This study investigates whether the design of weblinks in digital vaccination invitations affects how much people trust the message and their willingness to book an appointment. Researchers compare a standard third-party weblink used by the NHS ("accurx.thirdparty.nhs.uk/r/aafwaczmd5") with an improved, more transparent weblink (https://vaccine-booking.nhs.uk).

The study tests the hypothesis that the improved link will be perceived as more trustworthy and easier to read than the standard link. Further, the study tests whether participants would be more likely to correctly identify the sender and would be more willing to book a vaccine when shown the improved link.

To test these effects, the investigators planned to conduct a study with 600 participants from the United Kingdom and 600 from the United States. Participants will be randomly assigned to view one of the two hypothetical email versions. UK participants will see an email that appears to be from the NHS. US participants will see an email from a fictitious pharmacy (Pharma-US).This research aims to provide evidence on how to design more trustworthy digital health communications when recipients are asked to click a weblink.

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:

1. Emails containing the control "accurx" link will be rated as significantly less trustworthy than those with the improved weblink.
2. The improved link will increase the intention to book a vaccination appointment.
3. Participants will perceive the improved link as being more fluent (easier to read) than the control link.
4. 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

Anonymised study data is shared on the Open Science Framework.

The data and the study protocol were shared while the main scientific output was under review for publication. After acceptance for publication, all content will be made publicly available.

The data and study protocol will be freely accessible to the public after the main scientific output is accepted for publication.

This is a double-blind study in which participants were unaware of the specific experimental condition to which they had been assigned. Participants were randomly allocated to view only one of two possible versions of a hypothetical vaccination invitation email and were not informed of the existence or nature of the alternative stimulus. This ensures that their evaluations of trustworthiness, fluency, and booking intentions were not influenced by a direct comparison between the two link designs. As the study was conducted as an online survey via a fully automated platform, there is no direct interaction between the investigators and the participants during the intervention.