California's Obamacare exchange announced two weeks ago it had signed up more than 18,000 residents during the grace period there, or more than one-quarter of HealthCare.gov's current tally.
Kevin Counihan, CEO of HealthCare.gov, said in a press release Monday, "We understand the requirement to have insurance is new, which is why we're providing this last chance for those who are uninsured, are paying a fee and were not aware of or did not understand the implications of the fee with an opportunity to enroll in affordable coverage for the rest of the year."
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"We hope uninsured tax filers take the next few days to learn about the options and financial assistance that is available and to enroll in a plan that meets their needs, rather than taking the risk of choosing to get by without insurance for another year."
CMS did not make Counihan available for a requested interview with CNBC.
Larry Levitt, an Obamacare expert with the Kaiser Family Foundation, said, "This special enrollment period is a safety valve for people who were confused about the individual mandate penalty and are still uninsured, but it hasn't been used a whole lot so far."
"While not very many people have taken advantage of this tax-time enrollment opportunity, it's likely had the effect of turning down the heat on the potential controversy surrounding the individual mandate," Levitt said. "There hasn't been much of a hullaballoo over the health law's requirement to have insurance this tax season, even though it's the most unpopular part of the law."
Nearly 11.7 million people had signed up for Obamacare plans nationally on HealthCare.gov and state-run insurance exchanges by the close of open enrollment.