Young people were more likely to be uninsured than their elders, the N.C.H.S. found. Almost 17 percent of adults aged 25 to 34 lacked insurance, while less than 9 percent of those aged 45 to 64 were uninsured.

The lead author of the report, Robin A. Cohen, a statistician at the N.C.H.S., pointed out that high-deductible health plans have become more popular. In 2010, 25.3 percent of the insured under the age of 65 had high-deductible plans. By 2016, nearly 40 percent had them.

From 2010 to 2016, rates of the uninsured declined in all age groups, down 14.4 percent among those aged 18 to 24, 16.5 percent among those aged 25 to 34, 13.7 percent among those aged 35 to 44, and down 8.9 percent in the 45-to-64 age group.

While rates of the uninsured declined sharply among the poor over those years, 26.2 percent of the near-poor (those with an income of 100 to 200 percent of the federal poverty level) and 23.2 percent of the poor (an income of less than 100 percent of the poverty level) lacked health insurance.

There were stark differences by race and ethnicity. While 25 percent of Hispanics were uninsured in 2016, 15 percent of African-Americans, 8.6 percent of whites and 7.5 percent of Asians lacked health insurance.