Importance  Social anxiety disorder (SAD) is a prevalent childhood-onset disorder associated with lifelong adversity and high costs for the individual and society at large. Cognitive behavioral therapy (CBT) is an established evidence-based treatment for SAD, but its availability is limited.

Objective  To assess the efficacy and cost-effectiveness of therapist-guided internet-delivered cognitive behavioral therapy (ICBT) for SAD in youths vs an active comparator, internet-delivered supportive therapy (ISUPPORT).

Design, Setting, and Participants  This single-masked, superiority randomized clinical trial enrolled participants at a clinical research unit integrated within the child and adolescent mental health services in Stockholm, Sweden, from September 1, 2017, to October 31, 2018. The final participant reached the 3-month follow-up (primary end point) in May 2019. Children and adolescents 10 to 17 years of age with a principal diagnosis of SAD and their parents were included in the study.

Interventions  ICBT and ISUPPORT, both including 10 online modules, 5 separate parental modules, and 3 video call sessions with a therapist.

Main Outcomes and Measures  The Clinician Severity Rating (CSR), derived from the Anxiety Disorder Interview Schedule, rated by masked assessors 3 months after the end of treatment. The CSR ranges from 0 to 8, with scores of 4 or higher indicating caseness. Secondary outcomes included masked assessor–rated diagnostic status of SAD and global functioning, child- and parent-reported social anxiety and depressive symptoms, and health-related costs.

Results  Of the 307 youths assessed for eligibility, 103 were randomized to 10 weeks of therapist-guided ICBT (n = 51) or therapist-guided ISUPPORT (n = 52) for SAD. The sample consisted of 103 youths (mean [SD] age, 14.1 [2.1] years; 79 [77%] female). Internet-delivered cognitive behavioral therapy was significantly more efficacious than ISUPPORT in reducing the severity of SAD symptoms. Mean (SD) CSR scores for ICBT at baseline and at the 3-month follow-up were 5.06 (0.95) and 3.96 (1.46), respectively, compared with 4.94 (0.94) and 4.48 (1.30) for ISUPPORT. There was a significant between-group effect size of d = 0.67 (95% CI, 0.21-1.12) at the 3-month follow-up. Similarly, all of the secondary outcome measures demonstrated significant differences with small to large effect sizes, except for child-rated quality of life (nonsignificant). The cost-effectiveness analyses indicated cost savings associated with ICBT compared with ISUPPORT, with the main drivers of the savings being lower medication costs (z = 2.38, P = .02) and increased school productivity (z = 1.99, P = .047) in the ICBT group. There was 1 suicide attempt in the ISUPPORT group; no other serious adverse events occurred in either group.

Conclusions and Relevance  In this randomized clinical trial, internet-delivered cognitive behavioral therapy was an efficacious and cost-effective intervention for children and adolescents with SAD. Implementation in clinical practice could markedly increase the availability of effective interventions for SAD.

Trial Registration  ClinicalTrials.gov Identifier: NCT03247075