Digital Nudges to Increase Influenza Vaccination in Patients with Chronic Diseases.

Summary

In a nationwide randomized implementation trial of 308,978 Danish adults with chronic conditions, behaviorally informed electronic letters increased influenza vaccination by 12.4 percentage points versus usual care, with the strongest effect from a repeated cardiovascular‑framed message (15.0 percentage points). Effects were consistent across subgroups and in a second consecutive season.

Key Findings

• Any electronic letter increased influenza vaccination vs usual care (36.5% vs 24.1%; difference 12.4 percentage points; 99.29% CI 11.9–12.9; P<0.001).
• The repeated cardiovascular‑focused letter achieved the largest effect (39.1% vs 24.1%; difference 15.0 percentage points; 99.29% CI 14.2–15.7).
• Effects were consistent across subgroups and among individuals who had received a similar letter in the prior season.

Clinical Implications

Health systems can deploy repeated, cardiovascular‑framed electronic letters via national patient portals to raise vaccination coverage in chronic disease populations, likely reducing influenza morbidity and downstream cardio‑respiratory complications.

Limitations

• Conducted in a single country (Denmark), potentially limiting generalizability to other health systems
• Outcome was vaccine uptake rather than clinical endpoints (e.g., hospitalizations)

Future Directions

Evaluate message tailoring and frequency across diverse health systems, link to clinical outcomes (e.g., influenza hospitalizations), and test integration with multimodal outreach (SMS/app notifications).