Electronic nudge letters to increase influenza vaccination uptake in younger and middle-aged individuals with diabetes: a prespecified analysis of the NUDGE-FLU-CHRONIC trial

Abstract Background Despite evidence demonstrating that influenza vaccination is associated with reduced risk of adverse cardiovascular events and all-cause mortality, influenza vaccine uptake remains suboptimal in persons with diabetes mellitus (DM). Purpose In this prespecified analysis of the NUDGE-FLU-CHRONIC trial we assessed the effectiveness of electronically delivered nudges on influenza vaccine uptake according to DM status. Methods NUDGE-FLU-CHRONIC was a nationwide, randomized, pragmatic implementation trial among younger and middle-aged (18-64 years) Danish citizens with chronic disease during the 2023/2024 influenza season. All trial participants were eligible for a free influenza vaccination through the Danish governmental vaccine program. Participants were randomized in a 2.45:1:1:1:1:1:1 ratio to usual care (no electronic letter) or one of 6 different electronic nudge letters delivered on Sep 26, 2023. Baseline and outcome data were obtained using the Danish nationwide registries. The endpoint was receipt of a seasonal influenza vaccine on or before Jan 1, 2024. Results Of 299,881 NUDGE-FLU-CHRONIC participants, 57,666 (19.2%) had DM at baseline. Participants with DM had a median age of 51.6 years, 43.0% were female, median Hba1c of 53 mmol/mol, median DM duration of 10.0 years, and 51.2% were insulin dependent. During follow-up, 43.0% of those with DM vs. 34.6% of those without DM received the seasonal influenza vaccine (p<0.001). Any electronic letter vs. usual care was highly effective in increasing vaccine uptake in participants with DM at baseline (45.6% vs. 36.5%, difference: +9.1 percentage points [99.29%CI: 7.9-10.3], relative risk ratio [RR]: 1.42, 99.29%CI: [1.39-1.44]). However, DM status modified the effect of any electronic letter such that participants without DM at baseline experienced a slightly greater effect than those with DM (37.3% vs. 25.9%, difference: +12.3 percentage points [11.7-12.8], RR: 1.47 [1.45-1.50]; p-interaction <0.001). For each individual electronic nudge letter, the effect of the intervention was slightly lower in participants with DM compared to those without (Figure 1; all p-interaction < 0.015). When assessing DM subgroups, the effect of any electronic letter vs. usual care was greater in participants with DM and concomitant cardiovascular disease than in those without, greater in non-insulin treated than in insulin-treated individuals, and greater in those with shorter diabetes duration. Effectiveness of nudges were consistent across all other DM subgroups including HbA1c level (Figure 2). Conclusions Electronic nudge letters significantly boosted vaccine uptake in a broad range of individuals with DM, but the effect was slightly lower compared to those without DM. Future studies should test whether behaviorally designed nudges may positively influence other health behaviors in people with diabetes.