Nationwide Utilization of Danish Government Electronic Letter System for Confirming the Effectiveness of Behavioral Nudges in Increasing InFLUenza Vaccine Uptake Among Adults With Chronic Disease

NCT ID: NCT06600490

Last Updated: 2025-09-12

Basic Information

• Recruitment Status: COMPLETED
• Clinical Phase: NA
• Total Enrollment: 308978 participants
• Study Classification: INTERVENTIONAL
• Study Start Date: 2024-10-01
• Study Completion Date: 2025-05-31

Brief Summary

In randomized clinical trials and observational studies, influenza vaccination has been shown to be effective in reducing influenza-related illness, hospitalizations, cardiovascular events, and mortality in select populations. However, the real-world effectiveness of influenza vaccination is limited by its uptake. Conducted during the 2023/2024 influenza season, the first NUDGE-FLU-CHRONIC trial demonstrated the effectiveness of behavioral nudging letters in increasing influenza vaccination rates among adults aged 18-64 years with chronic diseases in Denmark. This present study will once again investigate whether digital behavioral nudges delivered via the official, mandatory Danish electronic letter system can increase influenza vaccine uptake among adults aged 18-64 years with chronic diseases including whether the effectiveness of the previously successful strategies can be confirmed during a subsequent influenza season.

Detailed Description

The study is a prospective, randomized, open-label implementation trial. The study population will consist of persons aged 18-64 years identified as eligible for free-of-charge influenza vaccination due to a chronic condition. Subjects will be identified through Danish nationwide health registries using codes from the International Classification of Diseases, 10th revision (ICD-10). Individuals will be randomized to 1 of 7 arms (1 usual care arm and 6 intervention arms) with each testing different nudging strategies employing various behavioural economic principles. The interventions will be delivered through the official, mandatory Danish electronic letter system. All subject data will be retrieved from the Danish nationwide registries with the exception of information on intervention allocation. Endpoints will be retrieved at prespecified dates using prespecified search algorithms.

Conditions

Influenza; Behavior and Behavior Mechanisms

Study Design

• Allocation Method: RANDOMIZED
• Intervention Model: PARALLEL
• Primary Study Purpose: OTHER
• Blinding Strategy: NONE

Study Groups

Usual Care

No letter

Group Type: NO_INTERVENTION

No interventions assigned to this group

Standard Letter

This group will receive a standard letter on the benefits of influenza vaccination without behavioral economic enhancement

Group Type: EXPERIMENTAL

Behavioral Economic Principles

Intervention Type: BEHAVIORAL

The control arm will receive no letter to reflect the background vaccination uptake. Intervention arms will test the effects of different letters developed using behavioral economic principles.

Repeated Letter

The standard letter sent out two times instead of once

Group Type: EXPERIMENTAL

Behavioral Economic Principles

Intervention Type: BEHAVIORAL

The control arm will receive no letter to reflect the background vaccination uptake. Intervention arms will test the effects of different letters developed using behavioral economic principles.

Cardiovascular Gain-Framing Letter

Text added to the standard letter highlighting potential cardiovascular benefits of influenza vaccination

Group Type: EXPERIMENTAL

Behavioral Economic Principles

Intervention Type: BEHAVIORAL

The control arm will receive no letter to reflect the background vaccination uptake. Intervention arms will test the effects of different letters developed using behavioral economic principles.

Repeated Cardiovascular Gain-Framing Letter

The cardiovascular gain-framing letter sent out two times instead of once

Group Type: EXPERIMENTAL

Behavioral Economic Principles

Intervention Type: BEHAVIORAL

The control arm will receive no letter to reflect the background vaccination uptake. Intervention arms will test the effects of different letters developed using behavioral economic principles.

Respiratory Gain-Framing Letter

Text added to the standard letter highlighting potential respiratory disease-related benefits of influenza vaccination

Group Type: EXPERIMENTAL

Behavioral Economic Principles

Intervention Type: BEHAVIORAL

The control arm will receive no letter to reflect the background vaccination uptake. Intervention arms will test the effects of different letters developed using behavioral economic principles.

Implementation Intention Prompt Letter

Implementation intention prompt added to the standard letter

Group Type: EXPERIMENTAL

Behavioral Economic Principles

Intervention Type: BEHAVIORAL

The control arm will receive no letter to reflect the background vaccination uptake. Intervention arms will test the effects of different letters developed using behavioral economic principles.

Interventions

Behavioral Economic Principles

The control arm will receive no letter to reflect the background vaccination uptake. Intervention arms will test the effects of different letters developed using behavioral economic principles.

Intervention Type: BEHAVIORAL

Eligibility Criteria

Inclusion Criteria

1. Age 18-64 years

2. Must be registered in the Danish nationwide registries as meeting at least one of the following eligibility criteria for free-of-charge influenza vaccination in the Danish public health system:

1. Chronic lung disease

2. Chronic cardiovascular disease other than hypertension

3. Type 1 or type 2 diabetes mellitus

4. Congenital or acquired immunodeficiency

5. Impaired breathing due to muscular weakness

6. Chronic renal or hepatic insufficiency

7. Other chronic conditions with an increased risk of severe influenza as determined by the treating physician

3. Access to the official, mandatory Danish electronic mailbox system

Exclusion Criteria

None

• Minimum Eligible Age: 18 Years
• Maximum Eligible Age: 64 Years
• Eligible Sex: ALL
• Accepts Healthy Volunteers: No

Sponsors

Tor Biering-Sørensen

OTHER

Sponsor Role: lead

Responsible Party

Tor Biering-Sørensen

Professor, MD, MSc, MPH, PhD

Responsibility Role: SPONSOR_INVESTIGATOR

Principal Investigators

Tor Biering-Sørensen, MD, MSc, MPH, PhD

Role: PRINCIPAL_INVESTIGATOR

Center for Translational Cardiology and Pragmatic Randomized Trials, Department of Cardiology, Copenhagen University Hospital - Herlev and Gentofte

Locations

Center for Translational Cardiology and Pragmatic Randomized Trials, Department of Cardiology, Copenhagen University Hospital - Herlev and Gentofte

Hellerup, Capital Region, Denmark

Countries

Denmark

Other Identifiers

NUDGE-FLU-CHRONIC-2

Identifier Type: -

Identifier Source: org_study_id