For over a decade prior to the ACA, the proportion of the American population that was uninsured had been growing steadily. According to estimates made by the U.S. Bureau of the Census, between 1999 and 2011, the percentage of Americans without any source of health insurance coverage increased from 14 percent to 16 percent (see Appendix A). In 2012, the last year for which Census estimates are available, almost 48 million Americans, or 15 percent, lacked health insurance coverage. This figure was slightly higher among states that have decided (as of the beginning of 2014) to not expand Medicaid (17 percent) than in states that have elected to expand Medicaid (14 percent). Lack of health insurance was more common among adults aged 18 to 64 than among Americans of all ages; in 2012 41 million adults aged 18 to 64, or 21 percent, lacked health insurance.
As a result of implementation of the major coverage provisions in the ACA, the rate of uninsured adults began to drop in the first six months of 2014. The decline was most pronounced in Medicaid expansion states. Data published in the New England Journal of Medicine, using the Gallup-Healthways Well-Being Index, suggested that the uninsured rate for Americans ages 18 to 64 fell 4.7 percentage points, from 21.0 percent in September 2013 to 16.3 percent by April 2014 (the end of open enrollment), and stayed stable through June 2014.24 The study also showed that in Medicaid expansion states, the uninsured rate for those at or below 138 percent of the Department of Health and Human Services Poverty Guidelines declined 6 percent, while in Medicaid non-expansion states it declined by only 3.1 percent.25 Similarly, the Urban Institute, using the Health Reform Monitoring Survey, reported that the uninsured rate across adults of all ages fell from 17.5 percent in Q1 2013 to 13.9 percent in Q2 2014 (14.8 percent to 10.1 percent in Medicaid expansion states, and 20.8 percent to 18.3 percent in Medicaid non-expansion states).26
Commensurately, Medicaid enrollment data strongly suggest that there has been a large increase in the percentage of adults covered by Medicaid in 2014. As of the end of July 2014, enrollment reports demonstrate 7,935,257 more people enrolled in Medicaid and CHIP than in the comparison baseline period of July to September, 2013. The increases in enrollment were overwhelmingly seen in Medicaid expansion states, with 6,888,391 more enrollees, versus 1,046,866 in non-expansion states.27
24 Sommers BD et al., N Engl J Med. 2014 Aug 28;371(9):867-74