The ACA originally included a provision that required states to accept federal funding to provide Medicaid to everybody below 138 percent of the federal poverty level. Many states impose far stricter income thresholds and do not offer Medicaid to childless adults. The Supreme Court struck down that provision, however, leaving the decision to expand Medicaid up to individual states.

There are now 31 states (and the District of Columbia) that have expanded eligibility to those at 133 percent of the federal poverty level, although some of those states only recently expanded.

Unsurprisingly, states that expanded Medicaid experienced a much more significant boost in coverage than those that turned down the federal dollars. Expansion states saw their uninsured rate drop from 15.3 percent to 7 percent, while non-expansion states' uninsured rate declined from 19.8 percent to 13.7 percent.

“Data show the Affordable Care Act dramatically increased access to affordable health insurance coverage for millions of people,” said Mona Shah, program officer at the Robert Wood Johnson Foundation, which funded the study. “Sustaining these gains will require ongoing outreach to affected individuals and governmental support that underscores a commitment to the importance of health insurance coverage.”

None of the data analyzed in the study comes from the 18 months since President Trump took office. It's not yet clear what effect the attacks on the ACA by the Trump administration and Congress have had on the uninsured rate.