Research has shown that the last week of January tends to draw younger enrollees - an important group, underrepresented in the ACA marketplaces, whose scant use of medical services can help keep insurance prices lower for everyone in a given health plan. For the last week of the sign-up period, HHS had planned a major advertising campaign specifically aimed at that younger cohort, as well as emails and text messages to people who have looked at the HealthCare.gov site in the past but not signed up.