BackgroundIn Australia, the first nationwide COVID-19 lockdown occurred in March 2020 bringing challenges for early intervention psychosis (EIP) services. Limited evidence exists on the impacts of the pandemic on treatment outcomes among EIP clients. MethodsThis prospective cohort study analysed routine data from 15 headspace Early Psychosis centres in Australia. Participants were 12 to 25 years, meeting criteria for First Episode Psychosis (FEP) or Ultra High Risk of psychosis (UHR) comparing those who commenced treatment ‘pre-COVID-19’ (between 16th August 2018 and 15th August 2019), and ‘during-COVID-19’ (between 1st March 2020 and 15th September 2020). Clinical symptoms at treatment commencement were assessed using the Brief Psychiatric Rating Scale (BPRS) and the Kessler Psychological Distress Scale (K10). with outcomes after 6 months compared between cohorts using linear mixed-effects regression, controlling for confounders. ResultsOf 1246 young people analysed (653 FEP, 596 UHR), significant improvements were observed with treatment in both groups (5 to 13-point reduction in BPRS score per 6-months treatment). Treatment effectiveness reduced during-COVID-19 for psychosis symptoms, with the FEP BPRS treatment effect lower by 4.3 points (95%CI: 0.5, 8.1). UHR clients had lower BPRS negative symptoms during-COVID-19 (p = 0.020). Service contacts increased during-COVID-19, with increased telehealth services (p < 0.001). ConclusionsEarly intervention remained effective for FEP and UHR, despite the pandemic and transition of EIP services to virtual service delivery. Reduced treatment efficacy in FEP psychosis symptoms may indicate potential limitations of telehealth. Further research to examine longer term clinical and functional outcomes due to the pandemic is required.