Optimizing Weblinks Used in Digital Vaccination Invitations to Raise Trust and Booking Intention: Online Experiment 1

This randomised experimental study investigates how weblink design influences the perceived trustworthiness of digital health communications. As digital invitations (e.g., emails and SMS) have become standard for public health initiatives such as vaccination programmes, the use of third-party or disfluent (hard-to-read) web links may inadvertently undermine trust.

Study Protocol and Intervention Participants will be randomly allocated to one of two experimental conditions. In both conditions, participants will view a hypothetical email invitation for a COVID-19 booster vaccine. The content of the emails will remain identical except for the format of the embedded booking link: Control Condition: Features the "accurx" web link (accurx.thirdparty.nhs.uk/r/aafwaczmd5), a third-party format previously used in COVID-19 vaccination invitations by the National Health Service (NHS) in the UK. Experimental Condition: Features an improved, transparent - "clear" - weblink that was designed to be easier to read and clearly identifies the host organisation (e.g., https://vaccine-booking.nhs.uk). The UK and US respondents will see different hypothetical emails tailored to their country of residence: UK participants will see invitations sent from the NHS, while US participants will see invitations from a fictitious pharmacy (Pharma-US). Unlike the UK, COVID-19 vaccination in the US was predominantly organised by local pharmacies.

Primary Hypotheses

The study is designed to test four primary hypotheses:

• Emails containing the control "accurx" link will be rated as significantly less trustworthy than those with the improved weblink.
• The improved link will increase the intention to book a vaccination appointment.
• Participants will perceive the improved link as being more fluent (easier to read) than the control link.
• The improved link will facilitate significantly higher accuracy in identifying the host organisation that sent the communication.

Measurement Following the presentation of the stimulus, participants will rate the emails on three 5-point Likert scales: trustworthiness (1: Very suspicious to 5: Very trustworthy), link fluency (1: Very difficult to 5: Very easy), and booking intention (1: Very unlikely to 5: Very likely). Host identification is assessed via a multiple-choice question with four options: the NHS, Pharma-US, Accurx, or "Not sure". The investigators aim to collect data from 600 participants from the UK and 600 from the US. Participants will be invited to complete the study via an online platform.

Data will be analysed using a within-subject analysis of variance (ANOVA) to examine the effects of link manipulation, with further robustness checks incorporating sociodemographic variables such as age, gender, and education. UK and US participants data will be analysed separately if needed. This study is conducted as part of a larger research project exploring the broader relationship between the trustworthiness of digital messages and national vaccination rates.

The study hypotheses, design, measures and analyses were preregistered before the start of data collection on a different platform (AsPredicted). This time stamped preregistration protocol is link as a document in this registration.

Inclusion criteria

• Residency: Must be a resident of either the United Kingdom or the United States

Exclusion criteria

• Attention Failure: Participants who failed the designated attention check included within the survey will be excluded from the analysis
• Speeding: Participants who completed the study "too fast"-indicating a lack of meaningful engagement with the experimental stimuli-were filtered out