More than 1 million people selected a health plan during the fourth week of the health law’s open enrollment and nearly 2.5 million have done so since it began Nov. 15, federal officials said Tuesday.

“And this was before an extremely busy weekend,” said Andy Slavitt, principal deputy administrator of the Centers for Medicare & Medicaid Services, which oversees the federal online marketplace used by 37 states.

Tuesday’s report did not include enrollment for the final three days before the Dec. 15 deadline for people to enroll if they want coverage to begin Jan. 1.

Jose Ramirez (L) and Mariana Silva speak with Yosmay Valdivia, an agent from Sunshine Life and Health Advisors, as they discuss plans available from the Affordable Care Act at a store setup in the Mall of the Americas in Florida. (Photo by Joe Raedle/Getty Images)

Just over half of those individuals who have selected plans since the health law’s second open enrollment season began are returning customers. Enrollment in the states running their own exchanges is not yet available.

As expected, interest in healthcare.gov soared in the final days before the mid-December deadline, with 1.6 million people phoning the call center from Dec. 13 through Dec. 15, officials told reporters.

To avoid longer waiting times, nearly 500,000 people who called just hours before the Dec. 15 midnight PST deadline left their contact information. Website officials have begun to call them back, Slavitt said, and they will be able to enroll in coverage to begin Jan. 1.

At its peak volume Monday, healthcare.gov had more than 125,000 concurrent users but “we did not run into capacity constraints,” Slavitt said. “In other words, we are able to handle even more volume in the coming months ahead.” One website “waiting room” was used for about 90 minutes for “several thousand” individuals creating new accounts, Slavitt said. Their average wait time was about three minutes. Returning customers or those doing “window shopping” were not affected, Slavitt added.

In a call with reporters, Slavitt and Kevin Counihan, the CEO of healthcare.gov, said federal officials have begun to automatically re-enroll 2014 customers who have not selected a new plan for 2015. Less than 5 percent of current enrollees could not be automatically re-enrolled, Counihan said.

Counihan said the website has been sending daily updates to insurers to let them know about people that have switched health plans, helping to avoid confusion that could lead to insurers double-billing consumers. Separately Tuesday, America’s Health Insurance Plans said they would give consumers additional time to pay premiums due Jan. 1 and would provide prompt refunds if individuals were mistakenly billed for two health plans.

Several states, including California and Minnesota, have extended enrollment deadlines for coverage to begin Jan. 1.

Earlier Tuesday the consulting firm Avalere Health estimated that 10.5 million people would enroll in the health law’s state and federal exchanges by the end of 2015. Administration officials have estimated that about 9 million people would enroll in the exchanges while the Congressional Budget Office has estimated 13 million.

This story was updated to clarify the number of enrollees whose coverage could not be automatically re-enrolled.