Optimizing Weblinks Used in Digital Vaccination Invitations to Raise Trust and Booking Intention: Online Experiment 2

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.

Study Overview

• Status: Completed
• Conditions: Vaccination
• Intervention / Treatment: Behavioral: Improved trust and engagement

Detailed Description

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.

• Study Type: Interventional
• Enrollment (Actual): 1203
• Phase: Not Applicable

Contacts and Locations

Study Locations

• United Kingdom
  • Essex
    • Colchester, Essex, United Kingdom, CO4 3SQ
      • University of Essex

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Adult; Older Adult
• Accepts Healthy Volunteers: Yes

Description

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

Study Plan

How is the study designed?

Design Details

• Primary Purpose: Prevention
• Allocation: Randomized
• Interventional Model: Parallel Assignment
• Masking: Triple

Number of Arms

3

Arms and Interventions

Participant Group / Arm	Intervention / Treatment
No Intervention: Control; Participants were assigned to view a hypothetical vaccination invitation email containing the control weblink (UK participants: accurx.thirdparty.nhs.uk/r/aafwaczmd5; US participants: accurx.thirdparty.pharmaus.com/r/aafwaczmd5)
Experimental: Shortened URL; Participants assigned to this arm are shown a hypothetical COVID-19 booster vaccination invitation email featuring a shortened URL using a bit.ly domain. To ensure local relevance, the email stimulus is tailored to the participant's geographic location: UK participants view an email appearing to be from the NHS, while US participants view an invitation appearing to be from a fictitious pharmacy, Pharma-US.	Behavioral: Improved trust and engagement; The intervention involved presenting a hypothetical invitation to a COVID-19 booster vaccination via email. Participants were randomly allocated to view one of three versions of the invitation to evaluate how the specific design of the embedded weblink influences recipient trust and booking behaviour. To ensure local relevance, the email stimulus was adapted based on the participant's country of residence: participants in the United Kingdom saw invitations that appeared to be from the NHS, while those in the United States saw invitations that appeared to be from a fictitious pharmacy, Pharma-US. Following the intervention, participants evaluated the email's trustworthiness, the link's fluency, and their own intention to book a vaccine appointment.
Experimental: Text-embedded link; Participants assigned to this arm are shown a hypothetical COVID-19 booster vaccination invitation email where the booking link was presented as a descriptive hyperlink (e.g., "Book your vaccine here") rather than a visible URL. Following the study's geographic customisation, UK participants viewed an email stimulus that appeared to be from the NHS, while US participants viewed an invitation that appeared to be from Pharma-US.	Behavioral: Improved trust and engagement; The intervention involved presenting a hypothetical invitation to a COVID-19 booster vaccination via email. Participants were randomly allocated to view one of three versions of the invitation to evaluate how the specific design of the embedded weblink influences recipient trust and booking behaviour. To ensure local relevance, the email stimulus was adapted based on the participant's country of residence: participants in the United Kingdom saw invitations that appeared to be from the NHS, while those in the United States saw invitations that appeared to be from a fictitious pharmacy, Pharma-US. Following the intervention, participants evaluated the email's trustworthiness, the link's fluency, and their own intention to book a vaccine appointment.

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Perceived Trustworthiness	Participants rated the perceived trustworthiness of the vaccination invitation email on a 5-point Likert scale, where 1 represents "Very suspicious", and 5 represents "Very trustworthy".	Immediately after the intervention
Booking Intention	Participants rated their likelihood of booking a vaccine appointment based on the email invitation using a 5-point Likert scale ranging from 1 ("Very unlikely") to 5 ("Very likely").	Immediately after the intervention

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Ease of reading	Participants assessed the reading ease (or fluency) of the embedded weblink on a 5-point Likert scale, where 1 is "Very difficult", and 5 is "Very easy".	Immediately after the intervention
Host Identification	Participants were asked to identify the organisation that sent the communication by selecting one answer from a multiple-choice list of four options: the NHS, Pharma-US, Accurx, or Unclear/I don't know.	Immediately after the intervention

Collaborators and Investigators

• Sponsor: University of Essex
• Collaborators: University of Southern California; Kingston University; Department of Health and Social Care, UK

Publications and helpful links

Helpful Links

• Open Science Framework: Project Repository containing data, protocols, and materials.

Study record dates

Study Major Dates

• Study Start (Actual): January 13, 2022
• Primary Completion (Actual): January 23, 2022
• Study Completion (Actual): January 23, 2022

Study Registration Dates

• First Submitted: March 24, 2026
• First Submitted That Met QC Criteria: April 8, 2026
• First Posted (Actual): April 16, 2026

Study Record Updates

• Last Update Posted (Actual): April 16, 2026
• Last Update Submitted That Met QC Criteria: April 8, 2026
• Last Verified: April 1, 2026

More Information

Terms related to this study

• Keywords: digital health; health communication; trust
• Other Study ID Numbers: ETH 2122-0265 Expe2; CRUSA210009 (Other Grant/Funding Number: British Academy COVID-19 Recovery grant)

Plan for Individual participant data (IPD)

• Plan to Share Individual Participant Data (IPD)?: YES
• IPD Plan Description: Anonymised study data is shared on the Open Science Framework
• IPD Sharing Time Frame: 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.
• IPD Sharing Access Criteria: The data and study protocol will be freely accessible to the public after the main scientific output is accepted for publication.
• IPD Sharing Supporting Information Type: STUDY_PROTOCOL; SAP; ICF

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No