Digital Nudges to Increase Influenza Vaccination in Patients with Chronic Diseases

Electronic letters have demonstrated effectiveness in improving vaccination uptake among patients with chronic diseases, but the effects of serial implementation over consecutive influenza seasons are unknown.

In a nationwide randomized implementation trial, conducted during the 2024-2025 influenza season, that included all Danish citizens 18-64 years of age who were eligible for free-of-charge influenza vaccination because of a chronic condition, we randomly assigned participants in a 2.45:1:1:1:1:1:1 ratio to usual care (no letter) or to one of six different behaviorally informed electronic letter strategies. All data were sourced from nationwide Danish health registries. The primary end point was receipt of an influenza vaccine on or before January 1, 2025, analyzed through seven coprimary comparisons (all intervention groups pooled vs. usual care, and each intervention group vs. usual care).

We randomly assigned 308,978 Danish citizens 18-64 years of age; 164,100 (53.1%) were female; the median age was 52.1 years (interquartile range, 39.7-59.2). Compared with usual care, influenza vaccination rates were higher among those receiving any intervention letter (36.5% vs. 24.1%; difference, 12.4 percentage points [99.29% confidence interval [CI], 11.9-12.9]; P<0.001). Significant increases in influenza vaccination were observed with each letter type compared with usual care. Among the six different electronic letter strategies, the largest effect size was observed with a repeated cardiovascular-focused letter (39.1% vs. 24.1%; difference, 15.0 percentage points [99.29% CI, 14.2 to 15.7]; P<0.001). There was no apparent difference in the effect size of letters compared with usual care across major subgroups, including among those who had also received a similar letter during the preceding influenza season.

Electronically delivered, letter-based nudges significantly increased influenza vaccination among 18-to-64-year-old individuals with chronic diseases compared with usual care, when delivered during a second consecutive season. The largest effect size was with a strategy where a letter focused on the potential cardiovascular benefits of influenza vaccination was sent twice. (ClinicalTrials.gov number, NCT06600490.).