The uninsured rate just experienced its sharpest drop ever under Obamacare

The uninsured rate in the US declined by nearly one-fifth with the Affordable Care Act implemented for a full year, according to data released Tuesday by the Centers for Disease Control and Prevention.

The CDC's National Health Interview Survey, considered the most reliable government estimate of the country's uninsured population, found the uninsured rate for adults under the age of 65 dipped from 20.4% in 2013 to 16.3% in 2014. Americans could start buying insurance through marketplaces set up by the Affordable Care Act beginning in October 2013.

It's the largest one-year drop in uninsured since the CDC began conducting the National Health Interview Survey in 1997. And it confirms other private studies that have suggested the law is succeeding at its central goal of insuring more Americans. Gallup, for example, found the uninsured rate plummeting to its lowest level ever tracked in the first quarter of 2015.

The survey comes just days before the Supreme Court is set to rule in the case King v. Burwell. If the court decides to strike down a key provision of the law that allows the federal government to provide subsidies for about 6.4 million low-income people to buy insurance, experts predict a large chunk of those people would no longer be able to afford coverage.

Here's a chart from the CDC study that shows the sharp drop:

Obamacare's effects were more pronounced in states that had adopted the expansion of the federal Medicaid program. States that expanded Medicaid saw more than a 5% drop in its uninsured population (from 18.4% to 13.3%). Those that did not expand the program experienced a decline from about 22.7% to 19.6%.