How low can Obamacare go? Fewer than 1 in 10 Americans lacked health insurance in 2015, the first time ever in the nation's history that the uninsured rate has fallen so low, and a clear sign of Obamacare's impact.

A woman sits with an insurance agent to pick an insurance health plan under the Affordable Care Act. Getty Images

The U.S. uninsurance rate fell to 9.1 percent last year, according to data released Tuesday by the federal Centers for Disease Control and Prevention.

It was the first time the percentage of people without some form of health coverage has gone into single digits, and a 2.4 percentage point drop from 2014. The number of people uninsured in 2015 was 28.6 million, which was 7.4 million fewer than 2014, according to the CDC. Since 2011, when the uninsured rate stood at 15.1 percent, that rate has fallen 6 full percentage points, according to the CDC's National Health Interview Survey. Obamacare began going into full effect in 2014, which was the first year that people could have private health insurance plans purchased on government-run marketplaces. That was also the first year that nearly all Americans were obligated to have some form of health insurance or face a possible tax penalty.

Before that, the Affordable Care Act had for the first time allowed adults under the age of 26 to stay on their parents' health plans, which was another factor that has played a role in driving down the uninsured rate.

A third factor in bringing down the uninsured rate has been the expansion of Medicaid benefits in a majority of states to cover many more poor adults than had been eligible under that joint federal-state health insurance program before the ACA became law. The Obama administration has said more than 20 million Americans have gained health insurance coverage since the ACA became law in 2010. The new report Tuesday was warmly greeted by the Obama administration, which last week lost a key court decision that, if upheld, could damage the Obamacare program. A federal court judge ruled that the administration violated the law by giving insurers money, without congressional approval, to offset the cost of covering subsidized out-of-pocket health costs of many Obamacare customers.

The administration also has been dealing with a flurry of news stories detailing possibly steeper Obamacare rate hikes for 2017, and the departure of some insurers from various markets across the nation.

Sylvia Burwell, secretary of health and human services, said, "Today's report is further proof that our country has made undeniable and historic strides thanks to the Affordable Care Act." "The report documents the progress we've made expanding coverage across the country," Burwell said. "Meanwhile, premiums for employer coverage, Medicare spending and health-care prices have risen at exceptionally slow rates. Our country ought to be proud of how far we've come and where we're going," she said.