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.