Purpose:
To estimate the prevalence and geographic variation of serious mental illness (SMI) among young adults enrolled in Medicaid and to identify individual and community-level factors associated with SMI.

Methods:
We used New York Medicaid data for over 1.6 million young adults aged 18–34 years with continuous Medicaid enrollment from April 2021 through March 2022. We merged 3 additional county and zip code-level datasets, including the Distressed Communities Index, Area Health Resource File, and New York homelessness data. We used descriptive analyses, data visualization methods, and multivariable logistic regression to estimate the prevalence of SMI and identify individual- (sex, race and ethnicity, age, disability aid, and homelessness) and community-level (geography, community distress, number of hospital beds, and community mental health centers per capita) factors associated with SMI.

Results:
The 12-month prevalence of SMI was 8.3% in New York State, with wide geographic variation by zip codes, ranging from 0% to 39%. Young adults identifying as Asian, Black, and Latinx (compared to White), and those living in zip codes with the most community distress were associated with lower odds of SMI, odds ratios (OR) ranged from 0.47 (Asian) (95% confidence interval [CI] [0.46–0.49]) to 0.95 (Latinx) (CI [0.93–0.97]). Young adults identifying as female (compared to male), receiving disability, experiencing homelessness, and rural residence were associated with higher odds of SMI, ORs ranged from 1.64 (female) (95% CI [1.62–1.66]) to 5.49 (homelessness) (95% CI [5.33–5.66]).

Discussion:
SMI prevalence varies by individual- and community-level factors, including homelessness and rural residence, signaling the need for specific interventions and policies.