Franziska Maria Keller

• The COVID-19 pandemic as a crisis has been associated with changes in daily interaction, social connectedness, mental health, support structures, and the provision of (mental) health care treatment. This thesis aims to examine 3 main targets: psychological well-being during crises, psychological mechanisms related to reactions and consequences of individuals and systems, and the effectiveness of digital interventions to support mental health. In 5 studies the following were examined: (1) triggers of preventable adverse events (pAEs) through evaluating the psychometric properties of a questionnaire; (2) hand hygiene behavior along the Health Action Process Approach (HAPA) through structural equation modeling; (3) differences in worries and mental health of the general population and psychosomatic rehabilitation patients; (4) evaluation of the intercorrelation between psychological variables along the Evolutionary Theory of Loneliness via a serial mediation model; (5) evaluation of the effectiveness of digital (psycho)therapeutic interventions. In study 1, 5 areas of triggers of pAEs were defined and the questionnaire showed good psychometric properties. Study 2 has shown that hand hygiene behavior could be explained along the HAPA with the pattern being invariant for mental health. Study 3 has demonstrated higher mental health-related symptoms for psychosomatic rehabilitation patients who also reported different worries than the general population. Study 4 has shown a serial and individual mediation effect of loneliness and anxiety between distress and depression. Adding digital interventions to the traditional therapy approach supported the reduction of psychological symptoms as shown in study 5. The findings of this thesis contribute to a greater understanding of psychological mechanisms evaluated by reactions and consequences associated with the COVID-19 pandemic and allow for several practical, theoretical, and methodological conclusions to be drawn.