Obamacare enrollment caught a bad cold in February—leaving officials scrambling to get nearly 2 million more people signed up by the looming March 31 end of open enrollment.
Total sign-ups in Obamacare plans sold on government exchanges topped 4.2 million people by the end of February, with about 943,000 people enrolling that month alone, officials said Tuesday.
The new data reveals a significant fall-off from January, when about 1.1 million people enrolled during the month.
Officials suggested the drop resulted in part from the fact there are fewer days in February, and said they were confident that the sign-up rate would pick up this month.
But their own original estimates had projected a significant spike in February enrollments over January, which failed to materialize.
The Obama administration last year estimated that 5.65 million people would be enrolled by the end of February, and that 1.27 million people would sign up in that month alone.
Those predictions were abandoned as unrealistic after the disastrous launch of the federally run Obamacare exchange HealthCare.gov on Oct.1. The technologically troubled site was unable to enroll more than 130,000 people by the end of the first two months of operation.
Another highlight—or low-light—of Tuesday's report was the disclosure that the rate of sign-ups by young adults, ages 18-34, had remained flat at 27 percent of total enrollment over the past two months.
That's well below the 40 percent level that some health-care experts have said would likely offset benefits paid out to older, sicker enrollees, which in turn would reduce pressure on insurers to significantly hike prices for 2015..
Officials including President Barack Obama and first lady Michelle Obama have been making media appearances directly targeting young adults to push them to sign up. Obama on Tuesday appeared in a snarky Funnyordie.com interview with comedian Zach Galifianakis, to tout the benefits of enrolling, an appearance the administration said led to tens of thousands of of direct clicks to HealthCare.gov from the Funnyordie video.
If the overall enrollment slowdown trend persists this month, it could leave the Obama administration short of the 6 million enrollees that Vice President Joe Biden recently set as a revised sign-up target for the March 31 deadline. Most Americans must have some form of health insurance by then or face a fine next year.
Officials reiterated Tuesday that there will not be any extension of the open-enrollment period, despite the fact they they previously have extended or adjusted a slew of other Obamacare deadlines and rules.
Under often-sharp questioning by reporters Tuesday, Centers for Medicare and Medicaid Services spokeswoman Julie Bataille repeatedly refused to say whether the Obama Administration was confident it could hit its revised 6 million goal, which is 1 million less than what it had originally estimated last year.
"We do believe that millions more Americans will come into enroll by the March 31 deadline," Bataille said, without directly answering whether officials believed the 6 million target would be hit.
(Read more: Did ya' hear the one about Galifianakis and Obama? )
Bataille and other officials also said they were unable to answer questions asking for the percentage of enrollees who had paid their first month's premiums, which is required for enrollment to be official. Some experts have estimated that up to 20 percent of would-be enrollees did not pay after selecting an insurance plan, a rate that would substantially drive down the official tally, and make it even more difficult to reach the 6 million goal.
Officials also could not answer what percentage of enrollees had health insurance before they purchased plans on the government-run marketplaces. That is a key question because Obamacare's prime goal is to provide health coverage to the currently uninsured.
Obamacare critics have suggested that a large percentage of the enrollees had prior coverage, and needed to buy new plans because their old policies were not compliant with Affordable Care Act requirements.
While not having data about enrollees who were previously uninsured, Obama administration officials pointed to a new Gallup poll which showed that the rate of Americans who lacked insurance as of February was at 15.2 percent, the lowest rate since 2008. It also represented a marked drop from the all-time high of 18 percent right before the Obamacare exchanges launched on Oct. 1.
Administration officials said they expect a bump in the rate of enrollment this month because other health care programs such as Medicare Part D and Massachusetts Romneycare program also saw pronounced spikes before key deadlines.
To hit 6 million enrollees, another 1.8 million people would have to buy Obamacare plans by the end of this month.
"It's not too late to sign up to get covered, but they'll want to do it today, as we approach that March 31 deadline," Health and Human Services Secretary Kathleen Sebelius.
(Read more: Fed watchdog probes 'flawed' Obamacare exchange )
"There's every reason to believe that they're right, that sign-ups are going to shoot up in March, and that they'll get to 6 million," said Timothy Jost, a law professor at Washington and Lee University and leading health-care reform expert.
Jost said it was not surprising that the enrollment rated dropped in February since millions of people had flocked sign-up by the end of December to ensure coverage beginning of 2014.
He said the Obama Administration's ongoing marketing push is likely to juice enrollment numbers.
A spokeswoman for the House Energy and Commerce Committee, whose Republican majority oppose Obamacare, said, "Until the administration provides details about who was previously uninsured and who has paid, these reports tell us nothing about the success or failure of the president's health care law."
"The president's recent media appearances suggest that the administration is panicked as the March 31 deadline fast approaches. Does the administration not think Americans can handle the truth regarding the status of their health care system? It is time for the facts," the spokeswoman said.
But Ann Filipic, president of the Obamacare advocacy group Enroll America, said "We know from our research and past enrollment efforts that many people, especially young folks, will wait towards the end to sign up, which is why we've ramped up our outreach efforts during this final push."
"Through the consumer-focused Get Covered America campaign, our hundreds of staff, thousands of partners, and tens of thousands of volunteers stand ready in communities across the country to host over 3,000 events throughout March—including daily enrollment events in all major cities throughout our 11 staffed states during the last two weeks—to ensure that onsumers have access to the information and in-person assistance they need to enroll."
(Read more: Maryland Obamacare site eyes eHealth deal )
—By CNBC's Dan Mangan. Follow him on Twitter @_DanMangan