Supplementary Material for: Effectiveness of an Internet-Based Self-Help Intervention versus Public Mental Health Advice to Reduce Worry during the COVID-19 Pandemic: A Pragmatic, Parallel-Group, Randomized Controlled Trial

<b><i>Introduction:</i></b> The mental health burden for the general population due to the COVID-19 pandemic has been highlighted. Evidence on effective, easily accessible public health interventions to reduce worry, a major transdiagnostic risk-factor for, e.g., anxiety and depression, is scarce. <b><i>Objective:</i></b> In a pragmatic randomized controlled trial, we aimed to assess whether an internet cognitive-behavioral self-help intervention could reduce worry more than public mental health advice in the general population. <b><i>Methods:</i></b> Eligible internet users above the age of 18 were recruited from the German general population and randomly assigned, to either get.calm-move.on (GCMO), a 10-day unguided, internet-based self-help intervention, or mental health advice waiting group (MHA-W, receiving officially endorsed mental health recommendations). The primary outcome was level of worry, using the Penn State Worry Questionnaire (PSWQ), 2 weeks after randomization. Baseline assessment and 2-month and 6-month follow-ups were conducted. The trial was registered at the German Clinical Trials Registry (DRKS00021153). <b><i>Results:</i></b> Between April 7, 2020 and December 11, 2020, we randomly assigned 351 individuals to receive either GCMO (<i>n</i> = 175) or MHA-W (<i>n</i> = 176). Participants receiving GCMO (PSWQ = 46.6; change –10.3) reported significantly less worrying at post-intervention (<i>F</i>_1,219 = 12.9; <i>p</i> &lt; 0.001; <i>d</i> = 0.38) than MHA-W controls (PSWQ = 51.6; change –5.1). Improvements were also seen on most secondary outcomes, including symptoms of anxiety and depression, general well-being, resiliency, and emotion regulation skills. Improvements made from baseline were stable until the 6-month follow-up. <b><i>Conclusions:</i></b> This internet-based self-help intervention providing cognitive-behavioral techniques to cope with the threatening pandemic situation is effective in reducing worry in the general population and should complement existing and potentially effective mental health recommendations.