3.6 million people have signed up for coverage through @HealthCareGov, up substantially over the 2.9 million at the same time last year. But it’s pretty apparent that signups will end up lower with the enrollment period being half as long.
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The idea of a shorter ACA open enrollment period originated with the Obama administration, but the Trump administration implemented it with a 90% reduction in outreach funding and a 41% reduction in navigator grants.

There have been two theories about how a shorter open enrollment period would alter the individual insurance market.

One theory is that sick people sign up immediately, while healthy people procrastinate and sign up towards the deadline. With a shorter open enrollment period, you lose some of the healthy people and worsen the risk pool.

Another theory is that a longer open enrollment period allows some healthy people to remain uncovered and sign up only if they end up getting sick before the end of the period. A shorter period mitigates this and could improve the risk pool.

At this point it looks very likely that a shorter open enrollment period is leading to fewer signups for ACA coverage and more people uninsured.

The effect of a shorter open enrollment period on the risk pool -- that is, the mix of healthy and sick people -- is less definitive until more data are available.

It's possible that with sustained outreach, a shorter open enrollment period might not have led to a drop in signups, especially given that weekly enrollment is up over last year.

One also has to wonder how much all the talk of repealing the individual mandate is suppressing enrollment. Of course, the tax bill doesn't repeal the mandate until 2019, so some people could be in for a nasty surprise.