Optimizing Weblinks Used in Digital Vaccination Invitations to Raise Trust and Booking Intention: Online Experiment 3

This study investigates how the design of weblinks in digital vaccination invitation emails influences recipient trust and their willingness to book an appointment. In this study, investigators compare three different link formats: a control third-party link previously used by the NHS, and two experimental weblinks: an improved version of the link, and a text embedded hyperlink.

The study tests primarily whether the two experimental weblinks will be perceived as more trustworthy and increase booking intention than the control weblink. Furthermore, the study examined whether the experimental weblinks are perceived to be more fluent (easier to read) and improve 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 planned to gather data from 2,000 participants from the United Kingdom and 2,000 from the United States. They will be assigned to view one of the three hypothetical email versions. UK participants will see emails that appear to be from the NHS, while US participants see emails that appear to be from a local fictitious pharmacy. Due to US emails appearing to come from a fictitious pharmacy, investigators also expected that the benefit of correctly identifying the host organisation would be more pronounced in the United Kingdom than in the United States.

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: Concealing the weblink included in vaccination email invitation; Behavioral: Improvement to the weblink included in the vaccination email invitation

Detailed Description

This randomised experimental study investigates how weblink design influenced 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 three experimental conditions. In all conditions, participants will view a hypothetical email invitation for a COVID-19 booster vaccine. The content of the emails remains identical except for the format of the embedded booking link:

Control Condition: Features the "accurx" web link previously used by the National Health Service (NHS) in the UK.

Experimental Condition 1: 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).

Experimental Condition 2: Features a text-embedded hyperlink.

To ensure relevance to the target populations, the stimuli are tailored to participants' countries of residence: UK participants see invitations appearing to come from the NHS, while US participants see slightly different versions of the email appearing to be sent by their local fictitious pharmacy. Unlike the UK, COVID-19 vaccination in the US was predominantly organised by local pharmacies.

Primary Hypotheses

The study is designed to test five 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 will 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. Identifying the organisation hosting the link will have a more beneficial effect on perception in the UK than in the US.

Measurement

Following the presentation of the stimulus, 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 is assessed via a multiple-choice question with four options: "the NHS", "your local pharmacy", "accurx", or "Not sure".

Investigators will gather data from 2,000 participants from the UK and 2,000 from the US. They will be invited to complete the study via an online platform. Participants who complete the study too fast or fail the attention check will be excluded.

This study will be conducted as part of a larger research project exploring the broader relationship between the content of health text messages and trust perception.

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): 4014
• 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: The weblink included in the email is cryptic: accurx.thirdparty+Host; 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.pharmacy.com/r/aafwaczmd5)
Experimental: The web link in the email is a text-embedded 'Book here' weblink; Participants assigned to this arm are shown a hypothetical COVID-19 booster vaccination invitation email in which the booking link is presented as a descriptive hyperlink - where the link is embedded in text (e.g., "Book your vaccine here").	Behavioral: Concealing the weblink included in vaccination email invitation; Participants were presented a hypothetical COVID-19 booster vaccination invitation email that included a booking weblink. The intervention involved concealing the weblink included in the email within text using a hyperlink (click HERE).
Experimental: The web link in the email is a "clear" weblink: https://vaccine-booking+Host; Participants were assigned to view a hypothetical email containing an improved version of the weblink that more clearly showed the website host and that was designed to be easier to read (UK participants: https://vaccine-booking.nhs.uk; US participants: https://vaccine-booking.pharmacy.com)	Behavioral: Improvement to the weblink included in the vaccination email invitation; Participants were presented a hypothetical COVID-19 booster vaccination invitation email that included a booking weblink. The intervention involved improving the weblink included in the email to make it easier to read and facilitate the identification of the website host.

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, your local pharmacy, 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): February 1, 2022
• Primary Completion (Actual): March 2, 2022
• Study Completion (Actual): March 2, 2022

Study Registration Dates

• First Submitted: March 24, 2026
• First Submitted That Met QC Criteria: April 15, 2026
• First Posted (Actual): April 20, 2026

Study Record Updates

• Last Update Posted (Actual): April 20, 2026
• Last Update Submitted That Met QC Criteria: April 15, 2026
• Last Verified: April 1, 2026

More Information

Terms related to this study

• Keywords: Trust; Digital Health Communications
• Other Study ID Numbers: ETH 2122-0265 Expe3; 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