(Adds California figures, final two paragraphs)
By David Morgan
WASHINGTON, March 17 (Reuters) - More than 5 million people have enrolled in private health insurance under the law known as Obamacare since open enrollment began on Oct. 1, the Obama administration said on Monday.
With the March 31 enrollment deadline only two weeks away, a top administration official reported a big upswing in public interest in subsidized health insurance and said traffic on the federal website HealthCare.gov reached 1 million visitors over the weekend.
"The last several days have been the busiest since December," Marilyn Tavenner, administrator of the U.S. Centers for Medicare and Medicaid Services, said in a government blog posting that announced the new total.
The 5 million enrollee total marked an increase of at least 800,000 people since March 1, indicating that total enrollment could approach the 6 million figure estimated by the nonpartisan Congressional Budget Office if enrollment surges in the final weeks as administration officials have predicted.
The CBO initially forecast 7 million enrollees in private health coverage, but scaled back its estimate after technical problems with the rollout that continue to afflict Obamacare insurance marketplaces operated by several states.
Monday's release did not say how many new enrollees were adults aged 18 to 34, a younger demographic whose participation is seen as essential for the success of President Barack Obama's signature domestic policy achievement.
Before the rollout, administration officials said their goal was for 38 percent of the new online private insurance marketplaces to consist of younger consumers, who compensate for older, sicker enrollees because they tend to be healthy and cheaper to ensure. But the data so far shows young adults accounting for only one-quarter of enrollees.
Another important section consists of people signing up for insurance for the first time, a group that also tends to be healthy. The administration says it has no data for that group, but the consulting firm McKinsey & Co in a report this month put the number at around 27 percent of the total enrollees
Too few healthy beneficiaries could cause healthcare costs to rise as insurers grapple with higher insurance risks posed by policyholders who are sick or older.
"Five million is only a good number if it's mostly people who didn't have insurance before," said Joe Antos of the conservative American Enterprise Institute.
MARCH 31 DEADLINE
The Patient Protection and Affordable Care Act is expected to extend health insurance to millions of uninsured Americans through the private marketplaces and an expansion of the Medicaid program for the poor.
But the 2010 law's implementation has been dogged by delays, legal challenges and political attacks from Republicans and other critics.
U.S. Health and Human Services Secretary Kathleen Sebelius told Congress last week that the administration would not extend the March 31 enrollment deadline.
But the law allows people who have difficulty signing up for coverage to arrange for special enrollment periods. With huge volumes expected in the final days of the open enrollment period, analysts say 2014 enrollment may not reach its final total until sometime after March.
Separately, California's state health insurance exchange said on Monday its enrollment passed the 1 million mark, with 1,018,315 people selecting a health plan through the exchange as of Saturday. Those numbers are part of the Obamacare enrollment figures.
"California continues to lead the nation in the implementation of the Affordable Care Act and the numbers released today illustrate strong progress in the push to expand health care coverage as we head into the final weeks of open enrollment," California Assocation of Health Plans' Chief Executive Patrick Johnston said in a statement. (Editing by Chris Reese, Mohammad Zargham and Peter Cooney)