The percentage of Californians without health insurance reached a record low 6.8 percent during the first six months of 2017, according to new estimates from the U.S. Centers for Disease Control and Prevention.

The figure, released Thursday, is down slightly from the 7.2 percent uninsured rate from a year ago, which was at the time a record low — and significantly lower than the 17 percent uninsured rate in 2013, before the Affordable Care Act took effect.

The uninsured rate is dropping faster in California than it is nationally; the U.S. uninsured rate fell from 14.4 percent in 2013 to 9 percent in 2017.

In California, most of the gains were through the expansion of Medi-Cal, the insurance program for the poor that added 3.7 million new enrollees since 2014 — mostly funded by federal dollars allocated under the ACA to states that chose to expand their Medicaid programs. Covered California, the state-based insurance exchange that was created by the Affordable Care Act, has about 1.3 million people enrolled in health plans.

In states — including California — that operate their own insurance exchanges, the collective uninsured rate fell from 18.7 percent in 2013 to 8.3 percent in 2017, according to the CDC data. In states that use the federal exchange HealthCare.gov, the uninsured rate fell less dramatically, from 22 percent in 2013 to 16.1 percent in 2017.

“These data show loudly and clearly that states like California that have sought to make the Affordable Care Act work for their residents are delivering,” California Health and Human Services Agency Secretary Diana Dooley said in a statement. “While California and many other states are helping lower health care costs and reducing the rate of the uninsured, the nation is being challenged by great uncertainty at the federal level.”

Of the remaining uninsured, officials estimate that about half are living in California without legal documentation and are not eligible for coverage.

Catherine Ho is a San Francisco Chronicle staff writer. Email: cho@sfchronicle.com Twitter: @Cat_Ho