Nirsevimab's First RSV Season in New York City, 2023–2024

At a glance

• Known before: nirsevimab, a monoclonal antibody that protects infants and young children from severe respiratory syncytial virus (RSV) infection by passive immunization, was approved for U.S. use for the 2023–24 RSV season.
• Found here: among New York City infants born during the recommended window (October 1, 2023–March 31, 2024) who got nirsevimab, 37% of those eligible for Vaccines for Children (VFC) and 45% of those not eligible got it within their first 7 days of life.
• What follows: making sure birthing hospitals are enrolled in VFC, and have protocols to offer nirsevimab before discharge, might raise the number protected in the first week.

A new antibody, and a shortage

RSV is the leading cause of infant hospitalizations in the United States. Nirsevimab — a new, long-acting, injectable human recombinant monoclonal antibody — became available for the 2023–24 season. In August 2023, the Advisory Committee on Immunization Practices recommended it for:

• infants aged 0–7 months;
• infants and children aged 8–19 months with risk factors for severe RSV disease;

given shortly before the season, or within the first week of life for babies born October–March in most of the country.

In October 2023, a nationwide shortage hit both the commercial market and the federally funded VFC program. CDC issued a health advisory to prioritize available doses for infants at highest risk, which forced a new VFC allocation plan.

• VFC: since 1994, a federal entitlement program giving vaccines to eligible uninsured or underinsured people aged 18 or younger. Nirsevimab isn't an active immunizing agent, but VFC is authorized to give it as an immunization.
• In New York City: about 75% of children are eligible for free public immunizations from the NYC Health Department's VFC program, but only about two thirds of birthing hospitals (26 of 38) were enrolled in VFC that season.
• The city's plan: reach VFC-eligible infants in their first week, at birthing hospitals.

What was measured

The team looked at 15,521 nirsevimab doses given from October 1, 2023, to March 31, 2024, to infants and children aged 0–19 months and reported to the NYC Citywide Immunization Registry, the city's immunization information system. Children counted as VFC-eligible if their reported insurance showed they were on Medicaid, covered by Child Health Plus B or Section 317 Immunization Program funding, uninsured or underinsured, or were Native American or Alaskan Native. Only first doses counted.

What was found

The large share with unknown eligibility among babies immunized at birthing hospitals may reflect how uncertain a newborn's VFC status is there.

Other details from the doses:

• Age: the median child was 23 days old; 97.4% were aged 0–7 months.
• Dose: 71.1% got 50 mg, 28.9% got 100 mg.
• Where: private ambulatory practices gave 31.3%, birthing hospitals 18.2%, hospital ambulatory clinics 16.6% and federally qualified health centers (or look-alikes) 9.9%; for 20.5% the setting was probably misreported.
• When: shortage periods followed CDC notices — before the shortage (October 1–22, 2023), after the shortage notice (October 23, 2023–January 11, 2024), and a return to routine eligibility (January 12–March 31, 2024). Most doses (55.0%) came in the second period.

What it means

About 15,500 doses reached children aged 0–19 months shortly before or during the season, so many were protected from severe RSV by this highly effective new immunization. About half of the nearly 14,000 doses given to babies born shortly before or during the season came in the first week of life.

• Supply: going into 2024–25, an adequate supply will be important.
• Hospitals: the NYC Health Department keeps working to enroll the few remaining birthing hospitals; by November 2024, 31 of 38 were enrolled.
• Before discharge: VFC enrollment plus protocols to offer nirsevimab to eligible babies before they leave the hospital might raise first-week protection.

Sources

Based on "Notes from the Field: Rollout of Nirsevimab to Protect Infants and Young Children During the Respiratory Syncytial Virus Season — New York City, 2023–2024," by Melanie S. Askari and colleagues of the New York City Health Department and CDC, Morbidity and Mortality Weekly Report, CDC; a work of the United States government in the public domain.