Nirsevimab Protected Alaska Native Babies From RSV: Yukon-Kuskokwim Delta, 2023–24

Summary

• Nirsevimab, a preventive monoclonal antibody against respiratory syncytial virus (RSV), is recommended for all infants under 8 months in or entering their first RSV season who aren't protected by maternal vaccination, and for children 8–19 months entering their second season who are at higher risk — including all American Indian and Alaska Native (AI/AN) children.
• In Alaska's Yukon-Kuskokwim Delta, nirsevimab was 89% effective against RSV hospitalization in infants' first season, and 76% and 88% effective against medically attended RSV illness in the first and second seasons.
• It can prevent severe RSV among AI/AN infants and children in both seasons.

Background

RSV is a leading cause of hospitalization in young children, and AI/AN children have long had high rates — three to seven times higher in the Yukon-Kuskokwim Delta than elsewhere in the United States. CDC's Advisory Committee on Immunization Practices recommended nirsevimab in August 2023; in trials it was 79% effective against medically attended RSV lower respiratory infection and 81% against RSV hospitalization for 150 days after a dose. In September 2023, the committee recommended that every infant be protected either by maternal vaccination or by nirsevimab. This study gives the first real-world estimates for AI/AN children in their first and second seasons.

The region

About 27,000 people live in the Yukon-Kuskokwim Delta of southwestern Alaska — 90% Alaska Native — in a regional hub and 48 remote villages with no road connections. The Yukon-Kuskokwim Health Corporation, a tribal health organization, runs a regional hospital that tests for RSV and 46 village clinics that send it swabs. Of an estimated 1,591 children under 20 months, it gave nirsevimab to 756 (48%) from October 16, 2023, to April 30, 2024.

How effectiveness was measured

The study used a test-negative design: among children under 20 months with a medically attended acute respiratory illness (October 23, 2023–June 30, 2024) and an RSV test, it compared how often those who tested positive and those who tested negative had received nirsevimab at least 7 days earlier. Children whose mothers were vaccinated, or who had received palivizumab or more than one nirsevimab dose on different dates, were excluded. Results were adjusted for age, sex, calendar month, type of community and high-risk conditions.

Results

472 children were included — 14% hospitalized, 70% seen in an emergency department, 16% in an outpatient clinic. 68 (14%) tested positive for RSV, with positives peaking in November 2023–January 2024. Their median age was 9 months; 292 were in their first season and 180 in their second. 98% were AI/AN, 73% lived in villages outside the hub, and 16% had a condition raising their risk.

48% had received nirsevimab at least 7 days before their visit — fewer in the second season (37%) and in the regional hub (35%) than in the first season (55%) and the villages (53%). Only 15% of RSV-positive children had received it, against 54% of RSV-negative children.

The median time from dose to illness was 91 days. Results held when children with COVID-19 or influenza were excluded. Of 64 children hospitalized with respiratory illness, 23 had RSV, and only three of them had received nirsevimab. The drop in effectiveness over time hints at waning, but the confidence intervals were wide and overlapping.

Children with medically attended respiratory illness, by RSV test result and month. Figure from the CDC report.

What it means

The results match earlier studies in infants and extend them to an AI/AN population at high risk, over a longer interval after the dose, and to children in their second season. Coverage here (48%) was relatively high for the United States: when supply ran short in October 2023–January 2024, Alaska prioritized all AI/AN infants under 8 months and rural AI/AN children aged 8–19 months. The region averaged 56 RSV hospitalizations a season among children under 2 in the nine previous seasons; in 2023–24 there were 28, only three in children who had received nirsevimab at least 7 days earlier. A steady, timely supply and higher coverage might cut hospitalizations further.

Limits: testing was at clinicians' discretion, and most visits were emergency or inpatient (84%), so milder illness may be missed; low RSV activity in spring may have skewed estimates; small numbers ruled out some estimates; dose (set by age and weight) wasn't recorded; and the findings come from one region.

Sources

Based on Brian Lefferts, Sara Bressler, James W. Keck and others, "Nirsevimab Effectiveness Against Medically Attended Respiratory Syncytial Virus Illness and Hospitalization Among Alaska Native Children — Yukon-Kuskokwim Delta Region, Alaska, October 2023–June 2024," MMWR, Centers for Disease Control and Prevention; authors from the Yukon-Kuskokwim Health Corporation and CDC. Published by CDC and in the public domain; rewritten in hubnx's own words. The report's social-media graphic, built around a photograph, is not reproduced.