Pediatric Influenza Vaccine Effectiveness Against Emergency Visits and Hospitalization in Canada: 2024-2025 and 2025-2026 seasons

Importance: Influenza causes substantial morbidity in children, yet Canadian data on influenza vaccine effectiveness (VE) in preventing emergency department (ED) visits and hospitalizations are limited. Objective: To estimate VE against influenza-associated ED visits and hospitalizations among children during the 2024-2025 and 2025-2026 seasons. Design: Test negative case control design. Setting: Fourteen Canadian pediatric tertiary-care hospitals participating in the Surveillance Program for the Rapid IdeNtification and TracKing of Infectious Diseases in kidS (SPRINT KIDS) network. Participants: Symptomatic children aged 6 months to <18 years tested for influenza from October 27, 2024, to June 7, 2025, and from November 2, 2025, to June 13, 2026. Exposure: At least one dose of current season influenza vaccine. Main Outcomes and Measures: VE against laboratory confirmed influenza A or B associated with an ED visit or hospitalization. VE was estimated using multivariable logistic regression adjusted for age, epidemiologic week, site, and high-risk comorbidity. Results: The 2024/2025 season included 17,316 encounters (median [IQR] age=41.0 [18.5, 82.3] months; 54.0% male), with 4,760 influenza-positive cases (8.9% vaccinated) and 12,556 influenza-negative controls (15.9% vaccinated). The 2025/2026 season included 13,417 encounters (median [IQR] age=33.5 [16.7, 74.8] months; 54.1% male), with 4,798 cases (7.6% vaccinated) and 8,619 controls (18.1% vaccinated). VE against any influenza was 47% (95%CI, 40%, 53%) and 53% (95%CI, 46%, 59%) in 2024/2025 and 2025/2026, respectively. VE against ED visits was 53% (95%CI, 45%, 59%) and 57% (95%CI, 49%, 63%), and VE against hospitalization was 33% (95%CI, 16%, 47%) and 48% (95%CI, 33%, 59%) in 2024/2025 and 2025/2026, respectively. VE was lower against influenza A as compared to B in both seasons: 31% (95%CI, 22%, 39%) vs. 75% (95%CI, 64%, 82%) (p<0.001) in 2024-2025; and 42% (95%CI, 32%, 50%) vs. 64% (95%CI, 55%, 72%) (p=0.001) in 2025/2026. VE was lower in children aged 10 to <18 years than in those <10 years, particularly among children with comorbidities. Protection against influenza A lasted up to 90 days, whereas it persisted beyond 120 days for influenza B. Conclusions and Relevance: Influenza vaccines were moderately effective against pediatric ED visits and hospitalizations in both seasons. Routine vaccination should continue to be recommended to prevent hospital-attended influenza.