Sara Collins and Tracy Garber of the Commonwealth Fund in New York said following the improvements made to HealthCare.gov and state-operated health insurance marketplace websites in November, Americans are continuing to sign up for the Affordable Care Act's new coverage options.
With coverage for many people beginning on Jan. 1, the non-profit examined the enrollment trends across the nation.
"The latest enrollment figures for marketplace plans from the 14 states and the District of Columbia running their own marketplaces show that enrollment has climbed to at least half a million people, about 525,000 nationwide, more than double our last estimate reported on Nov. 21," Collins and Garber said in a blog.
"The new total, which includes people who have selected plans but may or may not have paid their premiums, is up from an estimated 364,682 people who had selected a plan or enrolled by Nov. 30. This latest tally does not account for any new enrollment in the federal marketplaces in 36 states, which is also likely to have grown substantially in December."
However, this estimate was most likely to be under-reported in states such as California and New York where enrollment surged in recent days, they said.
Peter Lee, California's marketplace director told reporters more than 53,000 people signed up in the first three days of last week and then enrollments were about 15,000 a day.
In addition, many states report significant enrollment in Medicaid and the Children's Health Insurance Program. Among states running their own marketplaces and reporting Medicaid enrollment, more than 620,000 people have enrolled in the program, up from about 345,000 from November.
The federal government reported this month that about 269,000 people were found to be eligible for Medicaid or CHIP through the federal marketplaces, but did not report actual enrollment, the blog said.
State websites reported young adults ages 18-34 comprise 21 percent of enrollment in California, 19 percent in Colorado and 20 percent in Rhode Island. In Washington State, 18 percent of total enrollees are between the ages of 19-35. NBC News reported 41 percent of enrollees in Kentucky were age 35 and younger.