An interventional study of Letter in Influenza, Human, sponsored by Finnish Institute for Health and Welfare. Completed at 1 site in Finland. Open to participants aged 65 Years and older, including healthy volunteers. Per ClinicalTrials.gov, last updated 2025-08-20.
Sponsored by Finnish Institute for Health and Welfare · Not applicable, Interventional, and Prevention
The aim of this study is to test the effectiveness of communicating the concept of herd immunity on actual influenza vaccine uptake using a randomized controlled trial.
The investigators run a large-scale randomized controlled trial (RCT) to understand behavioural processes that guide decisions to vaccinate and test alternative strategies to communicate the concept of herd immunity. The experiment varies the text of postal letters send to individuals eligible for free influenza vaccines. The experiment is run in collaboration with the municipal health authorities.
The investigators identify from the Population Register all individuals who are born in year 1953 or before and resided in the participating municipality on October 01, 2018. The investigators will obtain the postal addresses of these individuals using the Population Register and randomly assign individuals to treatment conditions that differ in the wording of the information material and reception of a text message. Importantly, the extract from the Population Register will also include an individual identifier (social security number) that can be used to match the received type of letter (treatment) with the actual vaccination decision recorded in the Care Register for Health Care to objectively evaluate the impact of received information on vaccine uptake. The matching of postal addresses and vaccination data will be conducted using pseudonymized identifiers. The pseudonymization of individual identifiers will be performed separately for both datasets by professional data management and data security staff of the National Institute for Health and Welfare. The investigators conducting the data analysis will be able to access only pseudonymous data without a possibility to directly identify individuals from the data set. The data will be stored according to the data protection legislation and regulations of the National Institute for Health and Welfare during the research project.
The investigators randomly assign individuals to treatment conditions that differ in the wording of the letters. The authority's standard letter serves as the baseline condition (T1). This letter is extended by communicating the concept of herd immunity and appealing to pro-social preferences by highlighting the fact that vaccinations may effectively prevent infections among people who are not able to vaccinate themselves (especially babies) (T2). To summarize, this study includes a control treatment (T0, no letter) and two active treatment arms with the following contents:
T1 (Mailing) - Standard letter T2 (Herd) - Herd immunity letter
2,356 studies on the registry are indexed under Influenza, Human; 184 are open to participants now.
This study's enrollment of 48,125 is above the median of 227 across 1,957 interventional studies indexed under Influenza, Human.
Browse Influenza, Human studies →Finnish Institute for Health and Welfare is the lead sponsor of 42 studies on the registry; 3 are open to participants now.
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Exclusion Criteria:
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There is no intervention (letter) in the control arm. There is no control arm in the city of Espoo. 1/3 of subjects in all other municipalities are assigned to the control arm.
This treatment arm consists of a standard letter reminding elderly citizens (65 years and above) about influenza vaccines that are available free of charge from the local health centers.
Other: Letter
This treatment arm consists of a letter that highlights the herd immunity effects of vaccination and reminds elderly citizens (65 years and above) about influenza vaccines that are available free of charge from the local health centers.
Other: Letter
Intervention consists of two different types of postal letters reminding elderly citizens (65 years and above) about influenza vaccines that are available free of charge from the local health centers
Vaccine uptake
Number of influenza vaccinations in study population
Time frame: up to six months (beginning from November 1 / until April 30)
Heterogeneity
We run the statistical analysis by municipality to study ATE in areas with low/high historical vaccine uptake
Time frame: up to six months (beginning from November 1 / until April 30)
Plan to share: Undecided — This study is based in administrative register follow-up. We will share pseudonymized data according to current data availability regulations.
This study is completed, as verified in Aug 2025. You cannot join it, but the record below documents what was studied.
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Finnish Institute for Health and Welfare