In 2014, when Medicaid expansion and the health exchanges were implemented as provisions of the ACA, children's coverage soared over the previous year's numbers, according to the May study done by the Urban Institute with funding from the Robert Wood Johnson Foundation.
Participation in Medicaid and the Children's Health Insurance Program, or CHIP, increased between 2013 and 2014, according to the study.
The uninsured rate for children in 2014 dropped in both states that had expanded Medicaid and those that didn't, but the numbers were greater for the former: 4.9 percent of eligible children were not insured in expansion states in 2014, compared to 8 percent in non-expansion states.
The study also linked a reduction in premiums to rises in health insurance participation.
When children living in families earning between 100 and 138 percent of the federal poverty level were transferred from separate CHIP coverage to Medicaid in seven states, the uninsured rate dropped significantly, according to the study. In these seven states, the families no longer had to make premium payments for coverage, which indicates reduced premiums play a role in raising health insurance participation rates, the study said.
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The increased availability of Medicaid coverage for adults in those states may also have affected coverage for children, the study said.
The study found higher uninsured rates for Hispanics; American Indians and Alaskan Natives; children in homes with no English-speaking parent; and children in homes with no parents.
In 2013, 19.8 percent of children in a household with no parent were not covered by insurance; 12.6 percent of children in a home with no English-speaking parent were not covered; 11.4 percent of Hispanic children were not covered; and 11.8 percent of American Indians and Alaskan Natives were not covered.
This compared to 5.2 percent of white children in 2013; 5.9 percent of black children; and 7.2 percent of Asian and Pacific Islander children.
All of the rates for the number of uninsured children declined in 2014.
The South had the largest numbers of uninsured children. In 2013, this rate was 8.5 percent compared to the region with the lowest percentage of uninsured children, the Northeast, at 4.1 percent. In 2014, those numbers slid to 7.2 percent of children in the South compared to 3.7 percent in the Northeast.
Medicaid expansion states showed the highest participation rates for Medicaid and CHIP, and all states showed higher rates in 2014 than 2013.
[Also: Uninsured rate for Hispanic children hits record low]
States that expanded Medicaid showed Medicaid and CHIP participation rates of 89.9 percent in 2013 and 92.9 percent in 2014; compared to rates of 87.1 percent in 2013 and 89 percent in 2014 for states that did not expand Medicaid.
The lowest rates for participation in 2014 were found in Utah, a non-Medicaid expansion state, at 79.8 percent; Alaska, a non-expansion state, at 81.5 percent; Nevada, an expansion state, at 85.7 percent; and Wyoming, a non-expansion state at 82.9 percent.
This compares to rates of 97 and 99.9 percent, respectively, for Massachusetts and Vermont, which both adopted Medicaid expansion.
Medicaid expansion under the ACA expanded coverage to families and individuals who have incomes at or below 138 percent of the poverty level, or an estimated $27,724 annual salary for a family of three in 2015.
Families in states without expanded Medicaid are at a disadvantage and may fall into a gap because they do not meet Medicaid eligibility requirements in their state and may also be unable to qualify for tax credits to purchase coverage, the study said.
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The extension of CHIP in 2015 helped states enroll eligible children but it was reauthorized for only two years, so uncertainty remains about future coverage options which could put these gains at risk, the authors said.
The Urban Institute has been tracking and monitoring the effects of the ACA since May 2011 and will continue to do so, they said.
Even prior to implementation of most provisions of the ACA, the uninsurance rate for children was cut in half between 1997 and 2012, the study found. The expansion in eligibility and efforts to reach and enroll eligible children have contributed to the decline in children's uninsurance rates, the study said.
However, the ACA changed the way income and family size are calculated for Medicaid and CHIP coverage, shifting children of families with incomes below 138 percent of the FPL from separate CHIP programs to Medicaid.
The ACA has also provided tax credits for coverage in the marketplace exchanges, in some cases for families with incomes up to 400 percent of the federal poverty level.
Between 2013 and 2014, the children's uninsured rate declined from 7 percent to 5.8 percent, or from 5.4 million children to 4.5 million, respectively.
Of the remaining 4.5 million uninsured children, the majority, 62.1 percent in 2014, were eligible for Medicaid or CHIP but were not enrolled, according to the study.
Twitter: @SusanJMorse