043 - Uptake of Nirsevimab in Infants Aged 0-12 Months in San Diego County During the 2023-2024 RSV Season

Publication Number: 43.6852

Michele McDaniel, University of California, San Diego School of Medicine, San Diego, CA, United States; Nanda Ramchandar, University of California San Diego, San Diego, CA, United States; Edmund A.. Milder, University of California, San Diego School of Medicine, San Diego, CA, United States; Seema Shah, University of California, San Diego School of Medicine, San Diego, CA, United States; Danelle Wallace, County of San Diego, San Diego, CA, United States; Alice Pong, University of California, San Diego School of Medicine, San Diego, CA, United States

Background: 1-3% of infants under 12 months will be hospitalized due to RSV. While mortality due to RSV is rare, there is increased morbidity in infants with prematurity or medical complexity. Nirsevimab is a monoclonal antibody recommended to prevent RSV. Assessing the effectiveness of nirsevimab and evaluating uptake may lead to reduction of infant morbidity from RSV.

Objective: To assess uptake of nirsevimab among demographic groups in San Diego County and evaluate its efficacy in preventing RSV-associated hospitalization.

Design/Methods: To assess demographic factors associated with nirsevimab uptake, data was obtained from the San Diego County immunization information system. The study population included babies born between 1/1/2023 and 8/31/2024. Those who received nirsevimab were compared to controls who did not receive nirsevimab but who received at least one other vaccine. Associations were estimated using multivariate logistic regression. To evaluate effectiveness (VE), infants aged 0-12 months admitted to Rady Children’s Hospital for bronchiolitis from 10/1/2023-3/30/2024 were analyzed using a test negative design. Infants who received nirsevimab within one week of admission or after admission, those who received palivizumab, and those whose mothers received maternal RSV vaccine were excluded.

Results: Several racial minority groups (Asian, Hawaiian/Pacific Islander, and Multi-racial; OR=1.4, 1.9, 1.2; p< 0.001, p< 0.001, p< 0.05) were more likely to have received nirsevimab compared to the reference group of Whites. There was a negative relationship between healthy places index (HPI) and receipt of nirsevimab (highest HPI vs lowest, OR = 0.85, p < 0.05). 250 of 572 infants admitted to RCHSD for bronchiolitis during the study period tested positive for RSV via PCR. 11 of the 572 infants admitted for bronchiolitis received nirsevimab prior to admission; of those, two had breakthrough cases of RSV. VE for RSV-associated hospitalization was calculated to be 0.65 (95% CI 0.058-0.94).

Conclusion(s): In the first season of nirsevimab availability in San Diego County, infants from several minority groups and those with lower HPI were more likely to receive the drug. Vaccine effectiveness of nirsevimab against RSV-associated hospitalization in our cohort was 65%. The reason for higher uptake in minority and lower HPI groups is not clear but may have been related to availability of the drug in clinics where these children received care. This study is limited by a small sample size. Further studies assessing local uptake and effectiveness are needed.

Table 1 - Descriptive Statistics
Descriptive statistics of infants receiving nirsevimab and those who did not but who received at least one vaccine in the time period.

Table 2 - Multivariate Logistic Regression
Multivariate logistic regression of demographic factors associated with nirsevimab uptake.