Elsevier

The Lancet

Volume 401, Issue 10382, 1–7 April 2023, Pages 1103-1114
The Lancet

Articles
Electronic nudges to increase influenza vaccination uptake in Denmark: a nationwide, pragmatic, registry-based, randomised implementation trial

https://doi.org/10.1016/S0140-6736(23)00349-5Get rights and content

Summary

Background

Influenza vaccination rates remain suboptimal despite effectiveness in preventing influenza infection and related complications. We investigated whether behavioural nudges, delivered via a governmental electronic letter system, would increase influenza vaccination uptake among older adults in Denmark.

Methods

We did a nationwide, pragmatic, registry-based, cluster-randomised implementation trial during the 2022–23 influenza season in Denmark. All Danish citizens aged 65 years or older or turning 65 years by Jan 15, 2023 were included. We excluded individuals living in nursing homes and individuals who had an exemption from the Danish mandatory governmental electronic letter system. Households were randomly assigned (9:1:1:1:1:1:1:1:1:1) to usual care or nine different electronic letters designed on the basis of different behavioural nudging concepts. Data were sourced from nationwide Danish administrative health registries. The primary endpoint was receipt of influenza vaccination on or before Jan 1, 2023. The primary analysis assessed an analytical set of one randomly selected individual per household, and a sensitivity analysis included all randomly assigned individuals and accounted for within-household correlation. The trial is registered with ClinicalTrials.gov, NCT05542004.

Findings

We identified 1 232 938 individuals aged 65 years or older in Denmark and excluded 56 436 (4·6%) individuals living in nursing homes and 211 632 (17·2%) with an exemption from the electronic letter system. We randomly assigned 964 870 (78·3%) participants across 691 820 households. Compared with usual care, influenza vaccination rates were higher in the group receiving an electronic letter highlighting potential cardiovascular benefits of vaccination (81·00% vs 80·12%; difference 0·89 percentage points [99·55% CI 0·29–1·48]; p<0·0001) and the group receiving repeated letters at randomisation and at day 14 (80·85% vs 80·12%; difference 0·73 percentage points [0·13–1·34]; p=0·0006). These strategies improved vaccination rates across major subgroups including those with and without established cardiovascular disease. The cardiovascular gain-framed letter was particularly effective among participants who had not been vaccinated for influenza in the previous season (pinteraction=0·0002). A sensitivity analysis of all randomly assigned individuals accounting for within-household clustering yielded similar findings.

Interpretation

Electronically delivered letters highlighting potential cardiovascular benefits of influenza vaccination or sent again as a reminder significantly increased vaccination uptake across Denmark. Although the magnitude of effectiveness was modest, the low-touch, inexpensive, and highly scalable nature of these electronic letters might be informative for future public health campaigns.

Funding

Sanofi.

Introduction

Each year, seasonal influenza causes substantial morbidity and accounts for at least 500 000 excess deaths globally;1 groups at high risk including older adults and those with cardiometabolic risk factors might be disproportionately affected.2 Influenza vaccination reduces influenza infection and related complications3, 4 and is widely recommended by public health authorities and clinical practice guidelines. However, global vaccination rates remain suboptimal,5, 6 probably because of several factors including knowledge gaps, pre-existent beliefs around vaccination, safety concerns, and other inertial factors promoting vaccine hesitancy.7 Embedding behavioural nudges in public health messaging has potential to influence health-related behaviours, but the utility of nudges has largely been tested in select populations such as people with upcoming primary care visits, in certain employment, and those enrolled in confined health-care systems.8 The scalability and generalisability of nudging strategies to improve nationwide vaccination uptake is unknown. The Danish health system provides an optimal framework for large trials testing various behavioural insights; the universal health-care system provides free-of-charge influenza vaccination to groups at high risk; nationwide administrative health registries enable identification and high-fidelity follow-up of participants across the country;9, 10 and a mandatory governmental electronic letter system used for official communications ensures timely and reliable delivery. Influenza vaccination rates in Denmark are high, increasing from 75% in 2020–2021 to 78% in 2021–2022 in adults aged 65 years or older,11 ensuring a strong control group for this trial.

Research in context

Evidence before this study

Vaccination has been shown to reduce influenza infection and its attendant complications. As such, seasonal influenza vaccination is strongly endorsed as a core public health strategy. However, global influenza vaccination rates remain suboptimal, including among older adults at high risk. Although countries worldwide implement various influenza vaccination campaigns, these communication strategies have been largely untested in rigorous nationwide implementation trials. We assessed the existing influenza vaccination implementation trial literature by searching PubMed on Dec 30, 2022, with no date or language restrictions, using the search terms (“implementation” OR “behavioural science” OR “nudges” OR “letters” OR “reminders” OR “messages”) AND “influenza” AND (“vaccination” OR “vaccine”) AND (“trial” OR “randomised”).

Added value of this study

In a first-of-its-kind nationwide, pragmatic, registry-based, cluster-randomised implementation trial, 964 870 Danish adults aged 65 years or older received usual correspondence or one of nine electronic letters designed on the basis of different behavioural concepts to encourage influenza vaccination. A letter emphasising potential cardiovascular benefits of vaccination and a reminder letter sent again at 14 days after initial delivery both modestly but significantly increased influenza vaccination rates. Effectiveness extended across various subgroups, including those without influenza vaccination in the preceding season. Effectiveness in nudging influenza vaccination was consistently observed across all five major Danish regions.

Implications of all the available evidence

In a country with a high background rate of influenza vaccination, certain behaviourally informed, electronic nudges delivered in advance of vaccine availability incrementally increased vaccine uptake (approximately 1% absolute increase). Although the magnitude of effectiveness of the successful nudging strategies might seem modest, the low-touch, inexpensive, and highly scalable nature of these electronic letters might have important population-level public health implications.

We conducted the NUDGE-FLU (Nationwide Utilization of Danish Government Electronic Letter System for Increasing Influenza Vaccine Uptake) trial to test the hypothesis that behavioural nudges delivered via the Danish mandatory governmental electronic letter system could increase influenza vaccination rates in individuals aged 65 years or older.

Section snippets

Study design and participants

NUDGE-FLU was a nationwide, pragmatic, registry-based, randomised implementation trial conducted during the 2022–23 influenza season. The study design has previously been described.12 The study was sponsored by the Center for Translational Cardiology and Pragmatic Randomized Trials at Copenhagen University Hospital–Herlev and Gentofte, Copenhagen, Denmark, who assumed full responsibility for trial conduct and served as the data coordinating centre.

The trial was granted an exemption from ethics

Results

We identified 1 232 938 individuals aged 65 years or older in Denmark. We excluded 56 436 (4·6%) individuals living in nursing homes and 211 632 (17·2%) with an exemption from the electronic letter system. 964 870 (78·3%) individuals from 691 820 households met trial eligibility and were randomly assigned (appendix p 5). Participants lived across all 98 Danish municipalities (figure 1). Mean age was 73·8 years (SD 6·3), 496 482 (51·5%) were women, 468 388 (48·5%) were men, 264 392 (27·4%) had

Discussion

In this nationwide randomised implementation trial, two electronic-letter intervention strategies delivered in advance of the influenza season increased influenza vaccination uptake among adults aged 65 years or older in Denmark. The largest effects were found with an electronic letter highlighting potential cardiovascular benefits of vaccination and a repeated letter strategy with a 14-day reminder letter. The electronic letters showed effectiveness in increasing vaccination rates in addition

Data sharing

Individual-level data from the nationwide Danish administrative health registries cannot be made publicly available according to Danish law. Data access is obtained through the Danish Health Data Authority (https://sundhedsdatastyrelsen.dk/da/english) for researchers at authorised Danish research institutions.

Declaration of interests

MV has received research grant support or served on advisory boards for American Regent, Amgen, AstraZeneca, Bayer AG, Baxter Healthcare, Boehringer Ingelheim, Chiesi, Cytokinetics, Lexicon Pharmaceuticals, Novartis, Novo Nordisk, Pharmacosmos, Relypsa, Roche Diagnostics, Sanofi, and Tricog Health; has speaker engagements with AstraZeneca, Boehringer Ingelheim, Novartis, and Roche Diagnostics; and participates on clinical trial committees for studies sponsored by AstraZeneca, Bayer AG, Galmed,

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