Implementation and Adherence of a Custom Mobile Application for Anonymous Bidirectional Communication Among Nearly 4000 Participants: Insights from the Longitudinal RisCoin Study

<b>Background</b>: The longitudinal RisCoin study investigated risk factors for COVID-19 vaccination failure among healthcare workers (HCWs) and patients with inflammatory bowel disease (IBD) at a University Hospital in Germany. Since the hospital served as the study sponsor and employer of the HCW, we implemented a custom mobile application. We aimed to evaluate the implementation, adherence, benefits, and limitations of this study's app. <b>Methods</b>: The app allowed secure data collection through questionnaires, disseminated serological results, and managed bidirectional communication. Access was double-pseudonymized and irreversibly anonymized six months after enrollment. Download frequency, login events, and questionnaire submissions between October 2021 and December 2022 were analyzed. Multivariable logistic regression identified factors associated with app adherence. <b>Results</b>: Of the 3979 participants with app access, 3622 (91%) used the app; out of these, 1016 (28%) were "adherent users" (≥12 submitted questionnaires). App adherence significantly increased with age. Among HCW, adherent users were more likely to be non-smokers (<i>p</i> < 0.001), working as administrators or nursing staff vs. physicians (<i>p</i> < 0.001), vaccinated against influenza (<i>p</i> < 0.001), and had not travelled abroad in the past year (<i>p</i> < 0.001). IBD patients exposed to SARS-CoV-2 (<i>p</i> = 0.0133) and those with adverse events following the second COVID-19 vaccination (<i>p</i> = 0.0171) were more likely adherent app users. Despite technical issues causing dropout or non-adherence, the app served as a secure solution for cohort management and longitudinal data collection. <b>Discussion:</b> App-based cohort management enabled continuous data acquisition and individualized care while providing flexibility and anonymity for the study team and participants. App usability, technical issues, and cohort characteristics need to be thoroughly considered prior to implementation to optimize usage and adherence in clinical research.