Key result
Electronic nudges boost influenza vaccination by ~42% and may reduce CV events in chronic disease.
Why the study?
Does influenza vaccination reduce cardiovascular events in adults aged 18-64 with chronic disease?
RCT (n=299,881)
2.45:1:1:1:1:1:1
Yes
Does influenza vaccination reduce cardiovascular events in adults aged 18-64 with chronic disease?
Influenza vaccination may reduce the risk of cardiovascular events, particularly heart failure, in adults with chronic diseases, though concomitant COVID-19 vaccination confounds the observed effect.
Background Influenza infection is a transient risk factor for cardiovascular (CV) events. We did a prespecified instrumental variable (IV) analysis of the NUDGE-FLU-CHRONIC trial to estimate the causal effect of influenza vaccination on CV events. Methods NUDGE-FLU-CHRONIC was a randomized, nationwide, register-based trial of all Danish citizens aged 18-64 eligible for free influenza vaccination due to chronic disease during the 2023/2024 flu season. Participants were randomized 2.45:1:1:1:1:1:1 to no letter or one of 6 electronic letters encouraging vaccination. CV events were exploratory outcomes. We used IV analysis with two-stage least square regression to estimate the treatment effect of influenza vaccination on CV events, using randomization to any letter as the IV. Results A total of 299,881 participants were randomized, and 67,707 (20.6%) had CV disease at baseline. Randomization to any letter increased influenza vaccine uptake from 27.9% in the usual care group to 39.6% in the intervention group. COVID-19 vaccine uptake also increased from 18.7% in the usual care group to 26.5% in any letter group. IV analysis revealed a significant treatment effect of influenza vaccination on the risk of CV events (Table). Conclusion Influenza vaccination may reduce the risk of CV events in patients with chronic disease, particularly by lowering heart failure risk. However, due to high concomitant COVID-19 vaccine uptake, the effect may reflect a combined effect of influenza and COVID-19 vaccination.
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Modin et al. (2025) conducted an RCT in Chronic disease (n=299,881). Electronic letters encouraging vaccination vs. No letter (usual care) was evaluated on Cardiovascular events. Electronic letters encouraging vaccination increased influenza vaccine uptake (39.6% vs 27.9%) and may significantly reduce the risk of cardiovascular events in patients with chronic disease.
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