But in a marked contrast, two-thirds of the marketplaces run by states instead of the federal government saw increased enrollment between 2017 and 2018, including CT and NY. If they prove popular, that could siphon off more healthy people.
The academy's figures provide the initial evaluation of how enrollment fared in the state-based exchanges, which have more control over their outreach efforts and enrollment schedules.
Covered California announced Wednesday that 1.52 million people enrolled for 2018 insurance coverage, which is down about two percent from the prior year.
Sign-ups fell 5.3% in the 34 states that leave enrollment entirely to the federal exchange, healthcare.gov.
Fifteen states and the District of Columbia exceeded 2017 signups in 2018 - eight of these were state-based marketplaces, three were state-based marketplaces using the Healthcare.gov enrollment platform (KY, NV, and OR), and five were federal Healthcare.gov marketplaces.
The Centers for Medicare and Medicaid Services said it can not confirm the academy's data. Enrollment isn't final until consumers pay their first month's premium.
Even bigger enrollment increases were recorded in Washington state, which had 7.6 percent increase, and Rhode Island, which led all states with a 12.1 percent increase.
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Covered California - the state's health insurance marketplace - saw an increase in new enrollees past year.
Only four states - California, Idaho, Maryland and Vermont - saw sign-ups slip.
"There could be a number of different reasons for that", says James Scullary with Covered California.
The annual enrollment tally remains a relatively crude metric that doesn't account for what kind of consumers are signing up for coverage.
While the state exchange directors said Obamacare remained strong this year, they were concerned about Congress' elimination of the individual mandate starting in 2019.
Subsidized consumers paid less for health coverage in 2018 than 2017 due to the protective effect of their subsidy rising to offset higher premiums, while unsubsidized consumers in the individual market - both in and out of Covered California - saw their costs rise. Here in Texas, though, we saw some of the most significant drops of any state. It is in the process of relaxing federal regulations to allow the sale of more insurance policies that don't adhere to Obamacare's mandates, for instance.