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 out 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 number of cases 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 have undiagnosed 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 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
The CDC HCV surveillance data from 1982 to 2019 shows a peak in the estimated number of acute HCV cases in 1989, a steady decline from 1990 to 2005, a relative leveling off between 2006 and 2010, and then an increase from 2010 to 2019 (Figure 3).[2] In 2019, a total of 4,136 new cases of acute hepatitis C were reported to the CDC from 44 states; based on this number, the CDC estimated a total of 57,500 new acute cases of HCV in 2019.[2] From 2010 to 2019, the number of estimated annual acute HCV infections increased by 387% (Figure 4).[2] This steady and significant increase in new HCV infections from 2010 to 2019 is attributed primarily to the opioid epidemic, and associated injection drug use, particularly among young adults.[3,4,5,6] The following summarizes CDC HCV surveillance data in 2019 based on specific groups and demographic factors.[2]
- Gender: The number of reported cases of acute HCV infections in 2019 was higher in males (2,471) than in females (1,653), with males accounting for 60% of the acute HCV infections in 2019 (Figure 5).[2]
- Age Group: For 2019, the reported number and rate of acute HCV infections was highest among persons aged 30-39 years followed by those 20-29 years of age; the lowest number and rate was among those 0-19 years of age (Figure 6).[2]
- Race/Ethnicity: In 2019, the highest number of reported cases of acute HCV by race/ethnicity occurred in White persons, followed by Hispanic and Black persons (Figure 7).[2] Among all reported acute cases in 2019, persons of White race comprised 65% (2,683 of 4,136) of the cases.[2] The highest rate for acute HCV cases was in American Indian/Alaska Native persons, followed by White persons (Figure 8).[2] The trend from 2003 to 2019 shows a dramatic increase in the rate of reported acute HCV among American Indian/Alaska Native individuals during the years 2010 through 2019, as well as a steady increase in White persons during this time (Figure 9).[2]
- State: In the United States for the year 2019, the 5 states with the most number of reported cases of acute HCV were (in descending order): Florida (616), Indiana (325), New York (306), Ohio (281), and Pennsylvania (210).[2] Together, these 5 states comprised approximately 42% (1,738 of 4,136) of the total reported number of acute cases in the United States. In 2019, the 5 states with the highest rate (per 100,000 persons) of reported cases of acute HCV were (in descending order): Indiana, West Virginia, Utah, South Dakota, and Maine.[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 any implemented plans, and identifying focal outbreaks or regional patterns of infection. In addition, valuable information emerges when data is categorized by age group, gender, race/ethnicity, and risk factor for acquiring hepatitis C virus; these data may inform major prevention strategies.