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

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.

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 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.

• Study Type: Interventional
• Enrollment (Actual): 1226
• 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

2

Arms and Interventions

Participant Group / Arm	Intervention / Treatment
No Intervention: Control; Participants were assigned to view a vaccination invitation email containing the control weblink (UK participants: accurx.thirdparty.nhs.uk/r/aafwaczmd5; US participants: accurx.thirdparty.pharma-us.com/r/aafwaczmd5
Experimental: Clear Link; Participants were assigned to view an email containing an improved version of the link (UK participants: https://vaccine-booking.nhs.uk; US participants: https://vaccine-booking.pharma-us.com)	Behavioral: Improved trust and engagement; The intervention involves presenting a digital COVID-19 booster vaccination invitation via email. Participants are randomly allocated to view one of two versions of the invitation to evaluate the impact of the weblink design on trust and behaviour. To ensure local relevance, the email content is adapted based on the participant's country of residence; participants in the United Kingdom see invitations sent from the NHS, while those in the United States see invitations from Pharma-US

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 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 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 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 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 3, 2022
• Primary Completion (Actual): January 10, 2022
• Study Completion (Actual): January 10, 2022

Study Registration Dates

• First Submitted: March 24, 2026
• First Submitted That Met QC Criteria: March 31, 2026
• First Posted (Actual): April 8, 2026

Study Record Updates

• Last Update Posted (Actual): April 8, 2026
• Last Update Submitted That Met QC Criteria: March 31, 2026
• Last Verified: March 1, 2026

More Information

Terms related to this study

• Keywords: digital health; health communication; trust
• Other Study ID Numbers: ETH 2122-0265 Expe1; 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