A nationwide vaccine shortage is impeding efforts to contain a multistate outbreak of hepatitis A virus (HAV), according to a recent announcement by the Centers for Disease Control and Prevention (CDC), CNN reports.

Currently, GlaxoSmithKline and Merck are the only pharmaceutical companies approved by the Food and Drug Administration (FDA) to sell hepatitis A vaccines in the United States. Both companies say they have begun back-ordering single doses of the vaccine after an “unprecedented demand.” The two companies are also working closely with CDC and state officials on how best to distribute vaccines to those at greatest risk for infection.

Meanwhile, the country’s hep A crisis continues to grow. In California, hepatitis A has sickened at least 644 people and claimed 21 lives since November 2016. Elsewhere, southeast Michigan has seen an additional 495 cases of hepatitis A and 19 deaths since August 2016. The liver virus has also reportedly spread to Arizona, Utah and Colorado.

Typically, outbreaks of hepatitis A are small in this country, as most people in the U.S. population now get inoculated as children. However, this latest outbreak has largely spread among older homeless and drug user populations, many of whom were not vaccinated before the protective shot became available in the early ’90s. Health authorities say local law enforcement officers, medical personnel and men who have sex with men are also considered to be at increased risk for infection in several cities.