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

Inclusion Criteria:

• At least 65 year old citizens in the municipalities of Espoo, Maalahti, Korsnäs, Kristiinankaupunki, Kaskinen and Närpiö

Exclusion Criteria:

-

This study is based in administrative register follow-up. We will share pseudonymized data according to current data availability regulations.