When should my baby get a dose of Enflonsia (clesrovimab)?

If your baby is born during RSV season, they should get Enflonsia (clesrovimab) as soon as possible after birth. The [RSV season](https://www.cdc.gov/rsv/hcp/vaccine-clinical-guidance/infants-young-children.html) typically runs from fall (September/October) through spring (March/April), but can vary in different areas of the country. If your baby is born outside of RSV season, they should receive one dose before their first RSV season begins.

What’s the difference between Enflonsia (clesrovimab) and the RSV vaccine?

Enflonsia (clesrovimab) is a [monoclonal antibody](https://www.goodrx.com/classes/monoclonal-antibodies) made in a lab. It provides immediate protection from RSV because antibodies are directly injected into the body. [Abrysvo](https://www.goodrx.com/abrysvo/what-is), on the other hand, is an [RSV vaccine](https://www.goodrx.com/conditions/rsv/rsv-vaccine) approved for women between 32-36 weeks of pregnancy. After Abrysvo is given to the mother, their bodies make antibodies against RSV. And those antibodies are passed to their baby to help protect them from the virus.

How long does Enflonsia (clesrovimab) last?

In clinical studies, Enflonsia (clesrovimab) protected babies from RSV for one RSV season or about [5 months](https://www.nejm.org/doi/full/10.1056/NEJMoa2502984?query=featured_home%20https://www.nejm.org/doi/full/10.1056/NEJMc2506107). Ask your pediatrician if you have any questions about how long Enflonsia (clesrovimab) will protect your baby.

Enflonsia

clesrovimab

Used for RSV

Enflonsia (clesrovimab) is a monoclonal antibody that’s used to prevent respiratory syncytial virus (RSV) in babies who are born during or entering their first RSV season. It isn’t a vaccine. Your child will receive a single injection in the thigh muscle from a healthcare professional any time from birth up to 12 months old. This is usually done either right after birth or before RSV season starts. Common side effects include redness or swelling at the injection site and rash.

Reviewed by:

Samantha Kasting, PharmD Candidate

Daisy Chau, PharmD

Last reviewed on October 3, 2025

What is Enflonsia (clesrovimab)?

What is Enflonsia (clesrovimab) used for?

Prevention of respiratory syncytial virus (RSV) in newborns and infants (up to 12 months old) who are born during or entering their first RSV season

How Enflonsia (clesrovimab) works

Enflonsia (clesrovimab) is a type of monoclonal antibody. It works by attaching to a protein on the surface of the RSV virus called the F protein. When it attaches to the F protein, the medication prevents the virus from infecting healthy cells, which stops the infection from starting. And if your baby does get RSV, Enflonsia (clesrovimab) can help make the illness less severe.

When does Enflonsia (clesrovimab) start working?Time passed since treatment started:

HOURS

Initial effect

DAYS

Full effect

WEEKS

MONTHS

Drug facts

Common BrandsEnflonsia

Drug ClassMonoclonal antibody

Controlled Substance ClassificationNot a controlled medication

Generic StatusNo lower-cost generic available

AvailabilityPrescription only

Side effects of Enflonsia (clesrovimab)

The following side effects may get better over time as your body gets used to the medication. Let your healthcare provider know immediately if you continue to experience these symptoms or if they worsen over time.

Common Side Effects

Redness at the injection site (4%)
Swelling at the injection site (3%)
Rash (2%)

Less Common Side Effects

Diarrhea
Decreased appetite
Sleepiness
Irritability
Fever

Enflonsia (clesrovimab) serious side effects

Contact your healthcare provider immediately if you experience any of the following.

Severe allergic reaction: Swelling of the face or throat, hives, itching

When do Enflonsia (clesrovimab) possible side effects begin and end?

Time passed since treatment started:

HHOURS

DDAYS

WWEEKS

MMONTHS

Swelling at the injection site

Redness at the injection site

Rash

Fever

Source: DailyMed

The following Enflonsia (clesrovimab) side effects have also been reported

Side effects that you should report to your care team as soon as possible:

Allergic reactions—skin rash, itching, hives, swelling of the face, lips, tongue, or throat

Side effects that usually do not require medical attention (report these to your care team if they continue or are bothersome):

Pain, redness, or irritation at injection site
Skin rash

Pros and cons of Enflonsia (clesrovimab)

Pros

One-time injection into your baby’s thigh
Can be given at the same time as other vaccines
Dose is the same regardless of weight
It starts working right away to protect your baby and lasts for about 5 months

Cons

Must be given by a healthcare professional
Only FDA approved for children 12 months and younger
Injection can be uncomfortable
Risk of severe allergic reaction

Pharmacist tips for Enflonsia (clesrovimab)

Enflonsia (clesrovimab) is given as a one-time dose by a healthcare professional at birth or at the beginning of RSV season. It’s typically injected into the muscle of your baby’s thigh.
Enflonsia (clesrovimab) isn’t a vaccine. It is a lab-made antibody that gives babies immediate protection from RSV during their first RSV season.
Even though Enflonsia (clesrovimab) can lower your baby’s risk of getting RSV, they could still get sick from RSV. Pay attention to any symptoms of RSV in your baby such as severe congestion, runny nose, or a low-grade fever.
If your infant has heart surgery during RSV season, talk with your baby’s pediatrician, as they may need a second dose of Enflonsia (clesrovimab).
Your baby may also get other vaccines at the same time as Enflonsia (clesrovimab). Check with their pediatrician if you have questions about your baby getting this medication along with routine vaccines.
Reactions to Enflonsia (clesrovimab) are typically mild. If your baby experiences any redness or swelling around the injection site, contact their pediatrician. These side effects can happen up to 5 days after the injection.
Enflonsia (clesrovimab) can also cause a rash on your baby’s body, typically seen up to 14 days after the injection. If your baby shows any symptoms of a rash, contact their pediatrician.
Enflonsia (clesrovimab) can cause severe allergic reactions. Seek medical attention if your infant has any swelling of the lips, tongue, or throat or trouble breathing as these can be life-threatening.

Common questions about Enflonsia

If your baby is born during RSV season, they should get Enflonsia as soon as possible after birth. The RSV season typically runs from fall (September/October) through spring (March/April), but can vary in different areas of the country. If your baby is born outside of RSV season, they should receive one dose before their first RSV season begins.

Enflonsia is a monoclonal antibody made in a lab. It provides immediate protection from RSV because antibodies are directly injected into the body. Abrysvo, on the other hand, is an RSV vaccine approved for women between 32-36 weeks of pregnancy. After Abrysvo is given to the mother, their bodies make antibodies against RSV. And those antibodies are passed to their baby to help protect them from the virus.

Most of the time, Enflonsia is given as a one-time dose for your baby’s first RSV season. However, if your baby has heart surgery during RSV season, an additional dose may be recommended once your baby is stable after surgery. This extra dose helps make sure there are enough antibodies to protect your baby from an RSV infection.

No. Your baby will only get one dose of Enflonsia. At this time, it’s only approved for your baby’s first RSV season. Talk with your baby’s pediatrician to see if it is a candidate for this medication.

Yes. Your baby can get Enflonsia at the same time as other vaccines. It will likely be given at a different spot on the body than the other injections. So your baby can still get all their vaccines on schedule.

Two clinical studies tested how well Enflonsia works in babies. In one study, infants who got an injection of Enflonsia were 60% less likely than placebo to need medical care for RSV. In the same study, the number of babies needing hospital care for RSV dropped by 80%. In the other study, the results showed that infants who were given Enflonsia were less likely to get an RSV infection that required a visit to their pediatrician. Enflonsia also reduced the severity of RSV in a way that was similar to Synagis (palivizumab), another RSV antibody treatment

In clinical studies, Enflonsia protected babies from RSV for one RSV season or about 5 months. Ask your pediatrician if you have any questions about how long Enflonsia will protect your baby.

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Risks and warnings for Enflonsia (clesrovimab)

Enflonsia (clesrovimab) can cause some serious health issues. This risk may be even higher for certain groups. If this worries you, talk to your doctor or pharmacist about other options.

Severe Allergic Reactions

Enflonsia (clesrovimab) can cause severe allergic reactions like anaphylaxis. Anaphylaxis is a life-threatening reaction that causes swelling of the lips, tongue, or throat that can lead to trouble breathing. If your infant develops any symptoms of anaphylaxis after receiving the injection, seek medical help right away.

Enflonsia (clesrovimab) dosage

Typical dosage for Enflonsia (clesrovimab)

The typical dose is 105 mg injected once into your baby’s thigh muscle by a healthcare professional.

What are alternatives to Enflonsia (clesrovimab)?

There are a number of medications that your doctor can prescribe in place of Enflonsia (clesrovimab). Compare a few possible alternatives below.

Enflonsia (clesrovimab)

Used for:

Prevention of respiratory syncytial virus (RSV) in newborns and infants (up to 12 months old) who are born during or entering their first RSV season

View prices

Beyfortus (nirsevimab)

Used for:

Prevention of respiratory syncytial virus (RSV) in young children of certain ages
- Newborns and infants born during or entering their first RSV season
- Children up to 24 months old at risk of severe RSV illness through their second RSV season

View prices

Synagis (palivizumab)

Used for:

Prevention of serious respiratory syncytial virus (RSV) infections for:

Babies born premature and are 6 months or younger at the start of RSV season
Children diagnosed with bronchopulmonary dysplasia (BPD) within the last 6 months, and are 2 years or younger at the start of RSV season
Children with congenital heart disease (CHD) and are 2 years or younger at the start of RSV season

View prices

Enflonsia (clesrovimab) images

This medicine is Yellow Syringe.yellow - ENFLONSIA 105mg/0.7mL Solution for Injection

References

Best studies we found

Centers for Disease Control and Prevention. (2025). RSV immunization guidance for infants and young children.

McLellan, J. S., et al. (2013). Structure and function of respiratory syncytial virus surface glycoproteins. Current Topics in Microbiology and Immunology.

Merck Sharp & Dohme LLC. (2025). ENFLONSIA- clesrovimab injection, solution [package insert]. DailyMed.

GoodRx Health has strict sourcing policies and relies on primary sources such as medical organizations, governmental agencies, academic institutions, and peer-reviewed scientific journals. Learn more about how we ensure our content is accurate, thorough, and unbiased by reading our editorial guidelines.

Common questions about Enflonsia

In clinical studies, Enflonsia protected babies from RSV for one RSV season or about 5 months. Ask your pediatrician if you have any questions about how long Enflonsia will protect your baby.

Enflonsia