Cost-effectiveness and public health impact of RSV immunization for all infants in Japan: a mixed strategy with nirsevimab and RSVpreF

BACKGROUND: Respiratory syncytial virus (RSV) remains a leading cause of medically attended lower respiratory tract disease (MA-LRTD) among infants in Japan. With the approval of maternal immunization (MI) and nirsevimab, a long-acting monoclonal antibody for infant RSV prevention, Japan is transitioning from high-risk - targeted prophylaxis toward broader RSV prevention. This study evaluated the public health and economic impact of alternative MI and nirsevimab strategies within the Japanese National Immunization Program (NIP). RESEARCH DESIGN AND METHODS: A static decision-analytic cohort model, estimated clinical and economic outcomes from birth through the first year of life (and the second RSV season for eligible high-risk infants). Strategies included an NIP-aligned scenario reflecting expected MI and nirsevimab, mixed nirsevimab - MI coverage scenarios (20/80, 50/50, 80/20), and a nirsevimab-only strategy. Outcomes included MA-LRTDs, hospitalizations, costs, quality-adjusted life-years (QALYs), and number needed to immunize (NNI) from payer and societal perspectives. RESULTS: Compared with the NIP-aligned scenario, shifting coverage toward greater nirsevimab use reduced MA-LRTDs by 9.6% to 28.1%. A nirsevimab-only strategy reduced MA-LRTDs by 34.3% and hospitalizations by 33.4%. Under base-case assumptions, strategies with greater nirsevimab use improved outcomes and reduced costs, with lower NNI as coverage increased. Findings were robust in sensitivity analyses. CONCLUSIONS: For the Japanese setting, greater nirsevimab use improves outcomes and reduces costs, with nirsevimab-only strategies providing the greatest overall benefit.