Abstract: This is an update of a previous review. We included 27 studies with 3963 participants. The meta-analysis included 21 studies of 2721 participants. Seventeen studies compared multiple session early psychological intervention versus treatment as usual and four studies compared a multiple session early psychological intervention with active control condition. Low-certainty evidence indicated that multiple session early psychological interventions may be more effective than usual care in reducing PTSD diagnosis at three to six months' follow-up (RR 0.62, 95% CI 0.41 to 0.93; I2 = 34%; studies = 5; participants = 758). However, there was no statistically significant difference post-treatment (RR 1.06, 95% CI 0.85 to 1.32; I2 = 0%; studies = 5; participants = 556; very low-certainty evidence) or at seven to 12 months (RR 0.94, 95% CI 0.20 to 4.49; studies = 1; participants = 132; very low-certainty evidence). Meta-analysis indicated that there was no statistical difference in dropouts compared with usual care (RR 1.34, 95% CI 0.91 to 1.95; I2 = 34%; studies = 11; participants = 1154; low-certainty evidence) .At the primary endpoint of three to six months, low-certainty evidence indicated no statistical difference between groups in reducing severity of PTSD (SMD –0.10, 95% CI –0.22 to 0.02; I2 = 34%; studies = 15; participants = 1921), depression (SMD –0.04, 95% CI –0.19 to 0.10; I2 = 6%; studies = 7; participants = 1009) or anxiety symptoms (SMD –0.05, 95% CI –0.19 to 0.10; I2 = 2%; studies = 6; participants = 945). No studies comparing an intervention and active control reported outcomes for PTSD diagnosis. Low-certainty evidence showed that interventions may be associated with a higher dropout rate than active control condition (RR 1.61, 95% CI 1.11 to 2.34; studies = 2; participants = 425). At three to six months, low-certainty evidence indicated no statistical difference between interventions in terms of severity of PTSD symptoms (SMD –0.02, 95% CI –0.31 to 0.26; I2 = 43%; studies = 4; participants = 465), depression (SMD 0.04, 95% CI –0.16 to 0.23; I2 = 0%; studies = 2; participants = 409), anxiety (SMD 0.00, 95% CI –0.19 to 0.19; I2 = 0%; studies = 2; participants = 414) or quality of life (MD –0.03, 95% CI –0.06 to 0.00; studies = 1; participants = 239). None of the included studies reported on adverse events or use of health-related resources. While the review found some beneficial effects of multiple session early psychological interventions in the prevention of PTSD, the certainty of the evidence was low due to the high risk of bias in the included trials. The clear practice implication of this is that, at present, multiple session interventions aimed at everyone exposed to traumatic events cannot be recommended. There are a number of ongoing studies, demonstrating that this is a fast moving field of research. Future updates of this review will integrate the results of these new studies.

Plain-language summary: • We found low-certainty evidence that multiple session early psychological interventions may be more effective than treatment as usual in preventing PTSD diagnosis three to six months after receiving the intervention. • We found very low-certainty evidence that multiple session early psychological interventions may be neither more nor less effective than treatment as usual in preventing PTSD, either immediately after, or at seven to 12 months after, the intervention. We also found very low-certainty evidence that multiple session early psychological interventions may be neither more nor less effective than treatment as usual in reducing the severity of PTSD symptoms, either immediately or at subsequent points of follow-up. • We found low-certainty evidence that multiple session early psychological interventions may be associated with a higher dropout rate than other psychological interventions. • We found low-certainty evidence that multiple session early psychological interventions may be neither more nor less effective than other psychological interventions in diagnosing PTSD; reducing the severity of PTSD, depression and anxiety; or in maintaining the general functioning of participants receiving the intervention. • We found no studies that measured adverse effects. • We found no studies that measured use of health-related resources. The current evidence base is small. However, new studies are being conducted and future updates of this review will incorporate the results of these.