RSV monoclonal antibodies: nirsevimab, clesrovimab and infant protection

Explain passive protection in plain words

A vaccine teaches the immune system to make its own antibodies over time. A monoclonal antibody supplies the antibodies directly, so protection does not depend on the infant's immune response. CDC describes these products as passive immunity that does not activate the immune system. That is why a single injection of nirsevimab or clesrovimab can protect an infant through one RSV season.

These products prevent severe RSV illness; they do not treat an active infection, and a protected infant can still catch RSV. Parents should know that the antibody reduces risk, while hand hygiene, avoiding sick contacts and recognising breathing difficulty still matter during RSV season.

Know who qualifies and how maternal vaccination fits

CDC recommends a monoclonal antibody for infants under eight months entering their first RSV season when the mother did not receive RSV vaccine in pregnancy, her vaccination status is unknown, or the infant was born within 14 days of maternal vaccination. Most infants do not need both maternal vaccine and an infant antibody, so check the maternal record first.

Some children aged eight to nineteen months at increased risk, such as those with chronic lung disease from prematurity, severe immunocompromise, certain cystic fibrosis criteria, or American Indian and Alaska Native children, are recommended to receive nirsevimab before their second season; clesrovimab is not used for this group. Palivizumab, the older monthly product, stopped being available in the United States at the end of 2025, so expect nirsevimab and clesrovimab in current questions.

Timing is part of eligibility. In most of the continental United States, CDC places administration from October through the end of March, ideally before the season starts for older infants. Local timing can differ, so follow regional guidance where you work, and check the infant's record before every season to avoid duplicate or missed doses.

Give the intramuscular injection safely

These antibodies are given by intramuscular injection, typically into the anterolateral thigh in infants. Use standard infant IM technique, positioning and comfort measures. Timing usually aligns with RSV season, and infants born during the season are often immunised during the birth hospitalisation. The antibody can be given at the same visit as routine childhood vaccines.

Before giving, ask about bleeding or clotting disorders, low platelets and anticoagulant use, because an intramuscular injection may need extra care. Ask about previous serious allergic reactions to the product. Document the product, site and date, and give parents a record, because the second-season decision depends on accurate history.

Monitor for reactions and teach warning signs

Common effects include injection-site discomfort, redness and swelling and, with nirsevimab, rash. Serious allergic reactions are uncommon but possible: facial swelling, difficulty breathing, hives or bluish skin or lips need immediate action using the anaphylaxis protocol.

Teach parents the signs of RSV illness that still warrant care, such as fast or laboured breathing, poor feeding, fewer wet nappies or pauses in breathing. A protected infant can still become unwell, and early recognition remains essential, particularly in premature infants or those with heart or lung disease.

Work a hypothetical newborn question

Imagine a hypothetical healthy term newborn born in November. The mother received RSV vaccine four weeks before delivery. The parent asks whether the baby also needs nirsevimab. Options include giving nirsevimab to every newborn regardless of maternal history, explaining that the infant is likely protected through maternal vaccination, or delaying any decision until the baby is two months old.

Explaining maternal protection is strongest, because vaccination more than 14 days before birth means most infants do not need an antibody as well. The answer changes if the baby were born within 14 days of vaccination or the maternal record were unavailable. In practice the prescriber and local guidance confirm eligibility, including any exceptional risk factors.

Sources and further reading

CDC: RSV immunization guidance for infants and young children. Nirsevimab and clesrovimab, eligibility by maternal vaccination status, second-season high-risk groups for nirsevimab only, timing, co-administration, palivizumab no longer available and passive immunity.

MedlinePlus: Nirsevimab-alip. Monoclonal antibody not a vaccine, single IM injection per season, side effects, allergic signs and bleeding disorder precautions.

MedlinePlus: Clesrovimab-cfor Injection. Monoclonal antibody that is not a vaccine, single intramuscular thigh injection in the first RSV season, side effects and allergic warning signs.