The percentage of uninsured Minnesotans has dropped to the lowest level in state history, and the second-lowest level in the nation, following the end of enrollments under the Affordable Care Act.

About 180,500 Minnesotans gained health insurance from last September to this May, with the vast majority getting coverage through one of the state’s public health programs, a report from the University of Minnesota found.

That left just 4.9 percent of all Minnesotans lacking health coverage on May 1, about a month after the federal health law’s first major sign-up deadline. That’s down from 8.9 percent last Sept. 30.

“A change in the uninsurance rate like this is pretty much unprecedented in Minnesota,” said Julie Sonier of the university’s State Health Access Data Assistance Center and a co-author of the report.

MNsure, the state’s health insurance exchange, commissioned the study to measure the impact of the federal health law on coverage in Minnesota. The study was paid for with a grant from the Robert Wood Johnson Foundation.

For much of the past decade, the rate of uninsured Minnesotans hovered around 7 to 8 percent, though it jumped above 9 percent after the onset of the recession in 2009.

The results rank Minnesota second only to Massachusetts in the percentage of its population with health coverage. That state’s health care reform in 2007 sent its uninsured rate to around 3 percent and, according to a study released there this week, the rate may have fallen below 1 percent in the wave of enrollments driven by the federal law this year.

Democratic backers of health care reform trumpeted the new Minnesota findings.

“It really confirms the wisdom of the Affordable Care Act and MNsure’s part of that,” said Gov. Mark Dayton, who was in Washington, D.C. “People who have been knocking this the whole time really now need to look at the facts. This has been tremendously successful and it’s going to get better.”

Researchers cautioned that the report was a “snapshot” and that people move in and out of the insurance market. The data also doesn’t answer key questions, including the demographics of the newly insured, what factors drove people to seek coverage and how many people who were previously uninsured purchased coverage through MNsure. Some of those questions will be addressed in a follow-up report later this year.

Sen. Michelle Benson, R-Ham Lake, said the drop in the number of uninsured Minnesotans is welcome but that a key question remains: How big an impact did MNsure make in helping people buy insurance from private carriers?

“If it’s mostly people enrolled in public programs, we could have done that without spending $170 million on a website,” she said, adding that money could have gone toward improving the existing county-based infrastructure for public insurance programs. “Let’s not take a victory lap on MNsure until we start counting what MNsure did and then let’s find out if it’s worth it.”

Word of the Minnesota results quickly spread through social media, a sign of how much President Obama’s health care initiative remains a political hot button. White House spokesman Jay Carney tweeted the study results to his more 500,000 followers, and supporters of the law from Congress down to grass roots activists took note.

MNsure CEO Scott Leitz called the report “an important starting point.”

“We know reduction of the uninsured was substantial, but we also know we have some people we need to work to get into coverage,” he said. There are “communities where we know the uninsurance rate is higher, communities of color, certain rural areas.”

Technical problems hobbled the launch of MNsure and forced the agency to rely on manual workarounds to meet the March 31 enrollment deadline. So far, nearly 237,000 Minnesotans have used MNsure to get coverage. The report didn’t quantify the contribution of MNsure or the work of thousands of navigators and brokers who tried to help people get access to federal tax credits through the clunky website.

The report also revealed that the state’s employers have not stopped offering coverage to workers in any meaningful way.

Meanwhile, the number of people buying private individual insurance on or off the exchange grew by almost 36,000.

The main drivers behind the drop in the uninsured came from enrollment gains in state health insurance coverage, Medical Assistance and MinnesotaCare.

Researchers have long known that as many as two-thirds of the state’s uninsured were eligible for coverage through public programs, but either didn’t know it or didn’t take advantage of the aid.

Minnesota was one of 26 states that decided to expand Medicaid to include childless adults, which accounted for a surprising gain in new applicants, said Department of Human Services Commissioner Lucinda Jesson.

“We anticipated growth in children and families because we were simplifying the process and we were doing more outreach,” Jesson said. “But we had more adults without kids than we expected, and we had pretty healthy projections. That tells me the Medicaid expansion was even more significant than we anticipated, so far. We’re only a few months in.”

The state already offers a more generous subsidy for the working poor than other states through its MinnesotaCare program, so making Medicaid benefits available to more people doesn’t fully explain the accelerated growth in coverage.

“People haven’t enrolled in such large numbers before in such a short period of time, so there was something else going on during the open enrollment period,” Sonier said. “Whether it was awareness of the individual mandate or far more intensive, effective outreach ... something really got these people in the door where other attempts to do so had not been successful.”

Star Tribune Washington correspondent Corey Mitchell contributed to this report.