As of Monday, the United States has delivered more than half a million monoclonal antibody therapeutics to states to treat non-hospitalized Covid-19 patients, according to the US Department of Health and Human Services’ website, but it’s not clear how many have been used.

Since the therapies received an emergency use authorization from the US Food and Drug Administration in November, the department said it has delivered 454,087 courses of an Eli Lilly treatment, and 96,923 courses of Regeneron’s cocktail. 

Both treatments are authorized for people who are 12 years of age and older, who are at high risk for progressing to a severe form of Covid-19. Both were shown to reduce Covid-19-related hospitalization or emergency room visits.

It’s unclear exactly how many of the distributed antibody treatments have actually been used; it’s not posted by HHS or tracked on state dashboards. Health officials have said they aren’t being used enough; in Michigan, for example, less than 10% of available Covid-19 monoclonal antibody treatments have been used, Dr. William Fales, the medical director at the Michigan Department of Health and Human Services, said last week.

The treatments are complicated to administer, in part because hospitals or infusion centers need to create a separate space to treat patients.