Clearest sign Obamacare is working?

Regardless of what you think about Obamacare, it's working on one front: The number of uninsured American adults has dropped by 16.4 million.

The reduction in the number of Americans lacking health insurance means that only 13.2 percent of U.S. residents now lack coverage, down from 20.3 percent before the law went into effect, according to new data from the U.S. Department of Health and Human Services. Because the numbers only include adults, there are likely many more uninsured children who gained health coverage during the past year, although that data wasn't available.

"In my experience this is a historic drop in the uninsured," said Dr. Richard Frank, the agency's assistant secretary for planning and evaluation, in a conference call to discuss the study. "Nothing since the implementation of Medicare and Medicaid has come close."

Those who gained health coverage include 2.3 million young adults between the ages of 19 to 25, thanks to the provision in the health-care law that allows people to remain on their parents' plans until they turn 26, the study noted.

The reduction in uninsured Americans is "good for people and their health, and it's good for the economy," Frank said.

While the ACA is succeeding in the law's goal to reduce the number of uninsured Americans, it's also facing an uncertain future, thanks to a case now before the Supreme Court.

The case, King v. Burwell, argues that the Affordable Care Act only permits the federal government to supply subsidies to states that operate their own health care exchanges. If the high court rules in favor of the plaintiffs, it could mean that 34 states that use the federal government's exchange could lose their subsidies.

Given that the average subsidy pays for 72 percent of the premium for Obamacare enrollees, that could lead to many families opting to drop their insurance if they end up on the hook for the full cost.

Latinos and African Americans saw the greatest reduction in uninsured rates, the study found. Among Latinos, the uninsured rate dropped by 12.3 percentage points, while African Americans saw the uninsured rate drop by 9.2 percent. Latinos had the highest uninsured rate before Obamacare took effect, with 41.8 percent of all Latinos lacking health insurance beforehand, the study found. For whites, the uninsured rate dropped by 5.3 percent.

The reduction in uninsured rates were especially strong in states that expanded Medicaid coverage. Among families earning incomes of 138 percent of the poverty rate or less, the uninsured rate dropped by 13 percentage points in the expansion states, the study found.

The agency is hopeful that the trend will continue, although Frank said the HHS doesn't have a target for how low the uninsured rate might go.

"It's important to recognize that we're in entirely new territory here," he said. "We've never come this close to this." He added, "We are just starting to understand more completely who we brought in and who is left uncovered."