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Enflonsia Dosage: Why One 105 mg Injection Is Used

Enflonsia (clesrovimab-cfor) is a long-acting monoclonal antibody approved as one 105 mg intramuscular injection for newborns and infants born during or entering their first RSV season. CDC recommends clesrovimab for eligible infants younger than eight months who are not protected through maternal RSV vaccination.

Timeline

  1. 2025-06-09: FDA approved Enflonsia for prevention of RSV lower respiratory tract disease in newborns and infants born during or entering their first RSV season.
  2. 2025-06-25: CDC’s advisory committee voted to recommend clesrovimab for eligible infants younger than eight months.
  3. Before or during the first RSV season: A clinician determines eligibility and timing in light of maternal vaccination and local RSV activity.

Enflonsia is the brand name for clesrovimab-cfor, a long-acting monoclonal antibody that the FDA approved in June 2025 to prevent respiratory syncytial virus lower respiratory tract disease in newborns and infants born during or entering their first RSV season. The approved dose is 105 mg given once as an intramuscular injection by a health professional. [1][2]

The 105 mg amount is a fixed infant dose in the FDA label, supplied as 105 mg in 0.7 mL in a single-dose prefilled syringe. CDC guidance likewise lists the same clesrovimab dose for eligible infants younger than eight months regardless of weight. Families should not calculate, divide or administer the dose themselves; the product is a prescription biologic delivered in clinical care. [2][3]

Clesrovimab is not a vaccine. It supplies an antibody directed at the RSV fusion protein, giving passive protection rather than training the infant’s immune system to manufacture antibodies. The long-acting design is intended to cover a typical first RSV season after a single injection, but it does not treat an RSV infection that has already developed. [2][3]

FDA’s indication and CDC’s public-health recommendation answer different questions. The label describes the population for which the product is approved. CDC recommends that infants younger than eight months born during or entering their first season receive a long-acting RSV antibody when they are not protected through maternal RSV vaccination. Most infants do not need both maternal vaccination and an infant antibody. [2][3]

Timing depends on birth date and RSV season. In most of the continental United States, CDC says infant antibody is usually given from October through March, shortly before the season or within the first week of life for a baby born during it, ideally during the birth hospitalization. Guidance can differ in Alaska, tropical climates and jurisdictions with different RSV patterns. [3]

FDA based approval on two clinical trials covering preterm and full-term infants, including a trial in infants at increased risk because of prematurity, chronic lung disease or congenital heart disease. The FDA snapshot summarizes benefits and harms from the original approval program; it is not a substitute for the current label or a pediatric evaluation. [1][2]

The label contraindicates Enflonsia after a serious hypersensitivity reaction to one of its components and warns about possible hypersensitivity, including anaphylaxis observed with related antibodies. Injection-site reactions can also occur. Parents and caregivers should ask the infant’s pediatric clinician about current CDC eligibility, maternal RSV vaccination, product availability and what symptoms after injection require urgent attention. [2][3]

Sources

  1. FDA — Drug Trials Snapshot: Enflonsia
  2. FDA — Enflonsia prescribing information
  3. CDC — RSV immunization guidance for infants and young children

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