Millions of volunteers respond after disasters, with a 24% to 46% risk of developing posttraumatic stress disorder (PTSD). It is unclear which symptom trajectories develop and how they differ between core (volunteering before the disaster) and noncore volunteers (joining after the disaster) and which factors predict trajectories.

Symptoms of PTSD were assessed at 6-, 12-, and 18-months post-earthquake in449 volunteers in Indonesia. Demographics, previous mental health service use, self-efficacy, social acknowledgment, and type of tasks were assessed at 6 months. In both core and noncore volunteers, 2 PTSD symptom trajectories emerged: a resilient trajectory (moderate levels of symptoms with a slow decrease over time; 90.9%) and a chronic trajectory (higher levels of symptoms with an increase over time;9.1%). In both trajectories, core volunteers had fewer symptoms than noncore volunteers.

Core volunteers in the chronic trajectory were characterized by having sought prior mental help, reported lower levels of self-efficacy and social acknowledgment, and were more likely to have provided psychosocial support to beneficiaries (Cramér’s V = .17 to .27, partial η²= .02 to .06). Aid organizations should identify and follow up chronic PTSD trajectories in volunteers, including the noncore, who may be out of sight to the organization after the acute response phase.