Influenza Vaccine Effectiveness Against Influenza-like Illness in Post-Operative Children with Congenital Heart Disease: A Retrospective Cohort Study Using Inverse Probability of Treatment Weighting.

Background: Children with congenital heart disease (CHD) are highly vulnerable to influenza, yet real-world vaccine effectiveness (VE) data in this post-operative population are lacking, especially in China. This study estimated influenza VE against medically attended influenza-like illness (ILI) in post-operative children with CHD using inverse probability of treatment weighting (IPTW) to address confounding. Methods: We retrospectively enrolled 194 children who underwent CHD surgery in Shanghai (January-July 2023). Vaccination records were obtained from the Shanghai Immunization Information System, and ILI episodes were captured via structured interviews. The primary analysis used doubly robust quasi-Poisson regression on an IPTW pseudo-population. VE = (1 - RR) × 100%. Sensitivity analyses included multivariable Poisson regression, marginal structural models, and stabilized IPTW. Results: Vaccination coverage was 21.6% (42/194). After IPTW, all covariates were well-balanced (SMD < 0.10). The primary VE was 50.9% (95% CI: 11.0-72.9%). Sensitivity estimates ranged from 49.6% to 56.6%. Subgroup analyses showed consistently protective point estimates. Conclusions: Influenza vaccination provided moderate, robust protection (≈51% VE) against ILI in post-operative CHD children, comparable to healthy pediatric populations. Our results reinforce the urgent need to prioritize influenza immunization in this vulnerable group and offer critical baseline data for China.