Respiratory syncytial virus (RSV) is a very common virus that is contagious and spreads very easily. It affects the respiratory system. Most infants are infected within the first year of life. While most people have mild symptoms, others can develop more serious symptoms, such as pneumonia, which may require hospitalization. People at highest risk of serious symptoms include the very old, the very young, and people who are pregnant or have certain medical conditions that involve the immune system, heart, or lungs.
RSV vaccines are available for adults over the age of 60 years and for people who are pregnant. Monoclonal antibodies ("mabs") like Enflonsia (clesrovimab) are used to prevent RSV in newborns and infants.
Enflonsia (clesrovimab-cfor) can help prevent RSV disease in newborns and infants who are born during or entering their first RSV season. After it is given, Enflonsia stays in the body for a long time, and its effects can last for up to five months. Because of this, it only needs to be given once before the newborn or infant enters their first RSV season. Enflonsia is usually given by a health care provider.
Other mabs used to help prevent RSV in infants and children include nirsevimab (Beyfortus) and palivizumab (Synagis).
- Synagis is given once monthly and is for infants who were born premature (≤35 weeks) and who are 6 months old or younger at the start of the RSV season. It can also be used in infants and children who have been treated for bronchopulmonary disease (BPD) in the past six months or have congenital heart disease (CHD) and who are ≤24 months of age at the start of the RSV season. It is dosed based on body weight.
- Beyfortus is given once before the infant’s first RSV season starts, and the dose is based on the infant’s body weight. Beyfortus can also be given to children up to age 24 months who are still at risk of severe RSV disease during their second RSV season.
- Enflonsia is given once before the RSV season starts, and the dose is the same for every patient and does not depend on the infant’s body weight.
How Does Enflonsia Work for RSV?
The respiratory syncytial virus has a protein on its surface called the F protein, which helps it enter and infect cells in the respiratory tract or lungs. Enflonsia attaches to the F protein, which stops the virus from entering the human cells.
How Was Enflonsia Studied, and What Types of People Were in These Studies?
Two studies were done to test the effectiveness of Enflonsia in infants and children – Trial 004 and Trial 007.
Trial 004
This study included more than 3,000 infants who were at least 29 weeks old but less than a year old. Infants received either a single dose of Enflonsia (105 mg) injection or a placebo injection before their first RSV season. Placebo injections contained salt water instead of active medicine.
Most of the children in the study (80%) were younger than 6 months old, 16% were between 6 and 9 months old, and 4% were 9 months old or older. About half (51%) were male. About 18% were born between 29 and 35 weeks, and 82% were born at or after 35 weeks. With regard to racial distribution, 45% were White, 27% were Asian, 14% were Black or African American, 12% were multiracial, and 2% were American Indian or Alaska Native. With regard to ethnicity, 28% were Hispanic or Latino.
Trial 007
This study included about 900 infants who were either premature or who had congenital heart disease and were at increased risk of getting severe RSV disease. Infants received either Enflonsia or Synagis. Those who received Enflonsia got a single 105 mg dose followed by a placebo injection a month later. Those who received Synagis got 15 mg/kg of their weight every month for three to five months.
Most (89%) were less than 6 months old, 9% were between 6 and 9 months old, and 2% were 9 months old or older. Most (89%) were less than six months of age, 9% were between 6 and 9 months, and 2% were 9 months or older. Half (50%) were male. About 28% of the infants had chronic lung disease and 11% had congenital heart disease. With regard to racial distribution, 52% were White, 18% were Asian, 15% were Black or African American, 12% were multiracial, and 1% were American Indian or Alaska Native. With regard to ethnicity, about 32% were Hispanic or Latino.
What Are the Main Benefits of Enflonsia, Based on the Studies?
Trial 004
To find out if Enflonsia worked, this study counted the number of lung infections that met certain conditions, such as having at least one symptom of a lower lung infection (pneumonia) or severity. Also, cases had to be diagnosed by a health care provider in a medical setting such as a clinic, emergency department, urgent care center, hospital, or clinic study site. There were fewer lung infections in infants who got Enflonsia (60 infants) compared to infants who got a placebo (74 infants). This translated to a rate of 2.6% over five months for Enflonsia, compared with 6.5% over five months for the placebo. With regard to hospitalization, 0.4% (nine infants) who got Enflonsia were hospitalized, compared with 2.4% (28 infants) who received a placebo.
Trial 007
To find out how Enflonsia worked, compared with Synagis, this study used the same criteria as those used in Trial 004. Like trial 004, this study counted the number of lung infections that met certain conditions, such as having at least one symptom of a lower lung infection or severity. Also, cases had to be diagnosed by a health care provider in a medical setting such as a clinic, emergency department, urgent care center, hospital, or clinic study site. The number of people with these types of respiratory infections was about the same for infants in the Enflonsia and Synagis groups. About 3.6% of infants who got Enflonsia had these types of infections, and about 2.9% of infants who got Synagis had these types of infections. The number of infants hospitalized for RSV was also similar for the two groups. About 1.3% of infants who got Enflonsia were hospitalized, and about 1.5% of infants who got Synagis were hospitalized.
Your child’s results may differ from what was seen in clinical studies.
What Is the Place in Therapy for Enflonsia?
In June 2025, the Advisory Committee on Immunization Practices voted to recommend the use of Enflonsia for infants under 8 months of age who are entering their first RSV season.
Is Enflonsia right for me?
Enflonsia should not be given to children over the age of 12 months. It has not been studied in this age group.
How Can I Manage Side Effects?
The most common side effects reported in studies included redness or swelling where the needle went in and rashes. These types of side effects are usually temporary. You can apply a cold compress to the area to provide relief. Your health care provider or pharmacist may also recommend over-the-counter pain relievers such as acetaminophen.
This is not a complete list of side effects. If side effects do not go away or become bothersome, notify your health care provider.
You can report vaccine side effects to the U.S. Department of Health and Human Services at www.vaers.hhs.gov or by calling 800-822-7967.
