Electronic Nudges to Increase Influenza Vaccination in Immunosuppressed Adults Across the Age Spectrum: A Pooled Analysis of 2 Nationwide Randomized Trials

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BACKGROUND: Immunosuppressed individuals face higher risk of severe influenza complications, yet vaccination coverage remains low. We aimed to assess the effect of electronic letter-based nudges on influenza vaccination uptake by immunosuppression status. METHODS: We performed a participant-level pooled analysis of 2 methodologically harmonized, nationwide, pragmatic randomized clinical trials during the 2023-2024 influenza season. Adults aged ≥65 years (NUDGE-FLU-2) and adults aged 18-64 years with chronic conditions (NUDGE-FLU-CHRONIC) were included, with immunosuppression defined by an immunosuppressive diagnosis or a filled prescription for immunosuppressive therapy. Participants were randomized in a 2.45:1:1:1:1:1:1 ratio to usual care or 1 of 6 electronic letter interventions. The primary outcome was receipt of influenza vaccination by 1 January 2024 ascertained from nationwide administrative health registries. Effect modification by immunosuppression status was assessed using binomial regression. RESULTS: Among 1 181 254 participants (mean age 67 years; 73 830 [6.3%] immunosuppressed), 66.1% were vaccinated. Any letter increased vaccine uptake compared with usual care (absolute difference, 2.79% points; 95% confidence interval [CI], 2.60-2.98; P < .001), with greater effect among immunosuppressed individuals (+4.12 vs +2.70% points; Pinteraction = .002). In younger adults with chronic conditions, letter-based nudges increased vaccination rates by 11.7% points overall and by 13.3% points among those with immunosuppression (Pinteraction = .007). Among immunosuppressed individuals, nudging effects appeared greater for those with immunosuppressive conditions than for those receiving immunosuppressive treatment. CONCLUSIONS: Electronic letter-based nudges increased influenza vaccination rates, with greater benefit among immunosuppressed individuals. These findings support implementation of low-cost, letter-based nudging strategies to improve vaccine uptake in this high-risk population. Clinical Trials Registration. NCT06030726 and NCT06030739.

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