This study investigates how the design of web links in digital vaccination invitation emails affects recipient trust and their willingness to book an appointment. Researchers compare three different weblink formats: a control third-party link previously used by the NHS ("accurx"), and two experimental weblinks in which the original link is hidden: a shortened "bit.ly" link, and a text-embedded hyperlink.

The study primarily tests whether the two experimental weblinks will be perceived as more trustworthy and increase booking intention compared to the control weblink. Furthermore, the study examines whether the experimental weblinks is perceived to be more fluent (easier to read) and improves participants' ability to correctly identify the organisation (e.g., the NHS or a US pharmacy) that sent the hypothetical email.

To test these effects, investigators will gather data from 600 participants from the United Kingdom and 600 from the United States. They will be randomly assigned to view one of the three hypothetical email versions. UK participants will see emails appearing to be from the NHS, while US participants will see emails from a fictitious pharmacy (Pharma-US). This research aims to provide evidence on how to design more effective and trustworthy digital health communications.

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 programs, 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 three experimental conditions. In all conditions, participants will read a hypothetical email invitation for a COVID-19 booster vaccine. The content of the emails remains identical except for the format of the booking weblink included in the email:

Control Condition: Features the "accurx" web link (accurx.thirdparty.nhs.uk/r/aafwaczmd5) based on a third-party format used in official invitations from the UK's NHS.

Experimental Condition 1: (Concealed in shortened URL) features a shortened version of the link using a bit.ly domain.

Experimental Condition 2: (Concealed, text-embedded) shows the link presented as a descriptive hyperlink.

To ensure relevance to the target populations, participants will see a version of the hypothetical email 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 was designed to test six primary hypotheses:

1. The email containing the two experimental weblinks will be rated as significantly more trustworthy than the control "accurx" weblink.
2. Participants will report being more likely to use the link to book a vaccine appointment based on the experimental weblinks than the control "accurx" web link.
3. Participants will find the two experimental weblinks more fluent (easier to read) than the "accurx" control link.
4. Participants would be more likely to correctly identify the organisation that hosts the link based on the two experimental weblinks than based on the "accurx" control web link.
5. Trustworthiness, booking intention, host identification accuracy, and fluency would be positively correlated with each other.
6. The effect of the link will be mediated by the identification of the host organisation and the fluency of the link.

Measurement

Following the presentation of the hypothetical email, participants will rate the email on three 5-point Likert scales: trustworthiness (1: Very suspicious to 5: Very trustworthy), fluency (1: Very difficult to 5: Very easy), and booking intention (1: Very unlikely to 5: Very likely). Host identification will be assessed via a multiple-choice question with four options: the NHS, Pharma-US, Accurx, or 'Not sure'.

The investigators will aim to gather data from 600 participants from the UK and 600 from the US. They will be invited to complete the study via an online platform. Participants who complete the study too fast or failed the attention check will be excluded.

This study will be 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 linked 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 three possible versions of a hypothetical vaccination invitation email-featuring either a control link, a shortened URL, or a text-embedded hyperlink-and were not informed of the existence or nature of the alternative stimuli. This masking ensures that their evaluations of trustworthiness, fluency, and booking intentions were not influenced by a direct comparison between the different link designs. As the study was conducted as an online survey via a fully automated platform, there was no direct interaction between the investigators and the participants during the intervention. 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.