Princeton, N.J.—Data released today show 5.4 million U.S. adults gained health insurance since September 2013—the first available estimate of how many uninsured people acquired coverage since enrollment in the Affordable Care Act (ACA) marketplaces began.

Funded in part by the Robert Wood Johnson Foundation (RWJF) and conducted by researchers at the Urban Institute, the Health Reform Monitoring Survey (HRMS) shows that the percentage of uninsured Americans fell from 17.9 percent in September 2013 to 15.2 percent at the beginning of March.

“This represents a major step forward for 5.4 million previously uninsured people who now have health coverage,” said Risa Lavizzo-Mourey, MD, president and CEO of RWJF. “There is a great deal more work to be done in this area, but today’s news provides hope and encouragement to all of us who seek to build a culture of health in America by expanding access to coverage.”

The data suggest that most of the drop in uninsured numbers occurred in 2014 as enrollment in the marketplaces picked up. The survey shows that 17.5 percent of Americans were still uninsured in December 2013. While the survey represents the first look at the number of enrollees, experts say the numbers underestimate the total number of uninsured Americans who gained coverage. Because 80 percent of the survey was completed by March 6, the findings do not reflect the enrollment surge in late March.

Since President Obama’s announcement that 7.1 million Americans signed up for coverage during the ACA’s first open enrollment period, there’s been considerable attention on determining who the new enrollees are and whether or not they had health insurance before the ACA took effect. While questions still remain, researchers say these data lend the earliest insight into the enrollee population—illustrating that the state marketplaces and Medicaid expansion attracted individuals who were previously uninsured.

“The 15 percent drop in uninsurance among adults since September reveals a very promising start for the ACA’s key coverage expansion provisions,” said Sharon Long, an Urban Institute health economist and the coordinator of its Health Reform Monitoring Survey. “One can expect even more significant changes as the end-of-March surge in enrollments is accounted for.”

The data also show that declines in the number of uninsured Americans are considerably larger in states that expanded their Medicaid programs. As of early March, states that expanded the program averaged just 12.4 percent of residents remaining uninsured—in stark contrast with non-expansion states, where approximately 18.1 percent of residents remained without health coverage.

From September to early March, states that expanded Medicaid saw an average decline of 4.0 percentage points in their uninsured population. In comparison, states that did not expand the program saw an average drop of 1.5 percentage points over the same period. Researchers say that beyond the new Medicaid enrollees, this gap may also be attributed in part to stronger marketplace enrollment, as states that expanded Medicaid generally had higher rates of marketplace enrollment, too.

The HRMS is a nationally representative quarterly survey of approximately 7,500 adults under age 65. Support is provided by RWJF, the Ford Foundation and the Urban Institute.

View the report at http://hrms.urban.org/quicktakes/changeInUninsurance.html.