Optimizing Web Links Used in Digital Vaccination Invitations to Raise Trust and Booking Intention: 3 Online Randomized Controlled Trials.

People are encouraged to respond swiftly to digital health invitations, but they can be (rightly) skeptical about their legitimacy. Drawing on digital communication theory and psychological science, we hypothesized that easy-to-read web links that facilitate participants' ability to identify the health organization as the website host would improve trust in and user engagement with digital communications. In 3 double-blind, randomized online experiments, adult UK residents were recruited via the online platform Prolific (experiment 1: N1=569) or Qualtrics (experiment 2: N2=596; experiment 3: N3=1993). In each experiment, participants read a hypothetical email invitation for a COVID-19 vaccine from the UK's National Health Service (NHS). Participants reported trust in the email (primary outcome), whether the web link was easy to read, who they thought the website host was, and their intention to book an appointment via the web link. We manipulated the booking web link (between-participants, double-blind allocation via the Qualtrics randomizer). Across experiments, the control group read the email containing a deactivated NHS vaccination booking web link: "accurx.thirdparty.nhs.uk/r/aafwaczmd5." In experiment 1, participants read the email with the control link (randomized n=300, analyzed n=286) or an experimental clear link ("vaccine-booking.nhs.uk"; randomized n=301, analyzed n=283). In experiment 2, participants read the email with the control link (randomized n=203, analyzed n=201) or one of 2 experimental links: a shortened web link ("https://bit.ly/3GtTL0c"; randomized n=201, analyzed n=201) or a text-embedded link ("book here"; randomized n=196, analyzed n=194). In experiment 3, participants read the email with the control link (randomized n=655, analyzed n=655), the clear link (randomized n=669, analyzed n=668), or the text-embedded link (randomized n=670, analyzed n=670). Across experiments, the control web link was poorly perceived, with most participants (729/1142, 63%) unsure or unlikely to use it. Relative to the control web link, the clear web link improved trust (β coefficientExpe1=0.26, 95% CI 0.18-0.36; β coefficientExpe3=0.24, 95% CI 0.19-0.30). The text-embedded web link also improved trust (β coefficientExpe2=0.18, 95% CI 0.09-0.27; β coefficientExpe3=0.17, 95% CI 0.11-0.22), but the shortened web link did not (β coefficientExpe2=0.01, 95% CI -0.09 to 0.11). Across experiments, improved host identification and ease of reading explained increased trust, which was significantly associated with increased booking intention. The clear and text-embedded web link (vs control) effects were robust when controlling for demographics. We extend digital communication research by investigating how web link design can reduce people's justified suspicion in health messaging. Moving beyond the previous research focus on health message content, we experimentally test the role of web link wording. We bring causal evidence that easier-to-read links that have easy-to-identify host health institutions increased trust and intention to use the link. We provide simple, practical design guidance to improve real-world engagement with digital health communications.