WASHINGTON - Enrollment records for close to 15,000 HealthCare.gov shoppers were not initially transmitted to the insurance plans they selected, according to a preliminary federal estimate.

While these cases pose a challenge for the Obama administration, officials say they believe the situation is improving. Since early December, fewer than 1 percent of HealthCare.gov enrollments did not make their way to health insurance plans.

"The vast majority of the work is retroactive," Centers for Medicare and Medicaid Services (CMS) spokeswoman Julie Bataille said in an interview Friday. "We're making sure that as we do the intense data reconciliation, we identify the things that need to be resolved so consumers can confirm they're enrolled."

The preliminary estimate that fewer than 15,000 enrollments failed to reach carriers comes from a recently completed federal analysis that compared the number of shoppers who clicked "enroll" with the number of digital files HealthCare.gov fired off with health-insurance plans.

That analysis does not generate a list of specific shoppers whose enrollment files were never sent, but rather provides an estimate of the discrepancy between enrollments finished and reports generated. Still, the figure is one of the more concrete measures of the data errors that have plagued the back end of HealthCare.gov and challenged the website's ability to notify health-insurance providers when a new member enrolls in their products.

Those digital files are known in the insurance industry as "834 transmissions," and they have become the focus of much attention since insurers noticed some arriving with flaws or in duplicate.

Of particular concern to health insurers were "orphan files" - situations in which a shopper thought he or she had signed up but the exchange did not notify the carrier. While insurance plans could plausibly correct inaccurate data, they could not get coverage to a subscriber they did not know existed.

During the first two weeks of October, the new federal analysis estimates, nearly 10 percent of enrollments were missing an 834 transmission. That number rose to 15 percent in mid-October before declining through November.

From late November into early December, CMS estimates that less than 1 percent of sign-ups failed to send a record to the health insurer, indicating that software fixes have helped address the problem.