Definitions of HCV Incidence
The incidence of hepatitis C is defined as number of new infections in a specific region in a specific time period. The CDC typically defines the incidence of hepatitis C in the United States (or in each state) as the number of new hepatitis C virus infections that occur per year. The incidence rate is the number of cases per total population (typically defined as number of cases per 100,000 persons).
The CDC generates estimates for the total number of new cases that occur each year based on reporting data. For each new symptomatic acute HCV case that is reported, an estimated 3.3 cases of symptomatic acute HCV actually occur. In addition, for each new HCV case that is symptomatic and reported, the actual number of acute cases is approximately 13 times the number of new HCV infections reported. The CDC estimates that 30,500 persons were newly infected with hepatitis C in 2014. The number of new annual HCV infections has increased significantly from 2011 (16,500) to 2014 (30,500), but the number of annual new infections in recent years is markedly lower than during the 1980’s, when an estimated 230,000 persons were newly infected with HCV each year (Figure 1).
Importance of Incidence Data
The United States hepatitis C 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.
Method of Estimating Incidence
Most patients with acute hepatitis C do not have an acute illness and most do not seek medical care. In addition, many cases of diagnosed acute hepatitis C are not reported. Thus, determining the incidence of new infections per year requires sophisticated epidemiologic modeling techniques that use surveillance data to generate estimates for the number of new infections per year. The CDC provides several numbers related to the incidence of hepatitis C in the United States, including (a) number of reported acute cases, estimated number of acute clinical cases, estimated number of new infections, and the national rates per 100,000 persons.
Definition of HCV Prevalence
The HCV prevalence is defined as the number (or percent) of persons in the total population observed infected with hepatitis C. Most often, the HCV prevalence specifically refers to persons living with active (chronic) hepatitis C infection. Less frequently, the HCV prevalence data is given for all individuals with anti-HCV (number of persons living who have been infected with hepatitis C), which includes those with active HCV, persons who spontaneously resolved HCV, and those with HCV-treatment related cure. The prevalence rate of chronic hepatitis C is the number of persons living with HCV per population (typically defined as number of persons per 100,000 population).
HCV Prevalence Data Based on NHANES Data
In the United States, hepatitis C virus infection is the most common bloodborne infection. The best estimates of HCV prevalence derive from analysis of serum specimens taken from participants in the National Health and Nutrition Examination Survey (NHANES) (Figure 2). The first estimate of HCV prevalence in the United States was generated from NHANES conducted between 1988 and 1994 and it estimated 2.7 million persons had chronic HCV in the United States. In a similar subsequent NHANES analysis, which involved a survey conducted between 1999 and 2002, investigators estimated 3.2 million persons had chronic HCV, which corresponded to approximately 1.3% of the population. A recent follow-up NHANES study that involved survey data from 2003 to 2010 estimated prevalence of 2.7 million persons chronically infected with HCV, corresponding to a population prevalence of chronic hepatitis C of 1.0%. These NHANES surveys, however, did not sample certain populations, including the incarcerated, homeless, nursing home residents, persons on active military duty, and immigrants.
CDC Current HCV Prevalence Estimate
Other investigators have estimated a higher HCV prevalence in the United States, based on the knowledge that the NHANES surveys did not include certain populations in the survey, including incarcerated, homeless, nursing home residents, hospitalized individuals, and persons on active military duty. The CDC, after adjusting for populations not included in the NHANES surveys, estimates that 3.5 million persons are living with HCV infection in the United States.
HCV Prevalence by Year of Birth
The HCV prevalence is highest among persons born during 1945 to 1965 (Figure 3). Indeed, the CDC estimates that approximately three-fourths of all persons living with HCV infection in the United States were born during 1945 to 1965. The relatively high prevalence of HCV infection among persons born during 1945 to 1965 corresponds with the high HCV incidence (new infections) that occurred among young adults in the 1970s and 1980s.
Awareness of HCV Infection Status
An estimated 40 to 85% of persons infected with HCV are unaware of their HCV infection status. One study reported that among HCV-infected injection drug users who were 15 to 30 years old, 72% were unaware of their HCV infection status. A more recent NHANES study conducted from 2001 through 2008 found that 50.3% of persons infected with HCV were unaware of their status. In a study involving persons with access to medical care in four private health care organizations during the years 2006 to 2008, an estimated 43% were unaware of their HCV infection.
In the United States, approximately 70% of chronic HCV infections are caused by hepatitis C genotype 1, 15 to 20% by genotype 2, 10 to 12% genotype 3, 1% genotype 4, and less than 1% genotype 5 or 6. Among the genotype 1 infections, approximately 55% are genotype 1a and 35% 1b.