WASHINGTON -- The Obamacare health insurance exchanges appear to be doing a good job when it comes to one of their most important yet underappreciated functions: offering a fallback option to people who lose their health coverage during the year.

Already this year, almost half a million people have taken advantage of that safety net, a new government report shows.

Since the federal HealthCare.gov system and those run by states like California and Kentucky went live in October 2013, sign-ups during the annual open enrollment periods have garnered most of the attention, as has the rapid increase in the number of Americans with health insurance .

But a key function of these health insurance exchange marketplaces is to provide a place people can go if they lose their health coverage, such as when they lose a job and the benefits that came with it, or when they start working for an employer that doesn't offer a health plan.

On Thursday, the federal government for the first time released data suggesting Americans are taking advantage of this option. According to a report from the Centers for Medicare and Medicaid Services, half of the 944,000 new enrollees on the federally run exchanges in 37 states between Feb. 23 and June 30 signed up because they'd lost their previous coverage. The agency doesn't have data for states that fully operate their own marketplaces.

Losing health coverage is one of the qualifying events that allows individuals to access the exchanges outside of the regular yearly sign-up through what's called a special enrollment period, or SEP.