Nationwide Utilization of Danish Government Electronic Letter System for Increasing InFLUenza Vaccine Uptake Among Adults With Chronic Disease

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.

Inclusion criteria

• Age 18-64 years
• 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:

• Chronic lung disease
• Chronic cardiovascular disease other than hypertension
• Type 1 or type 2 diabetes mellitus
• Congenital or acquired immunodeficiency
• Impaired breathing due to muscular weakness
• Chronic renal or hepatic insufficiency
• Other chronic conditions with an increased risk of severe influenza as determined by the treating physician
• Access to the official, mandatory Danish electronic mailbox system

Exclusion criteria

None