RSV Protection for Babies, 2023–24: Nirsevimab and Maternal Vaccine Coverage

Summary

• Respiratory syncytial virus (RSV) is the leading cause of hospitalization among U.S. infants.
• In 2023, two new ways to protect babies were recommended: nirsevimab, a long-acting monoclonal antibody given to the infant, and an RSV vaccine given during pregnancy.
• In state immunization information systems covering 33 states and the District of Columbia, about 29% of babies born October 2023–March 2024 were protected by one or the other in that first season — from 11% to 53%, depending on the state.
• More work is needed to raise coverage, and these population-wide registries can track it.

The two options

• Nirsevimab: recommended by CDC's Advisory Committee on Immunization Practices (ACIP) on August 3, 2023, for all infants under 8 months born during or entering their first RSV season (October–March in most of the continental United States).
• Maternal vaccine (Abrysvo, Pfizer): recommended on September 22, 2023, as one dose at 32–36 weeks of pregnancy, given September–January, so the mother's antibodies pass across the placenta and protect the baby for its first 6 months.

One or the other is recommended for each mother and baby, not both, except in rare cases.

How coverage was estimated

Immunization information systems are confidential, population-based registries of immunizations given by health care providers in each jurisdiction; they began sending CDC data on all routine immunizations quarterly in 2023. Researchers counted:

• infants born October 1, 2023–March 31, 2024, who received nirsevimab in that period, plus
• women aged 18–49 who received RSV vaccine September 1, 2023–January 31, 2024 — assumed pregnant, since that is the only group it is recommended for in that age range —

and divided by the number of live births from CDC's WONDER natality data. Records from New York City and Philadelphia were merged with their states. Montana and Pennsylvania require opt-in consent for registry records for all ages, and New York and New York City for adults.

What was found

• 213,659 infants received nirsevimab and 119,879 women received the RSV vaccine. Nirsevimab doses peaked in December (20.6%), January (20.2%) and February (20.2%); maternal vaccine doses in December (31.6%) and January (36.2%).
• Coverage: 28.9% — 18.5% by nirsevimab and 10.4% by maternal vaccine.

Alaska, Connecticut, Maine, Minnesota, South Dakota, Vermont and D.C. protected about half their babies (43.4%–53.1%); Florida, Mississippi, Nevada and Oklahoma fewer than one in five (10.8%–19.7%).

Figure 1. Percentage of infants immunized against RSV, by state (A), and by method and state (B). Figure from the CDC report.

Timing:

• Nirsevimab coverage fell across the season, from 23.8% of babies born October–November 2023 to 9.2% of those born in March 2024.
• Only 38.1% of nirsevimab doses were given in the first week of life (0–6 days), the timing that gives maximum protection; 30.4% came between 7 days and 1 month, and 31.5% at 1 month or older.
• Babies born late in the season were far likelier to get it in their first 3 days (45.5% of March births, against 6.2% of October births).

Figure 2. Nirsevimab coverage by month and year of birth (A) and by age at receipt (B). Figure from the CDC report.

Vaccines for Children (VFC): of infants given nirsevimab, 43.0% were eligible for this program, which supplies vaccines free to uninsured and underinsured children and covers nirsevimab; 35.1% were not; 22.0% had no information. VFC-eligible babies were less likely to get it in the first 3 days (29.9%) than at 1 month or older (50.3%) — perhaps because few birthing hospitals are VFC providers.

Why coverage was low

• Supply: nirsevimab was scarce in some places, especially early in the season.
• Unfamiliarity: patients and providers were new to both products, and the pediatric and maternal recommendations are complex.
• Cost: private insurers have a 1-year grace period under the Affordable Care Act before they must cover ACIP-recommended vaccines, and some hospitals may have found nirsevimab too expensive to stock for routine newborn care.
• Counting: only doses given in the recommended months were included.

A CDC internet survey in spring 2024 put coverage at 55.8%, about double this estimate. The survey may overstate it (self-selection, recall, small sample), and the registries may understate it: consent rules and patchy reporting, and some birthing hospitals and obstetric providers that don't report to them. Single-hospital studies with targeted promotion also found higher coverage; a Wisconsin registry study found 36.2%, closer to this one.

Signs of progress: preliminary data suggest coverage reached 57% nationally in 2024–25 (for a slightly different birth period); first-week nirsevimab became more common as the season went on; in six states and D.C., at least 40% of babies were protected despite a late start, short supply and reimbursement problems; and a second monoclonal antibody, clesrovimab, has since been approved by the Food and Drug Administration and recommended by ACIP for infants under 8 months not protected by maternal vaccination.

Limits

• Registries don't record pregnancy, so vaccinated women who weren't pregnant would inflate coverage.
• Mother and baby records can't be linked, so a baby protected both ways would be counted twice — though that is rarely recommended or reported.
• Twins and other multiples (about 3% of live births) and stillbirths (about 0.6%) aren't identified.
• Nirsevimab given to babies of mothers under 18 or over 49 couldn't be removed from the count — likely a small number.
• Consent and reporting rules, especially for adults, may leave doses out.
• 22% of infant records lacked VFC status.
• 33 states and D.C. may not represent the whole country.

What's needed

More effort to protect babies from severe RSV through maternal or infant immunization; more birthing hospitals enrolled in VFC; and better reporting by obstetric providers to the registries, which give timely, state-level data to track coverage. With coverage rising and clesrovimab now recommended, access should widen in 2025–26. More: CDC's RSV guidance for infants and young children.

Sources

Based on Ellen O. Boundy, Hannah Fast, Tara C. Jatlaoui and others, "Respiratory Syncytial Virus Immunization Coverage Among Infants Through Receipt of Nirsevimab Monoclonal Antibody or Maternal Vaccination — United States, October 2023–March 2024," MMWR, Centers for Disease Control and Prevention; a work of the United States government in the public domain. The report's social-media graphic, built around a photograph, is not reproduced.