WASHINGTON — Over 600,000 people signed up for a plan through Obamacare's federal exchange in its first four days of open enrollment, significantly outpacing last year's sign-ups, according to official figures released Thursday.

The Centers on Medicare and Medicaid Services (CMS) said that 464,140 customers renewed their coverage while another 137,322 customers were new enrollees, for a total of 601,462. The number only includes people who signed up through the HealthCare.gov portal, which services 39 states.

By comparison, some 1,008,281 customers signed up through Healthcare.gov in the first 12 days of enrollment in 2016, the first period for which data is available. While not a perfect comparison, the daily rate of sign-ups is significantly faster this year: 150,366 per day in 2017, compared with 84,018 in 2016.

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But experts cautioned against assigning too much meaning to the early numbers.

The enrollment period, which ends Dec. 15, is significantly shorter this year, down to 45 days from three months in 2016 and the total number of sign-ups will depend a lot on how many marginal customers take part in the final days. The surge could reflect sicker patients with pent-up demand, rather than healthier patients who may be more reluctant to buy a plan but are crucial to keeping premiums down.

"We shouldn't read too much into this since there's five weeks to go and we need to get a lot of people signed up quickly," Lori Lodes, co-founder of Get America Covered, an outside campaign to enroll customers, told NBC News. "But it's a great start."

At the very least, the 600,000 sign-ups strongly clashed with President Donald Trump’s repeated insistence that Obamacare was "dead" and "finished" and would soon collapse.

Supporters of the law were afraid Trump's White House might make his claims a self-fulfilling prophecy: The administration slashed the advertising budget for enrollment by 90 percent, made deep cuts to customer service, and threw a wrench in the insurance market by cutting off cost-sharing payments owed to insurers just after they finalized their 2018 premiums.

"Bottom line, there are a lot of people out there who want and need good quality, affordable coverage, and no amount of political grandstanding changes that," said Sabrina Corlette, a research professor at Georgetown’s Center on Health Insurance Reforms.

Experts offered some possible explanations for better-than-expected enrollment. Trump's decision to cut off CSR payments raised overall premiums on paper, but actually lowered the cost of plans for many customers whose income qualifies them for federal help thanks to a quirk in the way the law calculates subsidies. The Kaiser Family Foundation estimates 54 percent of customers eligible for subsidies are able to purchase a bronze plan for zero dollars a month.

It's also possible coverage of repeal efforts has generated more awareness of the law’s benefits, filling in some of the gap created by advertising cuts.

"The administration has caused a lot of anxiety for people over the last year when it comes to health care, but there’s also been a more in depth conversation about what Obamacare really is about," Lodes said.