The COVID-19 pandemic has led to significant psychological distress among frontline healthcare workers (HCWs), with a particular increase in trauma-related symptoms. This study investigated the longitudinal course of trauma-associated symptoms and behaviors in HCWs and the effectiveness of a brief dialectical behavior therapy (DBT)-informed intervention in mitigating these symptoms over 12 months. The trial included 225 HCWs randomly assigned to one of three groups: no intervention (control), in-person DBT-informed intervention, or online DBT-informed intervention. Over time, a natural decrease in PTSD symptoms was observed in all groups. Contrary to expectations, no difference was found between the control and intervention groups. However, for participants with severe PTSD symptoms, the intervention significantly mitigated their distress. No differences emerged between in-person and online interventions, suggesting equal effectiveness. Females reported higher trauma-related symptoms, while no differences emerged among different professional roles. These findings underscore the importance of targeted interventions for HCWs experiencing severe symptoms and highlight the potential of online modalities. Further research is needed to optimize the deployment of mental health resources within the healthcare setting, particularly during crises.