+ years

The age a person must be to participate in a clinical trial.


Male or Female

The sex of people who may participate in a clinical trial. Sex is a person’s classification based on assignment at birth. Eligibility based on sex is distinct from eligibility based on gender.

Each clinical study has its own guidelines for who can participate, called eligibility criteria. However, only the research study staff can determine if you qualify to enroll in the study.

What to expect

People who are eligible for this clinical trial can expect to be enrolled in the study for about 24 weeks and will need to attend at least 10 study visits in-person or over the phone/video.

Participants will be randomly assigned to receive the study drug or placebo. The study doctor will not know who has been assigned to which drug.

PAXLOVID™ (nirmatrelvir and ritonavir) or placebo (drug with no active ingredient) and ritonavir, will need to be taken by mouth twice daily for 5 days (10 total doses).

Participants will need to track their doses and COVID-19 signs/symptoms in an electronic diary (eDiary) app on their mobile device or a device provided by the study team.

Length of study treatment

5 days (10 doses)

Length of study treatment is the length of time the study participants will receive the study treatment.

Number of study visits

At least 10

Each clinical trial’s design specifies the number of study visits and the total length of the trial.

Long-term follow up

24 weeks (6 months)

Some studies require the study team to stay in contact with the participant for a period of time after the participant completes the main part of the study. This long-term follow up is to collect additional information on the study drug over time.



PAXLOVIDTM is a combination of the medicines nirmatrelvir and ritonavir.

PAXLOVID has not been approved, but has been authorized for emergency use by FDA under an EUA, for the treatment of mild-to-moderate COVID-19 in adults and pediatric patients (12 years of age and older weighing at least 40 kg) with positive results of direct SARS-CoV-2 viral testing, and who are at high risk for progression to severe COVID-19, including hospitalization or death.

The emergency use of PAXLOVID is only authorized for the duration of the declaration that circumstances exist justifying the authorization of the emergency use of drugs and biological products during the COVID-19 pandemic under Section 564(b)(1) of the Act, 21 U.S.C. § 360bbb-3(b)(1), unless the declaration is terminated or authorization revoked sooner.

About COVID-19 symptom rebound

About COVID-19 symptom rebound

COVID-19 symptom rebound is a return of (or incomplete recovery from) COVID-19 symptoms that occurs in a small percentage of people, COVID-19 symptom rebound is not a separate infection caused by a re-exposure to COVID-19.

A previous study of high-risk participants who were treated with PAXLOVIDTM found a small number of participants (0.8%) experienced a return (rebound) of COVID-19 signs/symptoms after completing treatment. These participants’ symptoms initially improved with PAXLOVID, but later returned or worsened an average of 9 days after completing treatment. In most cases of COVID-19 rebound, symptoms are mild and do not increase the risk of severe illness or hospitalization.

Frequently asked questions

How long will I be enrolled in this study?

About 24 weeks (6 months).

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How likely am I to receive PAXLOVID as part of this study?

Participants will be randomly assigned to receive the study drug PAXLOVIDTM (nirmatrelvir/ritonavir) or placebo (drug with no active ingredient) and ritonavir. This means that you are twice as likely to receive PAXLOVID than not. Neither you nor the study doctor will know what you have been assigned to receive.

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How often will I need to attend study visits?

You will need to attend at least 10 study visits over about 6 months. Your first visit will be in-person at the study site. The next 7 visits may be a combination of in-clinic or at your home. Your last 2 appointments will be 3-month and 6-month follow-up phone calls.

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What if I change my mind about participating?

Your participation in this study is completely optional (your choice). You can choose to leave the study at any time, for any reason.

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What are possible side effects?

All clinical trials and treatments come with some risks. Your study doctor will discuss all the possible risks and benefits of participating in this study with you before you choose to enroll.

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Does it cost anything to participate?

No, the study treatment and study-related procedures will be provided at no cost to you.

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