A Washington state difference-in-differences cohort study using syndromic surveillance data from 2022 to 2025 found that introduction of infant nirsevimab and maternal RSV prefusion F vaccine was associated with a 43% relative decrease in the rate of RSV-associated hospitalizations and emergency department visits among infants aged 7 months or younger in the second season of use (2024–2025), with no significant effect observed in the first season when uptake was limited. The analysis included 16,775 events and showed a shift in disease burden toward older children, with median age increasing from 9 to 12 months.