Background/Objectives: According to a diathesis–stress model for the development of mental illness, it is assumed that, in addition to pre-existing individual vulnerability, the occurrence of acute strains is an etiological factor. The SARS-CoV-2 pandemic was a collective massive stressor, which could predispose to a first manifestation of a mental disorder or the exacerbation of a pre-existing mental disorder. The aim of this study was to investigate the effects of the pandemic on the cohort of patients admitted to hospital during the first year of the pandemic.

Methods: Patients admitted to inpatient treatment in a university psychiatric hospital in an urban region from April 2020 to March 2021 were interviewed using a systematic questionnaire assessing individual stress factors in the context of the pandemic. On the basis of the interview, clinical practitioners rated the influence of the pandemic on the admission.

Results: Six hundred and forty-five patients were interviewed. Only 6.4% showed a strong influence of the pandemic on inpatient admission. This group was characterized by a comparatively high level of socioeconomic functioning. Additionally, the majority of this group had a pre-existing mental disorder.

Conclusions: For the majority of patients, the pandemic had only a minor influence on their hospitalization; only for 6.4% was a high impact of the pandemic reported. We hypothesize that this group’s higher socioeconomic functioning in addition to a pre-existing mental disorder made them vulnerable to pandemic-associated limitations. These data confirm a complex diathesis–stress model for the development of mental illness in the context of an acute collective stressor.