Burwell says 1.9 million new customers have picked a plan through the federal market as of Dec. 19. It serves 37 states.
Another 4.5 million have renewed existing coverage, with most automatically re-enrolled.
The numbers don't include states running their own insurance exchanges.
Burwell said Tuesday the administration will release a full 50-state report next week.
With the HealthCare.gov website running much better this year, the administration has set a goal of 9.1 million customers signed up and paying premiums in 2015.
Open enrollment ends Feb. 15.
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