The goal of this individual-level pragmatic randomized policy trial is to learn about the impact of behavioural nudges via mailed letters on increasing influenza and travel vaccination uptake in Alberta, Canada. The primary outcome is the proportion of adults receiving influenza and/or travel vaccination. The secondary outcome is the number of stroke events in the 6 months following vaccination. As such, this research aims to answer two key questions:

1. Do behavioural nudges increase influenza and travel vaccination in Alberta?
2. If yes, is there a decrease in the number of stroke events in the 6 months following vaccination?

The two-arm individual level pragmatic randomized trial design includes two letter dimensions, a letter (a "nudge") with an influenza and cardiovascular risk reduction message and a letter with a travel vaccination benefit message, yielding the following groups:

Letter with an influenza and cardiovascular risk reduction message Letter with a travel vaccination benefit message

Study Population Setting This research project will take place in Alberta, Canada and in partnership with Mint Health and Drugs (Mint Pharmacy), an Alberta-based pharmacy corporation. Participants will be recruited through Mint Pharmacy patient records.

Inclusion Criteria 18 years of age and older by September 1, 2025, and visited a Mint Pharmacy site within 1 year of September 1, 2025.

Study Procedures Sampling Size and Strategy Mint Pharmacy will be sending a letter to all active clients (defined as an adult who filled a prescription at their pharmacy in the last year). The investigators estimate this will be 20,000 clients with 10,000 receiving the influenza vaccine nudge and 10,000 receiving the travel vaccine nudge. This population size will allow the investigators to detect an absolute difference of 2% between the two groups with 80% power and α= 0.05.

Recruitment and Screening Mint Pharmacy analysts will pull the study sample from their patient records, including first and last name, date of birth, and sex. The list of participants will be shared with Alberta Strategy for Patient Oriented Research (AbSPOR) analysts for randomization and data linkage. AbSPOR analysts will share the randomized list of participants, including mailing addresses, with the approved mail vendor to print and mail the appropriate nudge letter that the participant has been randomized to receive. The investigators will stratify be geographic zone (North, Edmonton, Central, Calgary, and South) because of the geographic variation in vaccine uptake.

Intervention The Alberta Government provides free influenza vaccines from October through March annually, which corresponds with peak travel vaccine season. Travel vaccines are typically low cost or covered by private insurance. Vaccines are frequently administered by pharmacists in private pharmacies. As a pragmatic trial, there will be no in-person encounters with patients during the trial. The intervention will be delivered through mailed letters with different messages described below, to be delivered in mid-October 2025. The date of the initial letter will be defined as study baseline.

Arm 1 (Letter with an influenza risk reduction message): This letter will be signed by the pharmacist at the Mint Pharmacy site that the participant accesses and will discuss the benefits of the influenza vaccine in preventing influenza infection and cardiovascular disease risk reduction.

Arm 2 (Letter with travel vaccination benefit message): This letter will be signed by the pharmacist at the Mint Pharmacy site that the participant accesses and will discuss the benefits of travel vaccines in preventing travel related disease, such as evidence that vaccination against Hepatitis A, a common travel vaccination, is 90-95% effective in preventing Hepatitis A infection.

Outcomes Primary outcome: receipt of an influenza and/or travel vaccine on or before March 1, 2026.

Secondary outcome: Number of hospitalizations and emergency department visits for influenza and the number of stroke events on or before September 30, 2026 (6 months following influenza vaccination programs)

Blinding Study researchers will be blind to the participants' group allocation. Participants will not be aware that the communications they receive are part of a research study. Because of this deception, the investigators are requesting a waiver of consent and not attaching a patient information form or informed consent form.

Waiver of Consent

A waiver of consent is necessary both logistically and to limit bias. It is not feasible to obtain informed consent with a sample size n=5190. Additionally, because this study aims to investigate health behaviour, obtaining prior consent alerts participants to the existence of other health messages and may affect their response to the health message they do receive, introducing response bias. Our firm commitment to patient engagement and robust patient engagement plan also reduces participant risk. As per the Tri-Council Policy Statement (TCPS) 2.0, Article 3.7A, alterations to the consent process (including waiver of consent) are appropriate when:

1. The research involves no more than minimal risk to the participant
2. The alteration to consent requirements is unlikely to adversely affect the welfare of participants
3. It is impossible or impractical to carry out the research and to address the research questions properly, given the research design, if the prior consent of participants is required

The proposed study meets the above TCPS 2.0 requirements for alterations to the consent process.

Data Collection Mint Pharmacy analysts will pull the study sample from their patient records, including first and last name, date of birth, and sex. The list of participants will be shared with Alberta Strategy for Patient Oriented Research (AbSPOR) analysts for randomization and data linkage. AbSPOR analysts will share the randomized list of participants, including mailing addresses, with the approved mail vendor to print and mail the appropriate nudge letter that the participant has been randomized to receive. AbSPOR analysts will gather vaccination data from the Alberta Health Immunization and Adverse Reactions Following Immunization (IMM/ARI) database by linking participant health numbers to determine how many participants received a vaccination. The IMM/ARI database is the most comprehensive vaccination database as it is where all vaccinations are required to be reported in Alberta with nearly 100% coverage. AbSPOR analysts will gather hospitalization, emergency department visit, and the number of stroke events from the Alberta Health Discharge Abstract Database (DAD) using participant health numbers. Influenza and stroke events will be identified using ICD-10 codes. This data collection will be continuous from April 1st, 2026, through September 30th, 2026, six-months following the influenza vaccination program. Data linkage and cross-reference will be conducted by AbSPOR, who will share de-identified data with the research team in either de-identified or aggregate form for analysis.

Data Analysis Data analysis will be conducted by members of the research team using the de-identified data provided by AbSPOR. Statistical comparisons will be performed according to the intention-to-treat principle. The investigators will report the proportion of adults who received influenza vaccination and travel vaccination (any vaccines, specific vaccine types, and full series of vaccines) in each group along with 95% confidence intervals. The investigators will model the risk difference for each type of nudge compared to the comparison group) in separate models, including adjustment for multiple comparisons. As a secondary analysis, the investigators will assess time to vaccination in the influenza letter group compared to the travel vaccine letter group. To assess impact of vaccine uptake on influenza and stroke events, the investigators will compare the number of events in one intervention group with the number of stroke events in the comparison group. The primary analysis will be conducted on the as-randomized populations (intention-to-treat analysis).

Inclusion Criteria:

• Visited a Mint Pharmacy site within 1 year of enrollment date
• 18 years of age and older by enrolment date