Digital letter interventions have proven effective in increasing influenza vaccination rates. In this trial, we sought to further refine these strategies and investigated whether the effectiveness of the strategies could be sustained across consecutive influenza seasons. We enrolled all eligible Danish citizens 65 years of age or older in a nationwide registry-based randomized implementation trial during the 2023–2024 influenza season. Households of participants were randomly assigned in a 2.45:1:1:1:1:1:1 ratio to usual care or six different behaviorally informed electronic letter-based nudges delivered before the influenza vaccination period. The primary endpoint was receipt of influenza vaccination. Statistical analyses accounted for household-level clustering. A total of 881,373 participants (mean age 74.1 ± 6.5 years, 52.1% female) were randomized across 649,487 households. The primary endpoint was met; influenza vaccination rates were higher in the pooled intervention letter group compared to usual care (76.32% versus 76.02%; difference, 0.31 percentage points; 99.29% confidence interval, 0.00–0.61; P = 0.007). Although no individual letter significantly increased influenza vaccination rates, the directionality of effect was consistent across all letters. Effectiveness was particularly pronounced in participants who had not received influenza vaccination during the preceding season (P_interaction = 0.010). Effectiveness was consistent regardless of whether participants had received a similar electronic letter-based nudge in the preceding season (P_interaction = 0.26). In summary, electronic letter-based nudges successfully increased influenza vaccination among older adults, and our results suggest that these highly scalable strategies can be implemented effectively and safely across consecutive vaccination seasons.

ClinicalTrials.gov registration: NCT06030726.

数字字母干预已被证明可有效提高流感疫苗接种率。在这项试验中，我们试图进一步完善这些策略，并调查这些策略的有效性是否可以在连续的流感季节中持续。在 2023-2024 年流感季节，我们招募了所有 65 岁或以上符合条件的丹麦公民参加一项全国性的基于登记的随机实施试验。参与者的家庭以 2.45：1：1：1：1：1：1：1 的比例随机分配到常规护理组，或在流感疫苗接种期之前提供六种不同的基于行为知情的电子信件提示。主要终点是接受流感疫苗接种。统计分析考虑了家庭层面的聚类。共有 881,373 名参与者 （平均年龄 74.1 ± 6.5 岁，52.1% 为女性） 被随机分配到 649,487 个家庭。达到主要终点;与常规护理相比，汇总干预信组的流感疫苗接种率更高（76.32% 对 76.02%;差异，0.31 个百分点;99.29% 置信区间，0.00-0.61;P = 0.007）。尽管没有单个字母显着提高流感疫苗接种率，但所有字母的效果方向性是一致的。在前一季节未接种流感疫苗的参与者中，有效性尤为明显 （P_交互作用 = 0.010）。无论参与者在上一季是否收到类似的基于电子信件的推动，有效性都是一致的 （P_交互 = 0.26）。 总之，基于电子信件的助推成功地增加了老年人的流感疫苗接种率，我们的结果表明，这些高度可扩展的策略可以在连续的疫苗接种季节中有效和安全地实施。

ClinicalTrials.gov 注册：NCT06030726。