The RSV antibody window is open. First-season and second-season are not the same shot.

Editorial context for parents, not medical advice. Eligibility and product choice belong with the baby's clinician and the hospital pharmacy.

## What changed

In most of the continental United States, the infant RSV antibody window runs October through March. 1 October 2026 is the first day of that window this season.

Two long-acting infant antibodies are now on the table for babies under 8 months entering their first RSV season: nirsevimab and clesrovimab. CDC does not prefer one over the other in that first-season group. For children 8 through 19 months at increased risk who are entering a second RSV season, only nirsevimab is recommended. Clesrovimab is not.

Palivizumab, the older monthly product, stopped being available after 31 December 2025. Families who remember "Synagis every month" are looking at a different product and a different schedule.

Maternal RSV vaccine (Pfizer's Abrysvo) is still given at 32-36 weeks of pregnancy. If that shot was given at least 14 days before birth, most first-season infants do not also need an infant antibody. If the mother was not vaccinated, the date is unknown, or the baby arrived within 14 days of the maternal dose, the infant antibody is the usual path.

## Background parents actually need

RSV is the winter virus that fills infant wards. The antibody is not a vaccine. It is passive protection that lasts about five months.

Timing:

- Born October-March: aim for the dose in the first week of life, ideally before discharge.
- Born April-September, now entering first season: October or November is the target.
- Second-season high-risk (8-19 months): nirsevimab shortly before the season.

Weight bands matter for nirsevimab under 8 months (50 mg under 5 kg; 100 mg at or above 5 kg). The clinic picks the vial.

## Competing viewpoints

**Give it in the birth hospital.** That is CDC's preferred path for October-March births.

**The outpatient backlog is real.** Not every nursery stocks both products. Some families leave with a call-your-pediatrician note and no appointment.

**Maternal vaccine first.** One prenatal shot covers most babies when the date is known and the 14-day gap is met. Dates get lost.

**Second-season confusion.** Not every toddler qualifies. Second-season nirsevimab is for a defined high-risk group.

## What to watch next

- Discharge paperwork: maternal RSV date written down, or unknown written down.
- October-November clinic capacity for April-September babies.
- Product on the shelf.

Sources: CDC RSV immunization guidance for infants and young children; CDC parent page on immunizations to protect infants.

Closes GitHub issue #147.