As the Dec. 15 enrollment deadline approaches, what will be the impact if there are less people with Affordable Care Act health insurance plans?

When it’s all said and done, it’s likely that fewer Americans will sign up for health insurance coverage this year in the Affordable Care Act (ACA) marketplaces.

In fact, it could be significantly fewer people.

If that happens, experts say you can expect higher insurance premiums and fewer insurance companies participating in the federal and state healthcare exchanges.

The warnings come as the deadline is less than two weeks away for most people to enroll in Obamacare insurance programs.

Dec. 15 is the final enrollment day for the 39 states that utilize the federal website.

That’s also the deadline in the state-run healthcare exchanges in Idaho, Maryland, and Vermont.

Connecticut and Rhode Island are leaving their exchanges open until late December.

Colorado, Minnesota, Washington, and Massachusetts are keeping their marketplaces open until mid-January.

California, New York, and Washington, D.C., are allowing sign-ups until Jan. 31.

There’s information on how to enroll on the federal website as well as on the sites of groups such as Get America Covered.

During the 2016 enrollment period, 12.2 million people signed up for health insurance programs under the ACA.

Of those, 9.2 million used the federal website and 3 million used state exchanges.

The number was down from the 12.7 million who signed up during the 2015 enrollment period.

Keep in mind that consumers aren’t considered officially enrolled in ACA plans until they make their first premium payment.

During the year, about a third of people who sign up will eventually drop their coverage for a variety of reasons, including getting a new job.

This year’s enrollment period began on November 1.

In the first 25 days, 2.8 million people signed up for ACA plans, according to estimates by Avalere Health, a Washington, D.C.-based consulting firm.

That compares with 2.1 million during the first 26 days of last year’s enrollment period.

In addition, almost 720,000 of this year’s sign-ups were new enrollees, compared to about 520,000 last year.

Of the 39 states that use the federal website, only West Virginia is showing a decrease in enrollees and that’s by a meager 1 percent.

However, Avalere officials point out, the enrollment period is only half as long this year as it was last year.

The latest enrollment figures of 2.8 million came with the enrollment period 57 percent complete.

At the 57 percent mark last year, 7 million people had signed up.

That has led S&P Global to project that between 10.6 million and 11.4 million people will enroll this year.

Charles Gaba, a blogger who tracks enrollment, predicts about 10 million people will sign up on the federal and state websites, according to CNN.

A big unknown is how many people will be automatically re-enrolled.

These are current enrollees who take no action to remove themselves from the ACA marketplace.

Caroline Pearson, senior vice president of policy and strategy at Avalere, said about 23 percent of all ACA participants last year were people automatically re-enrolled.

It’s unknown what percentage of this year’s pool will fall into this category.

That re-enrollment will occur on Dec. 16, about the same time it did last year.

A big difference, Pearson noted, is that people last year had time to change their ACA plan after being re-enrolled.

This year, in most states, re-enrolled participants will be stuck with whatever plan they get.

The other big unknown, Pearson added, is how many people will wait until the final week before Dec. 15 to sign up.

“There are a lot of variables when it comes to enrollment,” she told Healthline.

So what happens if there are 2 million fewer people in the ACA marketplaces?

A lot depends on who exactly has signed up.

Mary Ann Hart, an associate professor in the health administration graduate program at Regis College in Massachusetts, said a key is what percentage of that insurance pool are healthier people who don’t need as much medical treatment.

She said a significant crop of younger, healthy consumers can keep costs down for insurance companies and, in turn, keep insurance premiums from rising too dramatically.

However, fewer healthier people may enroll for 2018 coverage.

One big reason is the tax cut bill approved last week by the Senate. That bill contains a provision that eliminates the ACA’s individual mandate requirement.

Under that requirement, people who don’t have health insurance pay a penalty on their federal income tax returns.

Without the mandate, experts say healthy people will be less inclined to purchase health insurance.

That, Hart told Healthline, could drive up insurance premiums and convince some insurance companies to drop out of the marketplaces after their 2018 commitment is complete.

“It’s all about the stability of the insurance market,” she said.

Kurt Mosley, vice president of strategic alliances at Merritt Hawkins health consultants, sees a similar scenario.

He said that a smaller pool, especially one with fewer younger and healthier people, will most likely affect the insurance marketplace in a negative way.

He added that the confusion over whether Obamacare will survive after the individual mandate is repealed doesn’t help.

“A confused buyer never buys,” he told Healthline.

Pearson agreed.

“All this turbulence is making the market less attractive,” she said. “Some insurers are going to say it’s not worth putting up with the chaos.”

“A big drop in enrollment,” she added, “certainly does affect the exchange markets, which are already at risk.”