Definitions of HCV Incidence
The incidence of hepatitis C virus (HCV) infection is defined as the number of new infections in a specific region over a specific time period; the incidence data is typically reported for a 1-year period, often in conjunction with cumulative and comparative multiyear data (Figure 1). The Centers for Disease Control and Prevention (CDC) defines the incidence of HCV in the United States (or in each state) as the number of acute HCV cases that occur per year, which is the closest proxy of actual new infections. The incidence rate is the number of cases per total population (typically defined as the number per 100,000 persons).
Method of Estimating HCV Incidence
Most individuals with acute HCV infection do not have a clinically evident illness, and most do not seek medical care. In addition, many cases of diagnosed acute hepatitis C are not reported. Thus, determining the true incidence of new HCV infections per year based on the number of reported cases requires highly complex epidemiology modeling techniques.[1] For each new acute HCV case that is reported in the United States, the CDC estimates there are approximately 13.9 actual new acute HCV cases (reported and unreported) that have occurred.[1,2] This high ratio (total estimated cases to actual reported cases) is primarily a result of the large proportion of persons with acute HCV who have asymptomatic or minimally symptomatic infection and do not seek medical care, or who have undiagnosed HCV infection (Figure 2); the passive HCV reporting system likely also contributes to the low number of reported acute HCV cases. The CDC provides several numbers related to the incidence of hepatitis C in the United States, including the number of reported acute cases, estimated number of acute clinical cases, estimated number of new infections, and rates per 100,000 persons at the state and national level.[2]
HCV Incidence Data
In 2023, a total of 4,966 new cases of acute hepatitis C were reported to the CDC and this corresponds to an estimated 69,000 new acute cases of HCV in 2023.[2] From 2010 to 2020, the number of estimated annual acute HCV infections steadily increased, but then leveled off during the years 2020-2023.[2] The increase in new HCV infections from 2010 to 2021 and the persistent high number of cases has been primarily attributed to the opioid epidemic and associated injection drug use, particularly among young adults.[3,4,5,6] The following summarizes CDC HCV surveillance data for 2021 based on specific groups and demographic factors (Figure 3).[2]
- Sex: The number of reported cases of acute HCV infections was roughly twice as high in males (3,321) than in females (1,640), with males accounting for two-thirds of the acute HCV infections.[2]
- Age Group: The highest rate of reported acute infections involved persons 30–39 years of age, with the next highest number among persons 40–49 years of age.[2]
- Race/Ethnicity: The highest number of reported cases of acute HCV by race/ethnicity occurred in White persons, but the highest rates were in American Indian/Alaska Native persons.[2]
- Urban/Rural: Although 83% of cases were diagnosed in urban areas, the rate of acute HCV (cases per 100,000 population) was slightly higher in rural areas (1.8) versus urban areas (1.5).[2]
Importance of HCV Incidence Data
The United States HCV incidence data provide important information for monitoring trends in transmission patterns, developing hepatitis C prevention strategies, monitoring the effectiveness of implemented prevention plans, and identifying focal outbreaks or regional patterns of infection. In addition, valuable information emerges when data are categorized by age group, sex, race/ethnicity, and risk factor for acquiring HCV, as these data may inform major population-specific prevention strategies.