The official tally of QHP selections nationally during the 2015 Open Enrollment Period (from 11/15/14 - 2/22/15) was 11,688,074 people.
The actual number of people still enrolled in effectuated plans (i.e., active) as of March 31, 2015, according to the HHS Dept., was 10,187,197 people. That's a net reduction of exactly 1,500,877.
On the surface, that may look bad, but bear in mind that with a 90% payment rate (which I suspect is actually pretty close to the non-ACA industry standard, and which is about 2 points better than last year), that means only about 10.5 million of the original enrollees would have been expected to actually be enrolled in March anyway. That leaves another 332,000 people who presumably paid up for January, February and/or March, but had dropped their policies by the end of March.
