The number of uninsured Americans in 2017 saw its largest single-year increase since the implementation of the Affordable Care Act's coverage expansion, according to Gallup data released Tuesday.

According to the Gallup-Sharecare Well-Being Index, the number of Americans without health insurance rose by 1.3 percentage points in 2017, representing about 3.2 million people.

Among those who saw the biggest declines in coverage were people between the ages of 18 and 25, as well as black and Hispanic people and individuals with an annual household income of less than $36,000.

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The number of uninsured people fell to a record low in the final quarter of 2016, dropping to just 10.9 percent. Despite the increase in 2017, the percentage of people without insurance is still low compared to its peak of 18 percent in the third quarter of 2013 – just before the ACA's coverage expansion took effect.

According to the new Gallup-Sharecare data, the number of Americans who purchased their own health insurance coverage through the ACA's health-care exchanges also dropped by 1 percentage point to 20.3 percent.

That decline marks a reversal of a trend seen since the ACA's individual mandate, which required people to purchase health insurance or pay a penalty, took effect in the fourth quarter of 2013.

Between then and 2016, the number of adults who purchased their own coverage increased 3.7 percent, according to Gallup. A sweeping tax plan passed by Republicans and signed by President Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE last month repealed the ACA's individual mandate.

The Gallup-Sharecare data is based on phone interviews with 25,072 U.S. adults, and was conducted from Oct. 1 to Dec. 31, 2017. Its margin of error is 1 percentage point.