As of December 2020, 36 states and the District of Columbia had expanded Medicaid eligibility under the Affordable Care Act (ACA). We find that if the remaining 14 states had expanded eligibility in 2020, 4.4 million fewer people would have been uninsured that year. Absent the COVID-19 pandemic, the difference would have been 3.8 million. Thus, Medicaid expansion in the remaining states would increase health coverage even more under the pandemic. We also find young adults have the highest uninsurance rates under current law and would gain more coverage from Medicaid expansion than any other age group.