Ulysses Hernandez (L), an insurance agent from Sunshine Life and Health Advisors, speaks with Yuricel Duran as she shops for insurance under the Affordable Care Act at a store setup in the Mall of Americas on November 1, 2017 in Miami, Florida.

The pace of enrollments on the federal Obamacare insurance marketplace markedly slowed during the Thanksgiving week, officials said Wednesday.

A total of 504,181 people selected Obamacare plans on HealthCare.gov from Nov. 19 through Saturday, the fourth week of the ongoing open enrollment season, officials said.

That is about 294,600 fewer people than had selected a HealthCare.gov plan during the prior week, according to data from the federal Centers for Medicare and Medicaid Services.

So far this enrollment season, which began Nov. 1, a total of 2.78 million people have chosen an individual health plan sold by that federal exchange, which serves the 39 states that do not operate their own Obamacare marketplace.

Enrollment for insurance plans that take effect Jan. 1 is up more than 50 percent over last year's pace in Texas, Maine, Mississippi and Wyoming.

State-based exchanges report their enrollment tallies separately.

During roughly the same first four-week period last year, 2.14 million people signed up for a HealthCare.gov plan.

A slowdown in Obamacare enrollment is normal during a week that includes a major holiday.

However, the drop-off last week raises the question of whether the enrollment tally on HealthCare.gov will end below last season's total of 9.2 million.

Get America Covered, an Obamacare advocacy group founded by former Obama administration health officials, said that enrollments last week continued to outpace precious Thanksgiving weeks.

Sign-us this season are averaging 111,250 per day on HealthCare.gov, compared with 82,220 per day last season, according to Get America Covered.

But the group also noted that "the increase in sign-ups compared to last year is starting to get smaller."

"Due to the shortened open enrollment period, new consumer enrollment must significantly outpace last year for a comparable number of people to sign up this year as signed up last year," the organization said.

This is the first full open enrollment season conducted under the Trump adminstration, which opposes Obamacare, and which has actively sought the law's repeal and replacement with new health-care legislation.

The administration shortened the enrollment period on HealthCare.gov by half when it set a Dec. 15 deadline. Most other states that have their own exchanges have later deadlines this season.

Enrollments as a rule spike in the week leading up to sign-up deadlines.

Of the customers who selected a plan on HealthCare.gov last week, 351,938 people were returning customers, and 152,243 were new enrollees.

So far this season, 2.06 million people who selected a plan on the exchange were returning customers, and more than 718,000 people were new enrollees.

HealthCare.gov and the other state-based exchanges were created in 2013 to sell private individual plan health coverage to Americans who do not have insurance through a job, Medicare, Medicaid or other sources.

While the plans are offered by private insurance companies, most customers of Obamacare exchanges receive federal tax credits that lower their monthly premiums, and in many cases also receive discounts to their out-of-pocket health costs.

The Affordable Care Act, as Obamacare is formally known, requires most Americans to have some form of health coverage that complies with ACA minimum standards or pay a tax penalty.

The Senate is currently considering a tax bill that would repeal that ACA requirement.