The Long-Term Trend of the Affordable Care Act on Health Insurance Marketplace Enrollment.

The Affordable Care Act (ACA) expanded health care access in the United States. This study examines the long-term impact of the ACA on private health insurance enrollment using National Health Interview Survey (NHIS) data. A repeated cross-sectional study using NHIS data from 2015 to 2022 was analyzed. Given the repeal of the ACA's individual mandate in 2019, stratified analyses compared Marketplace enrollment before (2015, 2018) and after (2019, 2022) the repeal. The study included US adults aged 26-64 years. Unadjusted enrollment rates were compared across age, sex, race/ethnicity, social determinants of health (SDOH), chronic conditions, body mass index, and smoking. Multivariable logistic regression assessed enrollment trends and associated factors. Marketplace enrollment increased by 1.4 percentage points post-mandate (P < 0.001), with no significant change pre-mandate (0.5-point decline, P = 0.235). Some subgroups (ages 26-39, Midwest, West) saw declines pre-mandate, while many experienced increased enrollments post-mandate. After adjustment, individuals in 2022 had 27% higher odds of enrollment than in 2019 (adjusted odds ratio [aOR] = 1.27, 95% confidence interval [CI] = 1.13-1.43, P < 0.001), whereas no significant change occurred between 2015 and 2018 (aOR = 1.02, 95% CI = 0.89-1.16, P = 0.818). Age, racial minority status, and unfavorable SDOH were associated with higher post-mandate enrollment odds. Marketplace enrollment grew post-mandate, particularly among vulnerable populations. While the repeal of the individual mandate may have contributed, other policy changes-expanded enrollment windows, increased subsidies, enhanced outreach, and streamlined applications-likely played a role, particularly in response to COVID-19.