As the opioid epidemic rages, fueling a rise in hepatitis C virus (HCV) transmission through the sharing of injection drug equipment, a new analysis estimates that 2.4 million U.S. residents are living with HCV.

Researchers analyzed 2013 to 2016 data on all adults from a Centers for Disease Control and Prevention (CDC) national survey on health and nutrition. They also relied on scientific literature reviews and estimates of population sizes to estimate the prevalence of HCV among incarcerated people, homeless individuals, active-duty military personnel and nursing home residents.

An estimated 1.7 percent of all adults in the United States, or 4.1 million people, had HCV antibodies (which indicates a past or current infection) during the study period. (Note that hep C can spontaneously clear without treatment during the first months after infection.) And about 1 percent of adults, or 2.4 million people, had both HCV antibodies and a positive HCV RNA test, indicating a current infection.

“Hundreds of thousands of Americans have been cured, but many people infected with hepatitis C have not benefited from direct-acting antiviral [treatment for the virus] because they don’t know they are infected or cannot access treatment,” said the study’s first author, Megan Hofmeister, MD, MPH, a medical officer in the CDC’s Division of Viral Hepatitis. “Until hepatitis C testing and treatment are expanded to more Americans, we will be in a losing battle against this disease.”