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  • U.S. Centers for Disease Control and Prevention (CDC) Advisory Committee on Immunization Practices (ACIP) voted to recommend ENFLONSIA™ (clesrovimab-cfor) as an option for the prevention of respiratory syncytial virus (RSV) lower respiratory tract disease in infants younger than 8 months of age who are born during or entering their first RSV season

    This recommendation is provisional and will be official once reviewed and finalized by the CDC Director or the Health and Human Services Secretary (in the absence of a CDC Director)

ENFLONSIA is available to order
ahead of RSV season 2025

You will be able to order directly from Merck and
through select Merck authorized distributors

ENFLONSIA™ (clesrovimab-cfor) is Now FDA-Approved

ENFLONSIA is the first and only RSV preventive option administered to infants using the same dose regardless of weight

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See the data

See the data

Safety profile

Safety profile

Operational profile

Operational profile

Clinical data showed significant reductions in RSV disease incidence and hospitalizations

Incidence of RSV-Associated Disease in infants born at greater than or equal to 29 weeks GA from DAYS 1 Through 150 Post-Dose. Medically Attended Superscript b LRI (requiring > 1 indicator of LRI or Severity) Primary Endpoint. Criteria, Coughing/Difficulty Breathing and > 1 Indicator of LRI (Wheezing, Rales, Crackles) or > 1 indicator of severity (chest wall in-drawing/retractions, hypoxemia, tachypnea, and dehydration Due to Respiratory Systems). Incidence Rate (# of Cases) in ENFLONSIA (n=2,411): 0.026 (60). Incidence Rate (# of Cases) in Placebo (n =1,203): 0.065 (74). Efficacy Results (95% Cl) Superscript a (p-value): 60.5% (44.2, 72.0) (p<0.001). Hospitalization Secondary Endpoint. Criteria, Admitted for Respiratory Illness. Incidence Rate (# of Cases) in ENFLONSIA (n=2,411): 0.004 (9). Incidence Rate (# of Cases) in Placebo (n =1,203): 0.024 (28). Efficacy Results (95% Cl) Superscript a (p-value): 84.3% (66.7, 92.6) (p<0.001).Incidence of RSV-Associated Disease in infants born at greater than or equal to 29 weeks GA from DAYS 1 Through 150 Post-Dose. Medically Attended Superscript b LRI (requiring > 1 indicator of LRI or Severity) Primary Endpoint. Criteria, Coughing/Difficulty Breathing and > 1 Indicator of LRI (Wheezing, Rales, Crackles) or > 1 indicator of severity (chest wall in-drawing/retractions, hypoxemia, tachypnea, and dehydration Due to Respiratory Systems). Incidence Rate (# of Cases) in ENFLONSIA (n=2,411): 0.026 (60). Incidence Rate (# of Cases) in Placebo (n =1,203): 0.065 (74). Efficacy Results (95% Cl) Superscript a (p-value): 60.5% (44.2, 72.0) (p<0.001). Hospitalization Secondary Endpoint. Criteria, Admitted for Respiratory Illness. Incidence Rate (# of Cases) in ENFLONSIA (n=2,411): 0.004 (9). Incidence Rate (# of Cases) in Placebo (n =1,203): 0.024 (28). Efficacy Results (95% Cl) Superscript a (p-value): 84.3% (66.7, 92.6) (p<0.001).

Efficacy endpoints evaluated required an RSV-positive RT-PCR NP sample.

aEfficacy for MALRI (requiring ≥1 indicator of LRI or severity) and hospitalization based on relative risk reduction against placebo adjusted for hemisphere at randomization, gestational age group, and age group at randomization.

bEstimate and 95% CI of efficacy were estimated from the modified Poisson regression with robust variance method.

cMedically attended included all health care provider visits in settings such as outpatient clinic, clinical study site, emergency department, urgent care center, and/or hospital.

CI, confidence interval; IM, intramuscular; GA, gestational age; LRI, lower respiratory infection; MALRI, medically attended lower respiratory infection; NP, nasopharyngeal; RT-PCR, reverse transcription polymerase chain reaction.

Designed to protect infants against a spectrum of RSV disease severity, including worsening disease requiring hospitalization

Study design (CLEVER 004 Trial)1 The CLEVER 004 trial was a Phase 2b/3, randomized, double-blind, placebo-controlled trial to evaluate the safety and efficacy of ENFLONSIA in healthy early and moderate preterm infants (≥29 to <35 weeks gestational age) and late preterm and full-term infants (≥35 weeks gestational age) entering their first RSV season. Participants were randomized 2:1 to receive a single 105 mg dose of ENFLONSIA or saline placebo by IM injection.

Additional data from CLEVER trial1,5

See study limitations

Incidence of RSV-Associated Disease in in infants born at greater than or equal to 29 weeks GA from Days 1 Through Post-Dose Superscript 5. LRI Hospitalization, Exploratory Endpoint. Criteria, Admitted for respiratory illness AND Coughing/Difficulty Breathing AND > 1 Indicator of LRI (Wheezing, Rales, Crackles) or > 1 Indicator of Severity (Chest Wall in-Drawing/Retractions, Hypoxemia, Tachypnea, and Dehydration Due to Respiratory Systems) Incidence Rate (# of Cases) in ENFLONSIA (n=2,398): 0.002 (5). Placebo (n=1,201): 0.023 (27). Observed Results Per Exploratory Endpoints: 90.9%. Severe Medically Attended LRI Exploratory Endpoint. Criteria, Coughing/Difficulty Breathing and Severe Disease (Severe Hypoxemia or the Need for Supplemental Oxygen or Mechanical Ventilatory Support). Incidence Rate (# of Cases) in ENFLONSIA (n=2,398): 0.001 (2). Placebo (n=1,201) 0.01 (12). Observed Results Per Exploratory Endpoints: 91.7%.Incidence of RSV-Associated Disease in in infants born at greater than or equal to 29 weeks GA from Days 1 Through Post-Dose Superscript 5. LRI Hospitalization, Exploratory Endpoint. Criteria, Admitted for respiratory illness AND Coughing/Difficulty Breathing AND > 1 Indicator of LRI (Wheezing, Rales, Crackles) or > 1 Indicator of Severity (Chest Wall in-Drawing/Retractions, Hypoxemia, Tachypnea, and Dehydration Due to Respiratory Systems) Incidence Rate (# of Cases) in ENFLONSIA (n=2,398): 0.002 (5). Placebo (n=1,201): 0.023 (27). Observed Results Per Exploratory Endpoints: 90.9%. Severe Medically Attended LRI Exploratory Endpoint. Criteria, Coughing/Difficulty Breathing and Severe Disease (Severe Hypoxemia or the Need for Supplemental Oxygen or Mechanical Ventilatory Support). Incidence Rate (# of Cases) in ENFLONSIA (n=2,398): 0.001 (2). Placebo (n=1,201) 0.01 (12). Observed Results Per Exploratory Endpoints: 91.7%.

Efficacy endpoints evaluated required an RSV-positive RT-PCR NP sample.5

n=Number of participants eligible for inclusion in the full analysis set population.5

dResults presented reflect those within the CLEVER 004 Clinical Study Report. The denominators of these results, which reflect total evaluated participants for the exploratory endpoints, differ slightly from the denominators reflected in the US Prescribing Information, which inform the primary and key secondary endpoint.5

eEstimate and 95% CI of observed efficacy were estimated from the modified Poisson regression with robust variance method.5

Study Limitations5:

  • No formal statistical testing was planned for the prespecified exploratory endpoints
  • No conclusions can be drawn from these results and they should be interpreted with caution

Demonstrated safety profile of ENFLONSIA™ (clesrovimab-cfor) generally comparable to placebo

Adverse Reactions Reported and at an Incidence Higher Than Placebo (Trial 004). ADVERSE REACTION, Injection Site Erythema Superscript b. ENFLONSIA (n=2,412) Superscript a 3.8%. Placebo (N=1,202) Superscript a 3.3%. ADVERSE REACTION, Injection-Site Swelling Superscript b. ENFLONSIA (n=2,412) Superscript a 2.7%. Placebo (n=1,202) Superscript a 2.6%. ADVERSE REACTION, Rash Superscript c. ENFLONSIA (n=2,412) Superscript a 2.3%. Placebo (N=1,202) Superscript a 1.9%.Adverse Reactions Reported and at an Incidence Higher Than Placebo (Trial 004). ADVERSE REACTION, Injection Site Erythema Superscript b. ENFLONSIA (n=2,412) Superscript a 3.8%. Placebo (N=1,202) Superscript a 3.3%. ADVERSE REACTION, Injection-Site Swelling Superscript b. ENFLONSIA (n=2,412) Superscript a 2.7%. Placebo (n=1,202) Superscript a 2.6%. ADVERSE REACTION, Rash Superscript c. ENFLONSIA (n=2,412) Superscript a 2.3%. Placebo (N=1,202) Superscript a 1.9%.

These adverse reactions were also the most common. 

aSample size reflects the number of participants included in the safety analysis population.

bOccurring within 5 days post-dose; Solicited on Day 1 through Day 5 post-dose using an electronic diary device.

cOccurring within 14 days post-dose; Defined by the following grouped preferred terms: rash, rash erythematous, rash macular, rash papular, rash maculo-papular, rash vesicular, rash exfoliative, dermatitis allergic, drug eruption, and toxic skin eruption.

Most (≥97%) of the adverse reactions were toxicity grade 1 (mild) or grade 2 (moderate).

Convenient dosing and simple ordering process

First and only RSV preventive option administered to infants using the same dose regardless of weight2,3

30-month shelf life4

Purchased doses may be used for the next RSV season for infants entering their first RSV season

Simple ordering process to help meet the needs of your practice

Dosage and administration

The recommended dose is 105 mg administered as a single intramuscular (IM) injection.

Administer once starting from birth for infants born during the RSV season. For infants born outside of the RSV season, administer prior to the start of their first RSV season.

For infants undergoing cardiac surgery with cardiopulmonary bypass during the RSV season, an additional 105 mg dose is recommended as soon as the infant is stable after surgery to ensure adequate clesrovimab‑cfor serum levels.

References

  1. Identifier NCT04767373. Efficacy and safety of clesrovimab (MK-1654) in infants (MK-1654-004) (CLEVER). ClinicalTrials.gov. Last update posted May 6, 2025. Accessed June 5, 2025. https://clinicaltrials.gov/study/NCT04767373
  2. Beyfortus. Prescribing Information. Sanofi; 2024.
  3. Synagis. Prescribing Information. Sobi Inc; 2021.
  4. Data available on request from the Merck National Service Center via email at daprequests@merck.com. Please specify information package US-RSV-00285.
  5. Data available on request from the Merck National Service Center via email at daprequests@merck.com. Please specify information package US-RSV-00288.

Indications and Usage

ENFLONSIA is indicated for the prevention of respiratory syncytial virus (RSV) lower respiratory tract disease in neonates and infants who are born during or entering their first RSV season.

Selected Safety Information

Do not administer ENFLONSIA to infants with a history of serious hypersensitivity reactions, including anaphylaxis, to any component of ENFLONSIA.

 

Serious hypersensitivity reactions, including anaphylaxis, have been observed with other human immunoglobulin G1 (IgG1) monoclonal antibodies. If signs or symptoms of a clinically significant hypersensitivity reaction or anaphylaxis occur, initiate appropriate medications and/or supportive therapy.

 

The most common adverse reactions were injection-site erythema (3.8%), injection-site swelling (2.7%), and rash (2.3%).

 

Before administering ENFLONSIA, please read the accompanying Prescribing Information. The Patient Information also is available.

Indications and Usage
Selected Safety Information

ENFLONSIA is indicated for the prevention of respiratory syncytial virus (RSV) lower respiratory tract disease in neonates and infants who are born during or entering their first RSV season.

Selected Safety Information

Do not administer ENFLONSIA to infants with a history of serious hypersensitivity reactions, including anaphylaxis, to any component of ENFLONSIA.

 

Serious hypersensitivity reactions, including anaphylaxis, have been observed with other human immunoglobulin G1 (IgG1) monoclonal antibodies. If signs or symptoms of a clinically significant hypersensitivity reaction or anaphylaxis occur, initiate appropriate medications and/or supportive therapy.

 

The most common adverse reactions were injection-site erythema (3.8%), injection-site swelling (2.7%), and rash (2.3%).

 

Before administering ENFLONSIA, please read the accompanying Prescribing Information. The Patient Information also is available.