The plunge in that rate began in late 2013, as Obamacare exchanges began selling health insurance in earnest, and as enrollment in government-run Medicaid programs increased.
For its survey, Gallup randomly sampled more than 45,000 adults between the beginning of April and the end of June. The survey has a margin of error of +/- 1 percent.
Gallup said a big driver behind the second-quarter decline occurred at the beginning of April, when several million people signed up for Obamacare plans after an effective two-week extension of the March 31 enrollment deadline.
Since then, the uninsured rate has remained flat, suggesting that there will not be any further significant decreases in the rate until after Obamacare's second open-enrollment season begins in mid-November, Gallup noted.
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Much of the spike in the insured rate came from people who bought individual health plans either via government-run Obamacare exchanges or outside those exchanges, rather than from employer-based insurance that covers more than 40 percent of the population. The vast majority of people who bought individual plans on the government-run Obamacare exchanges received federal subsidies to help pay for that insurance.
Gallup noted that in its latest survey, 20.7 percent of adults under age 65 said they are insured through an individual plan purchased by themselves or a family member, compared with 16.7 percent in late summer 2013.
A smaller part of the increase in the number of people with health coverage came from Medicaid enrollment.