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Clesrovimab

Phase 3

RSV Infection | Monoclonal antibody | Infectious Disease |Merck & Company, Inc.|Last Updated: May 8, 2026

Success Probability

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Market & Valuation

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Trial Design

RandomizedDouble-BlindACTIVE_CONTROLLEDDMC
Total Trials1
Total Enrollment1,003

FDA Designations

No designations recorded

Clinical trial landscape

Clesrovimab · 4 trials · 5 indications

Phase 3 1Phase 2 2Phase 1 1
NCT04938830Clesrovimab (MK-1654) in Infants and Children at Increased Risk for Severe Respiratory Syncytial Virus (RSV) Disease (MK-1654-007)RSV Infection
COMPLETED1,003 Analytics
PHASE3COMPLETED
Clesrovimab (MK-1654) in Infants and Children at Increased Risk for Severe Respiratory Syncytial Virus (RSV) Disease (MK-1654-007)
RSV InfectionUnlock trial analytics

Study Endpoints

Primary Endpoints

Number of Participants With Solicited Injection-site Adverse Events (AEs) in RSV Season 1
Up to 5 days after each dose in RSV Season 1 (Up to ~33 days)

An AE is any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. Solicited injection-site AEs included erythema, pain, and swelling. The number of participants with solicited injection-site AEs in RSV Season 1 is reported.

Number of Participants With Solicited Daily Body Temperatures With Fever in RSV Season 1
Up to 5 days after each dose in RSV Season 1 (Up to ~33 days)

Fever was defined as maximum rectal temperature ≥102.2 °F (39.0 °C) or maximum axillary temperature ≥101.7 °F. The number of participants with solicited daily body temperature of fever in RSV Season 1 is reported.

Number of Participants With Solicited Systemic AEs in RSV Season 1
Up to 5 days after each dose in RSV Season 1 (Up to ~33 days)

An AE is any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. Solicited systemic AEs included decreased appetite, irritability, and somnolence. The number of participants with solicited systemic AEs in RSV Season 1 is reported.

Number of Participants With Anaphylaxis/Hypersensitivity AEs of Special Interest (AESI) in RSV Season 1
Up to 42 days in RSV Season 1

An AE was any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. The number of participants with anaphylaxis/hypersensitivity AESI in RSV Season 1 is reported.

Number of Participants With Rash AESI in RSV Season 1
Up to 42 days in RSV Season 1

An AE was any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. Rash AESI included urticaria and drug eruption. The number of participants with rash AESI in RSV Season 1 is reported.

Number of Participants With Nonserious AEs in RSV Season 1
Up to 42 days in RSV Season 1

An AE was any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. The number of participants with nonserious AEs in RSV Season 1 is reported.

Number of Participants With Serious AEs (SAEs) Through the Duration of Participation in RSV Season 1
Up to 365 days in RSV Season 1

An SAE is any untoward medical occurrence in a clinical study participant that results in death, is life-threatening, requires hospitalization or prolongation of existing hospitalization, results in persistent or significant disability/incapacity, is a congenital anomaly/birth defect, or is another important medical event. The number of participants with SAEs in RSV Season 1 is reported.

RSV Season 1: Number of Participants With Respiratory Syncytial Virus (RSV)-Associated Medically Attended Lower Respiratory Infection (MALRI)
From Day 1 (postdose) to Day 150

Outpatient and inpatient MALRI was defined as the presence of the following in a clinical setting: 1) cough or difficulty breathing; AND 2) 1 or more of wheezing, chest wall in-drawing/retraction, rales/crackles, hypoxemia, tachypnea, or dehydration due to respiratory symptoms; AND 3) RSV-positive reverse transcriptase polymerase chain reaction (RT-PCR) nasopharyngeal sample. Per protocol, the participants with outpatient and inpatient RSV-associated MALRI were reported together for RSV season 1 participants.

RSV Season 1: Percentage of Participants With Solicited Injection-Site Adverse Events (AEs)
From Day 1 (postdose) to Day 5

An AE was any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. Solicited injection-site AEs included redness/erythema, swelling, and pain/tenderness. Per protocol, the percentage of RSV season 1 participants with solicited injection-site AEs were reported.

RSV Season 1: Percentage of Participants With Fever
From Day 1 (postdose) to Day 5

Fever was defined as rectal temperature ≥102.2°F (≥39.0°C) or axillary temperature ≥101.7°F (≥38.7°C). Per protocol, the percentage of RSV season 1 participants with fever were reported.

RSV Season 1: Percentage of Participants With Solicited Systemic AEs
From Day 1 (postdose) to Day 5

An AE was any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. Solicited systemic AEs included irritability, somnolence/drowsiness, and appetite lost. Per protocol, the percentage of RSV season 1 participants with solicited systemic AEs were reported.

RSV Season 1: Percentage of Participants With Anaphylaxis/Hypersensitivity AE of Special Interest (AESI)
From Day 1 (postdose) to Day 42

An AE was any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. Anaphylaxis/Hypersensitivity AESI included anaphylaxis, angioedema, bronchospasm, drug hypersensitivity, dyspnea, hypersensitivity, dysphonia, and wheezing. Per protocol, the percentage of RSV season 1 participants with anaphylaxis/hypersensitivity AESI were reported.

RSV Season 1: Percentage of Participants With Rash AESI
From Day 1 (postdose) to Day 42

An AE was any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. Rash AESI included acute generalized exanthematous pustulosis, drug eruption, drug reaction with eosinophilia and systemic symptoms, erythema multiforme, generalized rash of exfoliative nature, Stevens-Johnson syndrome, toxic epidermal necrolysis, and urticaria. Per protocol, the percentage of RSV season 1 participants with rash AESI were reported.

RSV Season 1: Percentage of Participants With ≥1 Nonserious AE
From Day 1 (postdose) to Day 42

An AE was any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. Per protocol, the percentage of RSV season 1 participants with ≥1 nonserious AE were reported.

Percentage of Participants With Serious Adverse Events (SAEs)
RSV Season 1: Day 1 to Day 365, RSV Season 2: Day 366 to Day 515

An SAE was any untoward medical occurrence that results in death; is life-threatening; required inpatient hospitalization/prolongs existing hospitalization; results in persistent or significant disability/incapacity; is a congenital anomaly/birth defect; or other important medical event. Per protocol, the percentage of participants in RSV season 1 and RSV season 2 with SAEs were reported.

Area Under the Viral Load-time Curve (VL-AUC)
10 days; from Day 2 through Day 11 (inclusive) after viral inoculation (Study Day 31 through Day 40)

The VL-AUC will be determined by reverse transcription qualitative integrated cycler polymerase chain reaction (RT-qPCR) after viral inoculation.

Percentage of Participants Who Experienced At Least One Solicited Injection Site Adverse Event (AE)
Up to Day 5

An AE is any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. Solicited injection site AEs were monitored from Day 1 to Day 5.

Percentage of Participants Who Experienced At Least One Solicited Systemic Adverse Event (AE)
Up to Day 5

An AE is any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. Solicited systemic AEs were monitored from Day 1 to Day 5.

Percentage of Participants Who Experienced At Least One Serious Adverse Event (SAE)
Up to Day 545

An SAE is any untoward medical occurrence that, at any dose, results in death; is life-threatening; requires inpatient hospitalization or prolongation of existing hospitalization; results in persistent or significant injury/incapacity; is a congenital anomaly/birth defect; or is an other important medical event.

Secondary Endpoints

Number of Cases of RSV-associated Medically Attended Lower Respiratory Infection (MALRI) in RSV Season 1
Up to 150 days in RSV Season 1
Number of Cases of RSV-associated Hospitalization in RSV Season 1
Up to 150 days in RSV Season 1
Number of Participants With Solicited Injection-site AEs in RSV Season 2
Up to 5 days postdose in RSV Season 2 (Up to ~400 days)
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Study Design & Arms

AllocationRANDOMIZED
MaskingTRIPLE
ModelPARALLEL
PurposePREVENTION

Treatment Arms

ArmTypeDescription
ClesrovimabEXPERIMENTALParticipants will receive intramuscular (IM) injections of clesrovimab and placebo
PalivizumabACTIVE_COMPARATORParticipants will receive IM injections.
Clesrovimab 105 mgEXPERIMENTALParticipants receive a single intramuscular (IM) administration of clesrovimab 105 mg on Day 1. An eligible subset of participants from RSV Season 1 will enter RSV season 2.
PlaceboPLACEBO_COMPARATORParticipants receive a single IM administration of placebo on Day 1. An eligible subset of participants from RSV Season 1 will enter RSV season 2.
Clesrovimab 100 mgEXPERIMENTALParticipants receive a single IV infusion of clesrovimab 100 mg on Day 1.
Clesrovimab 200 mgEXPERIMENTALParticipants receive a single IV infusion of clesrovimab 200 mg on Day 1.
Clesrovimab 300 mgEXPERIMENTALParticipants receive a single IV infusion of clesrovimab 300 mg on Day 1.
Clesrovimab 900 mgEXPERIMENTALParticipants receive a single IV infusion of clesrovimab 900 mg on Day 1.
Panel A: Pre-term clesrovimab Dose 1EXPERIMENTALPre-term infants will receive clesrovimab Dose 1 via intramuscular (IM) injection and will be followed for up to 365 days.
Panel B: Pre-term clesrovimab Dose 2EXPERIMENTALPre-term infants will receive clesrovimab Dose 2 via IM injection and will be followed for up to 365 days.
Panel C: Pre-term clesrovimab Dose 3EXPERIMENTALPre-term infants will receive clesrovimab Dose 3 via IM injection and will be followed for up to 365 days.
Panel D1: Pre-term clesrovimab Dose 4EXPERIMENTALPre-term infants enrolled prior to AM4 will receive clesrovimab Dose 4 via IM injection and will be followed for up to 365 days.
Panel D2: Pre-term clesrovimab Dose 4EXPERIMENTALPre-term infants enrolled after AM4 will receive clesrovimab Dose 4 via IM injection and will be followed for up to 545 days.
Panel E1: Full-term clesrovimab Dose 4EXPERIMENTALFull-term infants enrolled prior to AM4 will receive clesrovimab Dose 4 via IM injection and will be followed for up to 365 days.
Panel E2: Full-term clesrovimab Dose 4EXPERIMENTALFull-term infants enrolled after AM4 will receive clesrovimab Dose 4 via IM injection and will be followed for up to 545 days.

Interventions

NameTypeDescription
ClesrovimabBIOLOGICALIM injection
PalivizumabBIOLOGICALIM injection
PlaceboBIOLOGICALIM injection
RSV-A Memphis 37bBIOLOGICALApproximately 4Log10 plaque-forming units (PFU)/mL RSV-A virus inoculation strain Memphis 37b administered via intranasal inoculation.
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Eligibility Criteria

Age RangeN/A to 1 Year
SexALL
Healthy VolunteersNo
Study Sites146

Inclusion Criteria: * Participants at increased risk for severe RSV infection recommended to receive palivizumab in accordance with national or local guidelines or professional society recommendations and meet defined criteria for the Early or Moderate Pre-term Group or the chronic lung disease (CL...

Countries:United StatesAustraliaCanadaChileColombiaCzechiaFinlandFranceGermanyGreeceHong KongHungaryItalyJapanMalaysiaMexicoNew ZealandNorwayPeruPuerto RicoSingaporeSouth AfricaSpainTaiwanThailandTurkey (Türkiye)United KingdomArgentinaBelgiumChinaDenmarkIsraelPhilippinesPolandRomaniaSouth Korea
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Recent Changes (Last 90 Days)

MEDIUMJun 8, 2026NCT04938830TRIAL_REMOVED: changed
MEDIUMJun 8, 2026NCT04938830TRIAL_REMOVED: changed
MEDIUMJun 8, 2026NCT04938830TRIAL_REMOVED: changed
MEDIUMJun 8, 2026NCT04938830TRIAL_REMOVED: changed
MEDIUMJun 8, 2026NCT04938830TRIAL_REMOVED: changed

Frequently asked questions about Clesrovimab

What is Clesrovimab used for?

Clesrovimab is an investigational monoclonal antibody being developed for the prevention of respiratory syncytial virus (RSV) infection, including respiratory tract infections caused by RSV. It is intended for use in infants and children at increased risk for severe RSV disease.

Who makes Clesrovimab?

Clesrovimab is being developed by Merck & Company, Inc., which trades under the ticker symbol MRK on the New York Stock Exchange. The company is conducting clinical trials to evaluate the safety and efficacy of this investigational monoclonal antibody for RSV infection.

What phase is Clesrovimab in?

Clesrovimab is in Phase 3 clinical development. A Phase 3 trial (NCT04938830) in infants and children at increased risk for severe RSV disease has been completed. The drug is investigational and has not been approved by regulatory authorities.

What clinical trials is Clesrovimab in?

Clesrovimab has been studied in several completed trials, including NCT03524118 (Phase 1 in infants), NCT04086472 (Phase 2a human challenge study in healthy adults), NCT04767373 (Phase 2 efficacy trial in infants), and NCT04938830 (Phase 3 trial in high-risk infants and children).

How does Clesrovimab work?

Clesrovimab is a monoclonal antibody designed to target respiratory syncytial virus (RSV). It works by binding to the virus to help prevent infection. The drug is being evaluated for its ability to protect infants and children from RSV-related respiratory tract infections.