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Digital nudges boost flu vaccine uptake in High-Risk adults

NCT ID NCT06600490

Prevention Sponsor: Tor Biering-Sørensen Source: ClinicalTrials.gov ↗

What the study statuses mean

This study's is highlighted.

Recruitment status, easiest to join first

Recruiting now
This trial is taking on new participants right now.
Not yet recruiting
Registered, but not yet taking participants.
By invitation only
Not open to general applications. Only people the study team invites can take part.
Paused
Paused for now. It may or may not start again.
Ongoing
Running, but no longer taking on new participants.
Completed This study
The trial has finished. Results may not be published yet.
Stopped early
Stopped early, before it reached the end. That can be for many reasons, including safety.
Cancelled
Cancelled before anyone took part.

Expanded access (not trials)

Expanded access
Not a trial. This treatment can be requested outside a study, case by case, for people who qualify.
Expanded access (paused)
Not a trial. The treatment can normally be requested outside a study, but is unavailable right now.
Expanded access (ended)
Not a trial. The treatment could once be requested outside a study, but no longer can.
Approved
The treatment has been approved, so it is available normally rather than through this programme.

When the status isn't known

Details not published
The full record has not been published yet, so there is little to show here.
Status unknown
This status has not been confirmed recently, so it may be out of date.

First seen Jun 25, 2026 · Last updated Jun 26, 2026 · Updated 1 time

Summary

This completed study tested whether sending behavioral nudges through Denmark's official electronic letter system could increase flu vaccination among adults aged 18-64 with chronic diseases. Over 300,000 participants were randomly assigned to receive different types of letters or no letter. The goal was to see if simple, well-designed messages could encourage more people to get vaccinated, potentially reducing flu-related complications in this vulnerable group.

What this could mean

Our plain-language read of the trial. This is informational only, not medical advice or a prediction.

Active substance
behavioral nudges (letters using behavioral economic principles)
What this could lead to
If effective, this approach could provide a low-cost, scalable way to boost flu vaccination rates in high-risk populations, reducing flu-related illness and hospitalizations.
What could go wrong
This is a completed trial confirming previous findings; results may not generalize to other countries or populations. The intervention is a nudge, not a vaccine, so its impact depends on individual behavior.

This is an AI summary of the original study and may miss details. Read our disclaimer.

Study facts

What this study's own registry entry says, in plain language.

Phase

Not a phased trial

Phase numbers describe drug development. The registry uses this when they do not apply, as it does for trials of devices, procedures or behaviour changes, and for observational studies.

Participants

308,978 people

The number who actually took part.

Started

Oct 2024

Finished

May 2025

Lead sponsor

Other sponsor

The registry's catch-all category, for sponsors it does not file as a company, a government agency, or a research network.

Who can take part

This study's own entry requirements. Only the study team can say for certain whether you qualify.

Ages

18 to 64 years

Sex

Anyone

Healthy volunteers

Not accepted

This study is not open to healthy volunteers. The entry requirements below say who it is open to.

Show the full entry requirements

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

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

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Conditions

The condition(s) this trial relates to.

Behavior cardiovascular disorder chronic kidney disease chronic liver failure congenital or acquired type 1 diabetes mellitus type 2 diabetes mellitus influenza prevention target

As listed by the trial registrant

The condition terms exactly as the trial's registrant entered them.

Behavior and behavior mechanisms Influenza

Contacts and locations

Locations

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

    Hellerup, Capital Region, 2900, Denmark

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