Unprecedented in scope and impact, the 9/11/2001 terrorist attacks on the World Trade Center (WTC) affected hundreds of thousands of residents, workers, and passersby in the WTC neighborhood and tens of thousands of rescue, recovery, and clean-up personnel (Murphy et al., 2007). Studies two decades later have shown that rates of posttraumatic stress disorder (PTSD) remain elevated in these populations, ranging from 14.3 to 21.9% (Bromet et al., 2016; Feder et al., 2016; Jordan et al., 2019). In our survey of the WTC Health Program General Responder Cohort (WTCHP GRC) an average of 12 years after 9/11/2001, prevalence of probable full and subthreshold WTC-related PTSD among police responders was 9.3% and 17.5%, respectively, and among non-traditional responders (e.g., construction workers) 21.9% and 24.1% (Chen et al., 2020). Both full and subthreshold WTC-related PTSD symptoms are associated with higher prevalence of comorbid psychiatric disorders, greater functional impairment, and lower self-reported qualify of life (Chen et al., 2020; Pietrzak et al., 2012). Particularly high rates of persistent WTC-related PTSD symptoms have been linked to highest trauma exposure, secondary stressors, and occupations with low disaster preparedness among WTC workers (Chen et al., 2020; Feder et al., 2016; Jordan et al., 2019; Welch et al., 2016).

Almost half of individuals with WTC-related PTSD report poor health-related quality of life and life satisfaction, and one-quarter report unmet mental health care needs (Jordan et al., 2019). Among WTC survivors, unmet mental health care needs despite accessing treatment were associated with more severe PTSD symptom trajectories (Welch et al., 2016). Despite the WTC Health Program (WTCHP) initiative to expand access to mental health services, considerable barriers remain (DePierro et al., 2021). The prevalence and chronicity of WTC-related PTSD in WTC-exposed individuals, including treated individuals, underscore the urgent need to develop and study new treatment interventions for PTSD in this population.

While trauma-focused cognitive behavioral therapies (CBT) are among the most effective and empirically supported PTSD treatments (Cusack et al., 2016), providing these therapies to symptomatic WTC workers and survivors is often limited by geography, stigma associated with treatment-seeking, and limited expert therapists (DePierro et al., 2021). This highlights the need to enhance access to evidence-based CBT while maintaining key interpersonal elements of face-to-face psychotherapy, such as therapeutic alliance. Less costly, more scalable, and potentially less stigmatizing than face-to-face CBT, integrative testimonial therapy (ITT) is a promising candidate. ITT is a therapist-assisted, Internet-based CBT conducted entirely via asynchronous patient-therapist written communications (Knaevelsrud et al., 2017, 2014; Wagner et al., 2012). Initially designed to treat individuals with persistent PTSD symptoms stemming from decades-old war-related trauma, ITT was developed from Interapy, a therapist-assisted, Internet-based psychotherapy, by adding an initial treatment phase during which patients construct a “chronological narrative” of their life – an approach based on narrative exposure therapy. This aimed to integrate a self-described, detailed written account of the index trauma into a “resource-oriented biographical narrative” (Knaevelsrud et al., 2014; Lange et al., 2003a, 2003b; Robjant and Fazel, 2010). ITT thus incorporates elements of both narrative exposure therapy and cognitive reappraisal central to CBT, an approach potentially well-suited to treat PTSD symptoms in other populations with decades-old trauma, such as WTC-affected individuals (Knaevelsrud et al., 2014). Initial RCTs with waitlist control comparisons in older adults who survived childhood trauma during World War II found that ITT yielded “moderate-to-large” magnitude effect size reductions in PTSD and related outcomes (d = 0.43–0.84) in trauma survivors (Knaevelsrud et al., 2017, 2014). To date, ITT efficacy has not been evaluated relative to an active control intervention (Cunningham et al., 2013).

This is the first RCT to compare the efficacy of ITT for WTC-related PTSD symptoms to an active control intervention, therapist-assisted, Internet-based modified present-centered therapy [I-MPCT] (Brinkman et al., 2021). Further, it is among the first studies to compare any Internet-based, therapist-assisted psychotherapy for PTSD to an active control intervention instead of a waitlist control (Kuester et al., 2016; Sijbrandij et al., 2016; Simon et al., 2021), and the first RCT to evaluate psychotherapy efficacy for PTSD in WTC workers and survivors. I-MPCT was modified from present-centered therapy (PCT), which was developed as an active control intervention and found to be modestly less than or as effective as trauma-focused PTSD therapies, with lower dropout rates (Shea et al., 2020). Based on prior ITT research (Knaevelsrud et al., 2017, 2014), we hypothesized ITT would yield, by convention, “moderate-to-large” (i.e., d = 0.50–0.80) magnitude effect size reductions in WTC-related PTSD symptoms (Cohen, 1988) and significantly greater reductions than I-MPCT. We further hypothesized that, compared to I-MPCT, ITT would be associated with significantly greater, “moderate-to-large” magnitude effect size improvements in comorbid depressive and anxiety symptoms, mental health-related functioning, quality of life, and posttraumatic growth.