question: INTRODUCTION: We designed a follow-up study of frontline health workers at COVID-19 patient care, within the same working conditions, to assess the influence of their general characteristics and pre-existing anxiety/depression/dissociative symptoms and resilience on the development of symptoms of post-traumatic stress disorder (PTSD), while monitoring their quality of sleep, depersonalization/derealization symptoms, acute stress, state anxiety, and burnout. METHODS: In a Hospital reconfigured to address the surge of patients with COVID-19, 204 frontline health workers accepted to participate. They completed validated questionnaires to assess mental health: before, during, and after the peak of inpatient admissions. After each evaluation, a psychiatrist reviewed the questionnaires, using the accepted criteria for each instrument. Correlations were assessed using multivariable and multivariate analyses, with a significance level of .05. RESULTS: Compared to men, women reporting pre-existing anxiety were more prone to acute stress; and younger age was related to both pre-existent common psychological symptoms and less resilience. Overall the evaluations, sleep quality was bad on the majority of participants, with an increase during the epidemic crisis, while persistent burnout had influence on state anxiety, acute stress, and symptoms of depersonalization/derealization. PTSD symptoms were related to pre-existent anxiety/depression and dissociative symptoms, as well as to acute stress and acute anxiety, and negatively related to resilience. CONCLUSIONS: Pre-existent anxiety/depression, dissociative symptoms, and coexisting acute anxiety and acute stress contribute to PTSD symptoms. During an infectious outbreak, psychological screening could provide valuable information to prevent or mitigate against adverse psychological reactions by frontline healthcare workers caring for patients.