RSV Prophylaxis — Long-Acting Monoclonal Antibody Pregnancy: §4.6 states 'Not applicable' — Beyfortus is for use in neonates and infants

Nirsevimab

Brand names: Beyfortus

Nirsevimab is a long-acting recombinant monoclonal antibody given for passive immunisation against respiratory syncytial virus (RSV) lower respiratory tract disease in infants.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: Not applicable — no adult dose is given in the SPC. Beyfortus (nirsevimab) is indicated for neonates and infants; the SPC states that safety and efficacy in children aged 2 to 18 years have not been established and no data are available

Route: Intramuscular injection only, preferably in the anterolateral aspect of the thigh

Frequency: Single dose per RSV season (see paedDose)

The gluteal muscle should not be used routinely as an injection site because of the risk of damage to the sciatic nerve; if two injections are required, different injection sites should be used. Infants during their first RSV season: 50 mg for body weight <5 kg, 100 mg for body weight >=5 kg, administered from birth for infants born during the RSV season, and ideally prior to the RSV season for those born outside it. Children who remain vulnerable to severe RSV disease through their second RSV season: 200 mg as two intramuscular injections (2 x 100 mg), ideally before the start of the second season. Dosing in infants with a body weight from 1.0 kg to <1.6 kg is based on extrapolation with no clinical data; exposure in infants <1 kg is anticipated to be higher than in heavier infants, so benefits and risks in infants <1 kg should be carefully considered. Limited data in extremely preterm infants (gestational age <29 weeks) less than 8 weeks of age; no clinical data in infants with a postmenstrual age of less than 32 weeks.

Paediatric dose

Route: Intramuscular injection only, preferably in the anterolateral aspect of the thigh

Frequency: A single dose per RSV season — from birth for infants born during the season, otherwise ideally before the season starts

Dosing is by weight band and season, not per kilogram, so dosePerKg is null. First RSV season: a single dose of 50 mg for infants with body weight <5 kg, and a single dose of 100 mg for infants with body weight >=5 kg. Second RSV season (children who remain vulnerable to severe RSV disease): a single dose of 200 mg given as two intramuscular injections (2 x 100 mg). Cardiac surgery with cardiopulmonary bypass: an additional dose may be given as soon as the individual is stable after surgery — if within 90 days of the first dose, give 50 mg or 100 mg according to body weight during the first season, or 200 mg during the second season; if more than 90 days have elapsed, the additional dose could be a single 50 mg dose regardless of body weight during the first season, or 100 mg during the second season.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Hypersensitivity to the active substance or to any of the excipients (§4.3)

Side effects

Rash (0.7%, uncommon) occurring within 14 days post dose, the majority mild to moderate
Pyrexia (0.5%, uncommon) within 7 days post dose
Injection site reactions (0.3%, uncommon; non-serious) — pain, induration, oedema, swelling
Hypersensitivity (frequency not known; from spontaneous reporting)

Interactions

No drug interaction data are given; the US label notes only that nirsevimab does not interfere with RT-PCR or rapid antigen detection RSV diagnostic assays using commercially available antibodies targeting antigenic site I, II or IV on the RSV fusion protein — where an immunological assay is negative but clinical observations are consistent with RSV infection, confirm with an RT-PCR-based assay (US BEYFORTUS label §7; the UK SPC §4.5 was not retrieved in this bundle)

Clinical monograph

How it works

It is a humanised IgG1 monoclonal antibody that binds a conserved epitope on the prefusion form of the RSV fusion (F) protein, neutralising the virus and preventing fusion with and entry into host cells.

Prescribing in practice

As with any biological agent, serious hypersensitivity including anaphylaxis is possible, so administer where reactions can be managed and avoid in known hypersensitivity to the product.
It provides passive immunity for an RSV season as a single intramuscular dose, rather than active vaccination, so it confers no lasting immunological memory.
Administer in line with the SPC and national immunisation guidance for the relevant infant cohorts.

Monitoring

Observe for immediate injection-site and hypersensitivity reactions after administration; no routine laboratory monitoring is required.

Counselling the patient

Explain that this single injection helps protect the baby against severe RSV chest infection for the RSV season.
It is given as a passive antibody and is not a routine childhood vaccine.
Report any rash, swelling, breathing difficulty or signs of an allergic reaction after the injection.

Evidence & guidelines

Randomised trials (including the MELODY and HARMONIE studies) demonstrated that nirsevimab reduces medically attended and hospitalised RSV lower respiratory tract infection in infants, supporting its adoption in national RSV programmes.

Reference: Hammitt et al. NEJM 2022 (MELODY trial); NICE TA885; MHRA SPC Beyfortus; JCVI RSV immunisation programme 2024; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways