Engagement of individuals with serious mental illness in outpatient mental health services and telehealth use during the COVID-19 pandemic

•Telehealth preserved engagement of adults with serious mental illess in services.•Those using only in-person were demographically different than telehealth users.•In-person users were in NYC, black and had schizophrenia or substance use disorders.•Addressing barriers to telehealth use may increase overall engagement in services. Questions remain regarding whether the transition and continued use of telehealth was associated with changes in treatment engagement among patients with serious mental illness (SMI). Using NYS Medicaid claims, we identified 116,497 individuals with SMI receiving outpatient mental health services from September 1, 2019–February 28, 2021 and a comparison cohort of 101,995 from September 1, 2017–February 28, 2019 to account for unmeasured and seasonal variation. We characterized engagement in three 6-month increments (T0-T1-T2) using clinically meaningful measures of high, partial, low, and none. Subgroup differences were compared, and telehealth users were compared to those with only in-person visits. Engagement, as characterized, was largely maintained during COVID. The 19.0 % with only in-person visits during COVID had different characteristics than telehealth users. Telehealth use was greater among younger people by T2 (33.1 %), women (57.7 %), non-Hispanic White people (38.9 %), and those with MDD (18.0 %), but lower among non-Hispanic Black people, in NYC, and those with schizophrenia or SUD. Most telehealth users were highly engaged (77.1 %); most using only in-person services had low engagement (47.5 %). The shift to telehealth preserved access to many outpatient services for this SMI population. Exploring reasons for not using telehealth may identify opportunities to increase care access.