For millions of Americans, time is running out to sign up for health insurance through the Affordable Care Act's online marketplace .

For those who will not receive health coverage beginning Jan. 1, 2020 through an employer or other programs like Medicaid, Medicare or the Children's Health Insurance Program — commonly referred to as CHIP — the deadline to purchase health insurance is Sunday, Dec. 15.

Health and Human Services Secretary Alex Azar tweeted a reminder: "If you decide that purchasing coverage through healthcare.gov is the right decision for you, make sure you select coverage by this Sunday."

Sign-ups for 2020 coverage in the first six weeks of open enrollment for the ACA, also referred to as Obamacare, are down slightly, trailing last year's totals by 6%. However, this decline is happening at a slower rate when compared to 2019 coverage sign-ups in the first six weeks. That decline dipped 12%, according to Modern Healthcare.

The publication also notes that the latest numbers "don't include the millions of people who will be automatically enrolled in coverage at the conclusion of open enrollment."

NPR's Health Policy reporter Selena Simmons-Duffin told NPR's Up Firstpodcast on Saturday that enrollment has been down every year since 2016.

"Last year more than 11 million people enrolled and we're on track to be slightly behind that this year," Simmons-Duffin said.

Many experts blame the drop in sign-ups on the Trump administration making sharp reductions in outreach efforts to connect would-be insurance purchasers to available plans.

"One of the actions that President Trump's administration took to change the [Affordable Care Act] law is to radically cut back the funding to do outreach and to do advertising to let people know that this exists," Simmons-Duffin said.

Kaiser Health News points out there is typically "a flurry in the last few days before the Dec. 15 deadline" when last-minute participants decide to sign-up.

Some states have seen double-digit declines. In Arizona, for example, enrollments are down 17% from this time a year ago, according to the Arizona Republic. The paper cites "apprehension among some Latino families over enrolling in anything government-related" as one possible cause for the drop off.

Meanwhile, Delaware Public Media reports a 1.7% decline from last year. It adds: "Lagging enrollment comes despite premiums in Delaware dropping for the first time since the ACA became law seven years ago."

Health officials in California announced Thursday more than 130,000 people signed up for new coverage plans this year — an increase of 16% compared to the open enrollment period last year.

For those who miss the open enrollment sign-up period, not all is lost. The health care law does allow, in specific cases, a special enrollment period where people can sign-up after the open enrollment period ends.

The government lists circumstances including losing health insurance, getting married, moving, having a baby or adopting a child as "life events" that would make applicants eligible.

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