Ojectives: Upon completion of this course the student will be able to identify the five major categories of Hepatitis, the symptoms, serologistic course, transmission methods, and prevention.
Hepatitis A Virus
Hepatitis A is caused by infection with HAV, a 27-nm RNA virus that is classified as a picornavirus. HAV infection appears to induce lifelong protection against subsequent infection. Only one serotype has been observed among HAV isolates collected from various parts of the world.
Hepatitis A: Clinical Features
The average incubation period for hepatitis A is 30 days with a range of 15 to 45 days.
The severity of clinical disease associated with HAV infection increases with increasing age; icteric disease occurs among <10% of children younger than 6 years of age, 40%-
50% of older children, and 70%-80% of adults. Complications of hepatitis A include fulminant hepatitis in which the casefatality rate can be >50% despite modern medical interventions such as liver transplantation, cholestatic hepatitis with very high bilirubin levels that can persist for months, and relapsing hepatitis in which exacerbations can occur months after apparent recovery. Chronic infection has not been reported to occur following HAV infection.
Age-specific Mortality Due to Hepatitis A
Hepatitis A produces occasional mortality, with an estimated 100 deaths each year in the United States due to fulminant hepatitis A. The overall case-fatality rate is approximately 0.4%; the highest case-fatality rate is among persons >49 years of age
(17.5 per 1000). In this age group, preexisting chronic liver disease appears to be associated with an increased risk of death from acute hepatitis A.
Hepatitis A Virus Infection: Typical Serologic Course
The diagnosis of acute HAV infection is confirmed during the acute or early convalescent phase of infection by the presence of IgM antibodies to HAV (IgM anti-
HAV). IgM anti-HAV is generally present 5-10 days into the incubation period and remains detectable for up to 6 months after onset of illness. IgG anti-HAV, which also appears early in the course of infection, remains detectable for the lifetime of the individual and confers lifelong protection against disease. The concentration of HAV in the stool of infected persons is highest during the 2-week period before onset of jaundice. Fecal HAV excretion generally persists for <1 week after onset of jaundice in adults; however, virus has been detected during disease relapse up to 2 months after illness onset. Children and infants may shed HAV for longer periods than observed in adults, possibly for several weeks after onset of clinical illness. In infants born
1
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2 prematurely, viral RNA has been detected intermittently by polymerase chain reaction
(PCR) in stool specimens for up to 6 months after infection. Chronic shedding of HAV in feces has not been demonstrated.
Concentration of Hepatitis A Virus in Various Body Fluids
Feces can contain up to 10 8 infectious virions per ml and are the primary source of
HAV. Viremia occurs during the prodromal phase of illness and HAV has been transmitted on rare occasions by transfusion. Virus has also been found in saliva during the incubation period in experimentally infected animals, but transmission by saliva has not been reported to occur.
Hepatitis A Virus Transmission
Transmission of HAV generally occurs as a result of ingestion by a susceptible person of virus shed in the feces of an infected person. Close personal contact is the most common mode of HAV transmission as demonstrated by high rates of infection among household and sex contacts of persons with hepatitis A and among children in day care center outbreaks. Contaminated food and water can also serve as vehicles of HAV transmission; the vehicles of transmission in foodborne outbreaks are most often uncooked foods or foods touched by human hands after cooking, but outbreaks have been reported in association with foods contaminated before wholesale distribution.
Because HAV can survive for 12 weeks to 10 months in water, infection can occur by ingestion of a variety of raw shellfish harvested from sewage-contaminated areas.
Waterborne outbreaks have also been reported both in association with drinking fecally contaminated water and with swimming in contaminated swimming pools and lakes. In addition, HAV transmission can occur as a result of blood exposures such as injecting drug use or blood transfusion because viremia can occur prior to onset of illness in infected persons; however, such transmission is rare.
Incidence of Hepatitis A, United States, 1952-1993
During the past several decades, the incidence of hepatitis A in the United States has been cyclic, with nationwide epidemics occurring every 10 to 15 years; the latest epidemic occurred in 1989. Between epidemics, hepatitis A continues to occur at relatively high levels. For example, in 1993, 24,238 hepatitis A cases were reported to the National Notifiable Diseases Surveillance System of the CDC; these cases are estimated to correspond to 75,000 cases of hepatitis A and 125,000 HAV infections, after correcting for underreporting and asymptomatic infections.
Age-specific Incidence of Hepatitis A, United States, 1983-1993
Children play an important role in the transmission of HAV in the United States. In recent years, peak rates of disease have been among children 514 years of age and nearly 30% of all reported cases are among children younger than 15 years of age.
However, reported disease rates do not reflect the extent of transmission among young children, because most children who are infected before age 5 have unrecognized asymptomatic infection.
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Sources of Hepatitis A Virus Infection by Mutually Exclusive Groups, United
States, 1983-1993
The most frequently reported source of infection for reported hepatitis A cases is personal contact (household or sexual) with an infected person, which is consistently reported by about 25% of persons with acute hepatitis A. About 15 percent of cases involve a child or employee at a day care center or a contact of a day care-related case, and 5-7% involve persons with a history of recent international travel or of being part of a recognized food or waterborne outbreak. Injecting drug use and homosexual activity are generally associated with <5% of cases; however, prolonged outbreaks in which person-to-person contact has been thought to be the predominant source of infection have occurred among persons with these risk factors. A large percentage
(approximately 45% in 1993) of persons with reported hepatitis A do not have a recognized risk factor for acquiring infection; this is a reflection of the fact that most hepatitis A in the United States occurs in communitywide outbreaks with extended person-to-person transmission. Contact with children may be an important source of infection for persons without identified risk factors because children <5 years of age reside in approximately 50% of these case-households.
Global Patterns of Hepatitis A Virus Transmission, Geographic Distribution of
HAV Infection
Worldwide, four different patterns of HAV transmission can be defined on the basis of age-specific seroprevalence data. In general, these transmission patterns correlate with socioeconomic and hygienic conditions. In many developing countries where environmental sanitation generally is poor, nearly all children <9 years of age have evidence of prior HAV infection. In these areas, distinct outbreaks rarely occur and clinical disease related to HAV infection is uncommon. As hygienic conditions improve, transmission shifts to older age groups and the incidence of clinically evident disease increases. In most industrialized countries, low levels of endemic HAV transmission occur. The relatively high prevalence of prior HAV infection among older age groups in these areas is likely to be related to the presence of lower socioeconomic and hygienic conditions in the past. Because most of the population is susceptible to HAV infection, disease outbreaks are relatively common in most of these countries. However, in a few industrialized countries hepatitis A outbreaks are uncommon, and nearly all HAV transmission occurs among drug users and travelers to high endemic areas.
Hepatitis A Vaccine Efficacy Studies
The hepatitis A vaccine currently licensed in the United States, HAVRIX
ｮ * (manufactured by SmithKline Beecham Pharmaceuticals), is a formalin-inactivated vaccine. Application has been made for licensure of another inactivated vaccine,
VAQTA™ (manufactured by Merck & Co., Inc.). Both of these vaccines are highly immunogenic, inducing anti-HAV titers above those observed following administration of
IG in more than 99% of adults. The efficacy of HAVRIX ｮ was evaluated in a doubleblind, placebo-controlled, randomized clinical trial in Thailand among 38,157 children aged 1-16 years living in an area with high endemic rates of hepatitis A. Following two doses of vaccine (360 EL.U. per dose) given 1 month apart, the efficacy of vaccine in protecting against clinical hepatitis A was 94% (95% confidence interval [CI], 79%-
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4
99%). A double-blind, placebocontrolled, randomized clinical trial using VAQTA™ was conducted among 1037 children aged 2 to 16 years living in a single U.S. community with a high endemic rate of hepatitis A. After 18 days following one dose (25 units) of vaccine, the efficacy in protecting against clinical disease was 100% (95% CI, 85%-
100%).
Hepatitis A Vaccination Strategies: Epidemiologic Considerations
Epidemiologic features of HAV infection that need to be considered in developing hepatitis A vaccination strategies include the following:
1.Most hepatitis A in the United States occurs due to extended community-wide outbreaks in which a large proportion of cases have no identifiable risk factor. In these outbreaks, the highest rates of disease occur among children 5-14 years of age and person-to-person transmission, especially from children to adults, is the primary mode of transmission.
2.The primary groups with an increased risk of infection include the following:
Travelers to developing countries.
The risk among such travelers who do not receive IG is about 3-5/1000 per month of stay.
Men who have sex with men.
Hepatitis A outbreaks among homosexual men have been reported frequently and serosurveys have demonstrated a prevalence of HAV infection among homosexual men several-fold higher than among control populations.
Injecting drug users
During the past decade outbreaks were reported in the United States among injecting drug users and during the late 1980s 10%-20% of persons with hepatitis A reported a history of injecting drug use.
Routine Childhood Hepatitis A Vaccination
Because of their important role in transmission of HAV, children should be considered the primary target of immunization strategies to lower disease incidence and the most effective means of achieving control of hepatitis A is likely to be routine hepatitis A vaccination of infants or young children. This strategy has a number of potential benefits: 1) an established vaccine delivery system is in place, 2) vaccination would occur before the greatest risk for clinical disease, and 3) the potential exists to interrupt transmission among children, which could result in a significant impact on the disease burden in other age groups. However, a number of issues need to be resolved before routine childhood vaccination can be recommended: 1) determining the appropriate dose and timing of vaccination in the first or second year of life, and determining the importance of the effect of maternal antibody on immunogenicity, 2) developing combination vaccines that incorporate hepatitis A vaccine, 3) determining the duration of protection following vaccination in children, and 4) determining the cost-effectiveness of such a vaccination strategy.
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ACIP Recommendations - Preexposure Hepatitis A Vaccination
Until routine vaccination of children against hepatitis A is feasible, the Immunization
Practices Advisory Committee (ACIP) has recommended hepatitis A vaccine for the following persons who are at increased risk for infection:
Persons who plan to travel repeatedly or reside for long periods in countries with high or intermediate endemicity of HAV infection.
These persons should be vaccinated before departure. Persons can be assumed to be protected by 4 weeks after receiving the initial vaccine dose, although the second dose
6 to 12 months later is necessary for long-term protection. Individuals who will travel to high-risk areas <4 weeks after the initial dose of vaccine should also be given IG, but at different injection sites. Travelers to high-risk areas who are allergic to a vaccine component or otherwise elect not to receive vaccine should receive IG.
Sexually active homosexual and bisexual men.
These persons should be vaccinated.
Injecting drug users or persons who use non-injection street drugs.
These persons should be vaccinated if local epidemiologic and surveillance data indicate that persons with such risk behaviors are at increased risk for HAV infection.
Persons with chronic liver disease.
Susceptible persons with chronic liver disease, including persons awaiting or having received liver transplants, may be at increased risk for complications of hepatitis A and should be vaccinated.
In addition, the ACIP has recommended that routine childhood vaccination be used as the primary vaccination strategy in communities with high endemic rates of HAV infection and periodic hepatitis A outbreaks (e.g., Alaska Natives and American
Indians). In these communities, children should be routinely vaccinated at or after 2 years of age. In addition, previously unvaccinated older children should be vaccinated to prevent epidemics of hepatitis A. Vaccination of school children prior to school entry should receive highest priority, followed by vaccination of older children. The upper age for catch-up vaccination should be determined using age-specific seroprevalence data for HAV infection, if available. Vaccination is not warranted in age groups with the lowest rates of disease because the prevalence of immunity is high (e.g., adults).
Features of Community-wide Hepatitis A Outbreaks
Most hepatitis A in the United States occurs through person-to-person transmission during community-wide outbreaks. As a frame of reference for considering the use of hepatitis A vaccine, communities that experience hepatitis A outbreaks can be classified into two types: high rate and intermediate rate, based on epidemiologic characteristics, including age-specific rates of infection and temporal patterns of disease incidence.
High-rate communities typically have epidemics every 5-10 years that may last for several years, have very high rates of disease (as high as 2,000 per 100,000 per year), and have few cases among persons >15 years of age. Seroprevalence data show that many children (30%-40%) in these communities acquire infection before age 5 years
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6 and nearly all persons become infected before reaching young adulthood. These communities are relatively well defined or isolated, either geographically or ethnically, and include American Indian, Alaska Native, Pacific Islander, and certain religious communities.
Compared with epidemics in high-rate communities, those in intermediate-rate communities produce lower rates of disease. In these communities, hepatitis A rates of
50-200 per 100,000 per year can persist for several years; cases occur in children, adolescents, and young adults; and disease rates are highest in census tracts or neighborhoods with low socioeconomic levels. In some communities, injecting drug users or other adult risk groups may account for a relatively large percentage of cases.
Available surveillance data often indicate that these epidemics occur with regular periodicity; however, single prolonged outbreaks do occur. Intermediate-rate communities tend to be less geographically or ethnically defined than high-rate communities, but cases are often concentrated in specific census tracts or neighborhoods within a larger community.
ACIP Recommendations - Control of Community-wide Outbreaks: High-Rate
Communities
The primary vaccination strategies recommended by ACIP to prevent or control outbreaks in high-rate communities are 1) routine vaccination of young children, and 2) accelerated implementation of catch-up vaccination of older children. Several studies have examined the effectiveness of hepatitis A vaccine in controlling outbreaks in these communities. In a study conducted in Monroe County, New York, vaccination of children
2 to 16 years of age with VAQTA™ resulted in a substantial decrease in community hepatitis A rates. In addition, vaccination of susceptible persons <30 years of age with one dose of HAVRIX
ｮ
(720 EL.U.) in several Alaska villages experiencing hepatitis A outbreaks resulted in a rapid decrease in the number of cases.
ACIP Recommendations - Control of Community-wide Outbreaks: Intermediate-
Rate Communities
Hepatitis A vaccination of children and/or adolescents may have the potential to control outbreaks occurring in communities with intermediate rates of disease. However, these communities are often large cities or counties, and therefore widespread vaccination may not be possible. Targeting vaccination among subpopulations or groups with the highest rates of disease may be more feasible, but the effectiveness of using vaccine in these settings has not been determined.
Use of hepatitis A vaccine can be considered to control outbreaks that occur in intermediate-rate communities. To determine possible target groups for vaccination, local surveillance and epidemiologic data should be used to define populations (e.g., age groups, drug users) or areas within the community (e.g., census tracts) with the highest rates of disease. Occasionally, when day care centers play a role in sustaining a communitywide outbreak, consideration should be given to adding hepatitis A vaccine to the immunoprophylaxis regimen for children and staff in involved centers. In addition, persons who work as foodhandlers may develop hepatitis A during communitywide outbreaks, and the food industry may want to consider vaccination of these employees.
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In making a decision to attempt vaccination to control outbreaks in communities with intermediate rates of infection, several factors should be considered including: 1) the feasibility of rapidly vaccinating the target populations of children, adolescents, and young adults; 2) program cost; and 3) the ability to continue ongoing vaccination of young children to maintain high levels of immunity and prevent future outbreaks.
ACIP Recommendations - Prevaccination Serologic Testing
Several factors should be considered in deciding whether prevaccination testing may be indicated in a particular setting including: 1) the cost of vaccination compared with the cost of serologic testing (including the cost of an additional visit); 2) the expected prevalence of immune individuals; and 3) the likelihood that prevaccination testing will not interfere with completion of the vaccine series. For example, if the cost of screening
(including laboratory testing and office visits) is one-third the cost of the vaccine series, then screening of potential recipients from populations where the prevalence of infection is >33% should be cost-effective.
Prevaccination testing is generally not indicated before vaccination of children.
Populations for whom prevaccination screening will likely be most cost-effective include adults who were born or lived for extensive periods of time in geographic areas with a high endemicity of HAV infection; older adolescents and young adults in certain population groups (i.e., American Indians, Alaska Natives, Pacific Islanders); and adults in certain risk groups with high prevalence of infection (e.g., homosexual men, older adults). Among adults >40 years of age in the United States, the anti-HAV prevalence is generally >33% (regardless of race/ethnicity or income level). Therefore, prevaccination screening for any adult >40 years of age would be likely to be cost-effective (assuming the cost of screening is one-third the cost of the vaccination series). Commercially available tests for total anti-HAV should be used for prevaccination testing.
ACIP Recommendations - Postvaccination Testing
Postvaccination serologic testing is not indicated because of the very high rate of vaccine response among both children and adults. In addition, a commercially available test with the sensitivity to detect the lower anti-HAV concentrations associated with protection is not approved in the United States.
Recommended Doses and Schedules of Hepatitis A Vaccine
The dose and schedule of administration for HAVRIX
ｮ varies according to age. For adults (>18 years of age), 1440 ELISA units (EL.U.) per dose are given in a two-dose schedule 6-12 months apart. For children and adolescents
(2 to 18 years of age), 360 EL.U. per dose are given in a three-dose schedule at 0, 1, and 6-12 months. The vaccine should be given by intramuscular injection into the deltoid muscle.
Hepatitis A Prevention - Immune Globulin
Immune globulin is 80%-90% effective in preventing clinical hepatitis A when administered before exposure or early in the incubation period. The primary indication for use of IG for preexposure prophylaxis is for travelers to high HAV endemic regions.
Postexposure prophylaxis with IG is effective if administered within 14 days of
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8 exposure. The primary routine indication for postexposure prophylaxis is for household or other intimate contacts of persons with hepatitis A. In addition, postexposure prophylaxis may be indicated when hepatitis A cases occur in some institutional settings
(e.g., child day care centers), and for some common-source exposures (e.g., eating food prepared by an infected food handler). Local and/or state health departments should be consulted regarding the use of IG for postexposure prophylaxis in these settings.
Hepatitis B Virus
HBV is a 42nm, doubleshelled deoxyribonucleic acid (DNA) virus of the class
Hepadnaviridae . The outer surface membrane contains hepatitis B surface antigen
(HBsAg), which also circulates in blood as 22-nm spherical and tubular particles. HBsAg is the primary component of hepatitis B vaccine; this antigen induces a protective, neutralizing antibody that provides long-term protection against HBV infection. The inner core of the virus contains hepatitis B core antigen (HBcAg), hepatitis B e antigen
(HBeAg), a single molecule of partially doublestranded DNA, and DNAdependent DNA polymerase.
Hepatitis B - Clinical Features
The incubation period for hepatitis B ranges from 45-180 days (mode, 60-90 days), and the onset of acute disease is generally insidious. Clinical illness associated with acute infection is age-dependent with jaundice occurring in< 10% of children less than 5 years of age and in 30%-50% of older children and adults. The case-fatality rate for reported acute cases in the United States is approximately 0.5%-1%. Most acute HBV infections in adults result in complete recovery with clearance of HBsAg from the blood and the production of anti-HBs designating immunity from future infection. However, approximately 30%-90% of young children and 2%-10% of adults who are infected with
HBV develop chronic infection, and most of the serious sequelae associated with HBV occur in these persons. Persons with chronic HBV infection are often asymptomatic, but these persons are at high risk for developing chronic hepatitis, and approximately 15%-
25% may die prematurely from either cirrhosis or liver cancer.
Acute Hepatitis B Virus Infection with Recovery: Typical Serologic Course
Serologic markers of HBV infection vary depending on whether the infection is acute or chronic.
The first serologic marker to appear following acute infection is HBsAg, which can be detected as early as 1 or 2 weeks and as late as 11 or 12 weeks (mode, 30-60 days) after exposure to HBV. In persons who recover, HBsAg is no longer detectable in serum after an average period of about 3 months. HBeAg is generally detectable in patients with acute infection; the presence of HBeAg in serum correlates with higher titers of
HBV and greater infectivity. A diagnosis of acute HBV infection can be made on the basis of the detection of IgM class antibody to hepatitis B core antigen (IgM anti-HBc) in serum; IgM anti-HBc is generally detectable at the time of clinical onset and declines to subdetectable levels within 6 months. IgG anti-HBc persists indefinitely as a marker of past infection. Anti-HBs becomes detectable during convalescence after the disappearance of HBsAg in patients who do not progress to chronic infection. The
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9 presence of anti-HBs following acute infection generally indicates recovery and immunity from reinfection.
Progression to Chronic Hepatitis B Virus Infection: Typical Serologic Course
In patients with chronic HBV infection, both HBsAg and IgG anti-HBc remain persistently detectable, generally for life. HBeAg is variably present in these patients.
The presence of HBsAg for 6 months or more is generally indicative of chronic infection.
In addition, a negative test for IgM anti-HBc together with a positive test for HBsAg in a single serum specimen usually indicates that an individual has chronic HBV infection.
Rate of Reported Hepatitis B by Age Group, United States, 1990
In the United States, hepatitis B is largely a disease of young adults. The rate of reported cases is highest for persons 20-39 years of age. Rates among children <15 years of age are relatively low; however, the rate of infection in this age group is higher than reflected from reported cases because infection in children is often asymptomatic and therefore not detected through disease surveillance.
Age at Acquisition of Acute and Chronic HBV Infection, United States, 1989
Estimates
Although most acute HBV infections in the United States occur among adults, about
24% of chronic HBV infections are acquired perinatally and about 12% are acquired during early childhood .
Outcome of HBV infections by Age at Infection
The outcome of acute HBV infection varies substantially depending on the age at which infection occurs. In children less than 5 years of age, <5% of acute HBV infections are symptomatic; however, chronic infection occurs in about 80%-90% of infants infected during the first year of life and in about 30%-50% of children infected between 1-4 years of age. In comparison, 30%-50% of adults with acute HBV infection are symptomatic, but only 2%-10% develop chronic infection.
Global Patterns of Chronic HBV Infection, Geographic Distribution of Chronic
HBV infection*
Approximately 45% of the global population live in areas with a high prevalence of chronic HBV infection (> 8% of the population is HBsAg-positive); 43% in areas with a moderate prevalence (2%-7% of the population is HBsAg-positive); and 12% in areas with a low prevalence (< 2% of the population is HBsAg-positive). In high prevalence areas, the lifetime risk of HBV infection is >60%, and most infections are acquired at birth or during early childhood when the risk of developing chronic infection is greatest.
In these areas, because most infections in children are asymptomatic, very little acute disease related to HBV occurs, but rates of chronic liver disease and liver cancer in adults are very high. In moderate prevalence areas, the lifetime risk of being infected is
20%-60% and infections occur in all age groups. Acute disease related to HBV is common in these areas because many infections occur in adolescents and adults; however, the high rates of chronic infection are maintained mostly by infections occurring in infants and children. In low prevalence areas, the lifetime risk of infection is
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<20%. Most HBV infections in these areas occur in adults in relatively well defined risk groups.
Concentration of Hepatitis B Virus in Various Body Fluids
HBV is transmitted by percutaneous or permucosal exposure to infectious blood or body fluids from persons who have either acute or chronic HBV infection. The highest concentrations of virus are in blood and serous fluids; lower concentrations are found in semen, vaginal fluid, and saliva. Therefore, blood exposure and sex contact are relatively efficient modes of transmission. Saliva can be a vehicle of transmission through bites; however, transmission has not been documented to occur as a result of other types of exposure to saliva, including kissing. HBsAg has also been detected in low concentrations in other body fluids, including tears, sweat, urine, feces, breast milk, cerebrospinal fluid, and synovial fluid; however, these fluids have not been associated with transmission.
Hepatitis B Virus: Modes of Transmission
In the United States, the most important route of HBV transmission is by sex contact, either heterosexual or homosexual, with an infected person. Direct parenteral inoculation of HBV by needles during injecting drug use is also an important mode of transmission. Transmission of HBV may also occur by other percutaneous exposures, including tattooing, ear piercing, and acupuncture, and by needlesticks or other injuries from sharp instruments sustained by medical personnel; however, these exposures account for only a small proportion of reported cases in the United States. In addition, transmission can occur perinatally from a chronically infected mother to her infant, most commonly by contact of maternal blood to the infants mucous membranes at the time of delivery.
Risk Factors for Acute Hepatitis B, United States, 1992-1993
Of persons with acute hepatitis B reported in the CDC Sentinel Counties Study of Acute
Viral Hepatitis in 1992-1993, 50% had a sexual risk factor (>1 sex partner during the previous 6 months, sex contact with a person with hepatitis, or homosexual activity),
15% reported injecting drug use, and 4% had other risk factors (e.g., household contact with a hepatitis patient, health care employment). No risk factor could be identified for
31% of reported cases.
Elimination of Hepatitis B Virus Transmission, United States: Goals and
Objectives
The current hepatitis B vaccination strategy in the United States has an overall goal of eliminating HBV transmission. Because most of the serious consequences related to
HBV occur among persons with chronic HBV infection, the primary objectives of this strategy are to prevent chronic HBV infection and its consequences, cirrhosis and liver cancer. A secondary objective is to prevent symptomatic illness associated with acute
HBV infection.
Elimination of Hepatitis B Virus Transmission, United States: Strategy
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The hepatitis B elimination strategy includes the following components: 1) preventing perinatal HBV transmission by screening all pregnant women for hepatitis B surface antigen (HBsAg) and providing hepatitis B immune globulin (HBIG) and hepatitis B vaccine to infants of HBsAg-positive mothers; 2) providing hepatitis B vaccine to all infants as part of the routine childhood vaccination schedule; 3) making special efforts to provide catch-up vaccination for children in high risk groups, including Alaska
Natives, Pacific Islanders and infants of immigrants from countries with a high prevalence of chronic HBV infection; 4) providing hepatitis B vaccine to adolescents, including a) all previously unvaccinated children at 11-12 years of age and b) adolescents of all ages who are at high risk for infection (see adult high-risk groups below); and 5) vaccinating adults in high-risk groups including a) men and women with a history of other sexually transmitted diseases and persons who have a history of multiple sex partners (>1 partner/6 months), b) household contacts and sex partners of persons with chronic HBV infection and health care and public safety workers who have exposure to blood in the workplace, d) clients and staff of institutions for the developmentally disabled, e) international travelers who plan to spend >6 months in countries with high rates of HBV infection and who will have close contact with the local population, f) injecting drug users, g) sexually active homosexual and bisexual men, and h) recipients of clotting-factor concentrates.
Estimated Incidence of Acute Hepatitis B, United States, 1978-1993
Since hepatitis B vaccine was licensed for use in the United States in 1981, the incidence of acute hepatitis B has changed dramatically. During the 1980s, use of hepatitis B vaccine had little impact on disease incidence, which actually increased during the early 1980s. The incidence declined between 1985 and 1988, and between
1988 and 1991, when fewer cases were reported among homosexual men and among injecting drug users, respectively. In both of these groups the decline in incidence was likely related to behavioral changes in response to the epidemic of acquired immunodeficiency syndrome (AIDS) rather than to vaccine use. The incidence has continued to decline during the first half of the 1990s; a likely contributing factor for this most recent decline is the widespread use of hepatitis B vaccine in response to regulations issued by the Occupational Safety and Health Administration. Future declines in incidence are expected to occur as a result of the implementation of routine infant and routine adolescent vaccination, which have been recommended in the 1990s.
Hepatitis C Virus
HCV is an enveloped, single-stranded RNA virus, approximately 50 nm in diameter, that has been classified as a separate genus in the Flaviviridae family. Before HCV was visualized, the virus genome was cloned and sequenced. The 5' end of the genome codes for core and envelope proteins, followed by nonstructural proteins, which extend to the 3' end. The ability of HCV to undergo rapid mutation in a hypervariable region(s) of the genome coding for envelope protein allows it to escape immune surveillance by the host; thus, most persons infected with HCV develop chronic infection.
Hepatitis C: Clinical Features
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The average incubation period for hepatitis C is generally 6-7 weeks with a range of 2 weeks to 26 weeks. About 30% to 40% of persons with acute infection develop symptomatic illness and 20% to 30% have jaundice. Chronic liver disease develops in about 70% of persons who become infected with HCV and nearly all (85%-100%) persons with acute HCV infection become persistently infected; these persons are at risk for developing cirrhosis and liver cancer. No protective antibody response has been identified following HCV infection.
Primary Etiology of Chronic Liver Disease: Mutually Exclusive Groups, Jefferson
County, Alabama, 1989
The findings of a study conducted in Jefferson County, Alabama, suggest that viral hepatitis may be responsible for at least 50% of all chronic liver disease in the United
States and HCV may be as important as alcohol as a cause of chronic liver disease. In this study, 40% of identified patients with chronic liver disease had evidence of HCV infection, 41% had a history of excessive alcohol intake, 14% had chronic HBV infection, and 5% had other causes of chronic liver disease; for 17% no etiology could be determined. If these proportions are applied to the estimated 32,000 deaths each year in the United States from chronic liver disease, 8-10,000 deaths may be related to chronic HCV infection each year about twice the number that are attributable to chronic
HBV infection.
Hepatitis C Virus Infection: Typical Serologic Course
HCV RNA is the earliest marker of HCV infection and is detectable by polymerase chain reaction (PCR) testing as early as 2 weeks after exposure. However, because PCR testing for HCV RNA is not licensed for use in the United States, diagnosis of HCV infection is generally made on the basis of detection of antibody to HCV (anti-HCV).
Anti-HCV is detectable by 5 to 6 weeks after onset of hepatitis in 80% of patients and by
12 weeks in 90%. Because the appearance of anti-HCV may be delayed in patients with acute HCV infection, testing should be repeated if hepatitis C is suspected in a patient and initial testing is negative for anti-HCV. No tests are available to differentiate acute from chronic infection. Diagnosis of chronic hepatitis C in an anti-HCV positive individual is generally based on the presence of elevated liver enzymes for >6 months.
Risk Factors Associated with Transmission of HCV
Percutaneous exposures, including transfusion and transplantation from an infectious donor and injecting drug use, are the most efficient modes of HCV transmission. The overall prevalence of antiHCV among persons with these exposures generally exceeds
60%. Other types of percutaneous exposures, including hemodialysis and needlestick injuries, have also been associated with HCV transmission. The risk of HCV transmission following a needlestick exposure to an antiHCV-positive patient is approximately 5% to 10%. Other risk factors that have been associated with HCV transmission include sexual or household exposure to an antiHCV-positive contact, having multiple sex partners, and being an infant of an HCVinfected mother. However, the magnitude of the risk associated with these exposures has not been well defined.
Estimated Incidence of Acute Hepatitis C, United States, 1982-1993
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Based on data from the CDC Sentinel Counties Study of Acute Viral Hepatitis, the incidence of acute hepatitis C remained relatively stable through much of the 1980s at
12-18 cases per 100,000 persons per year. However, between 1989 and 1993, a decline in incidence of >75% occurred, from 18 cases per 100,000 to< 5 cases per
100,000. Much of the recent decline in incidence can be accounted for by a decline in cases among injecting drug users that may be related to safer needle using practices. A substantial decline has also occurred in the number of cases of transfusionassociated hepatitis C. The latter decline began in 1985 when changes in donor selection practices were instituted in response to the AIDS epidemic and continued through the early 1990s after screening of the blood supply for antiHCV was initiated. Trends in other risk factors, including sexual and household exposure and occupational exposures, have remained relatively stable over time.
Risk Factors for Acute Hepatitis C, United States, 1990-1993
In the CDC Sentinel Counties Study of Acute Viral Hepatitis, persons with percutaneous exposures, including blood transfusion (4%), injecting drug use (38%), occupational exposure to blood (2%), hemodialysis (1%), and sexual or household exposures (10%), accounted for 55% of cases of acute hepatitis C between 1990-1993. Persons with no specific source for their HCV infection also account for a substantial amount of disease, and low socioeconomic level is associated with a large proportion of these patients.
More than 50% of these persons report histories of some type of high-risk behavior or contact, including imprisonment, use of noninjection illegal drugs, one or more sexually transmitted diseases, contact with a sex partner or household member who used injection drugs, but not in the 6 months preceding illness. Low socioeconomic level and other high-risk attributes identified among these patients have been associated with transmission of a number of infectious diseases, and probably serve as surrogate indicators for routes of transmission; however, the nonspecific nature of these factors make disease prevention difficult.
Prevalence of HCV Infection among Blood Donors
*
Knowledge of the geographic distribution of HCV infection is based primarily on seroprevalence studies among blood donors because few population-based seroprevalence studies have been conducted worldwide. An extremely low anti-HCV prevalence (<0.1%) has been reported among blood donors in the United Kingdom and
Scandinavia; a slightly higher prevalence (0.2-1%) has been reported in other Western
European countries, Australia, and North America; an intermediate prevalence (1.1%-
5%) has been reported in South America, Eastern Europe, Mediterranean countries,
South Africa and Asia; and the highest prevalence (as high as 20% in Egypt) has been reported in the Middle East.
Prevention of Hepatitis C
There are currently no vaccine or immune globulin products available to prevent transmission of HCV and developing such products is proving to be difficult because no effective protective antibody response has been demonstrated following infection with
HCV. In the absence of immunoprophylaxis, the primary measures available to prevent
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14 hepatitis C are screening of blood, organ, and tissue donors; modification of high-risk behavior; and use of blood and body fluid precautions. Blood donors have been screened for antiHCV in the United States since May 1990. Using first-generation antiHCV tests, the risk of HCV infection per recipient is estimated to have declined from about 5% to 1% and using second generation tests, the risk is <0.1% per recipient.
High-risk behavior modification, such as safer needle using practices, seems to have reduced the number of hepatitis C cases among injection drug users. However, there is uncertainty regarding whether this downward trend will be maintained, and many persons who acquire hepatitis C cannot be readily identified as belonging to this or other high-risk groups.
Postexposure Prophylaxis for Hepatitis C: ACIP Revised Recommendation,
February 1994
In February 1994, the ACIP made the following recommendation regarding use of IG for postexposure prophylaxis for hepatitis C: Recent studies indicate that IG does not protect against infection with HCV. Thus, available data do not support the use of IG for prophylaxis of hepatitis C. There are no data on the efficacy of IG for post-exposure prophylaxis of other (non-HCV) parenterally-transmitted, non-A, non-B hepatitis.
Postexposure Prophylaxis for Hepatitis C: Issues and Considerations for
Recommendations
Data considered by ACIP in making this recommendation include the following:
1.No protective antibody response has been identified following HCV infection. The extraordinarily high rate of persistent infection observed in humans and the lack of protection against rechallenge with homologous and heterologous HCV strains demonstrated in experimental studies in chimpanzees suggest that HCV fails to induce an effective neutralizing antibody response.
2.Prior studies of IG use to prevent posttransfusion non-A, non-B hepatitis may not be relevant in making recommendations regarding postexposure prophylaxis for hepatitis
C. In some of these studies, IG seemed to reduce the rate of clinical disease (although not overall infection rates); and in one, patients were less likely to develop chronic hepatitis. However, the results of these studies are difficult to compare and interpret because 1) the first dose of IG was given after exposure in only one of these studies, making it difficult to assess the value of IG for postexposure prophylaxis; 2) uniform diagnostic criteria were not used; 3) mixed sources of donors (volunteer and commercial) were used; 4) different study designs (some lacking blinding and placebo controls) were used; and 5) none of these data have been reanalyzed since anti-HCV testing became available.
3.Experimental studies in chimpanzees have demonstrated that neither standard IG nor
IG with a high titer of anti-HCV prevents infection when used for postexposure prophylaxis.
Public Health Service Recommendations for Counseling Anti-HCV-Positive
Persons
Anti-HCV-positive persons should:
1.Be considered potentially infectious.
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2.Keep cuts or skin lesions covered to prevent the spread of infectious secretions or blood.
3.Be informed of the potential for sexual transmission of HCV. HCV transmission by sex contact appears to occur, but this route of transmission appears to be much less efficient than for other bloodborne sexually transmitted diseases (e.g., HBV, HIV). For persons with a steady sex partner, data are insufficient to recommend changes in sex practices. However, to prevent many sexually transmitted diseases, including hepatitis and HIV infection, persons with multiple sex partners should follow safer sex practices, including reducing the number of sex partners and using barriers (e.g., latex condoms) to prevent contact with body fluids.
4.Be informed of the potential for perinatal transmission of HCV. The risk of perinatal transmission appears to be low (<10%). At the present time, there is no evidence to support advising against pregnancy or breastfeeding based on anti-HCV status alone, or to advise any special treatments or precautions for pregnant women or their offspring.
Anti-HCV-positive persons should not:
1.Donate blood, body organs, other tissue, or semen.
2.Share personal articles such as toothbrushes and razors that could be contaminated with blood.
Hepatitis D (Delta) Virus
HDV is a defective single-stranded RNA virus that requires the helper function of HBV to replicate. HDV requires HBV for synthesis of envelope protein composed of HBsAg, which is used to encapsulate the HDV genome.
Delta Hepatitis: Clinical Features
HDV infection can be acquired either as a coinfection with HBV or as a superinfection of persons with chronic HBV infection. Persons with HBV-HDV coinfection may have more severe acute disease and a higher risk of fulminant hepatitis (2%-20%) compared with those infected with HBV alone; however, chronic HBV infection appears to occur less frequently in persons with HBV-HDV coinfection. Chronic HBV carriers who acquire
HDV superinfection usually develop chronic HDV infection. In long-term studies of chronic HBV carriers with HDV superinfection, 70%-80% have developed evidence of chronic liver diseases with cirrhosis compared with 15%-30% of patients with chronic
HBV infection alone.
Delta Hepatitis: Modes of Transmission
The modes of HDV transmission are similar to those for HBV, with percutaneous exposures the most efficient. Sexual transmission of HDV is less efficient than for HBV.
Perinatal HDV transmission is rare.
HBV-HDV Coinfection: Typical Serologic Course
The serologic course of HDV infection varies depending on whether the virus is acquired as a coinfection with HBV or as a superinfection of a person with chronic HBV infection. In most persons with HBV-HDV coinfection, both IgM antibody to HDV (anti-
HDV) and IgG anti-HDV are detectable during the course of infection. However, in about
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15% of patients the only evidence of HDV infection may be the detection of either IgM anti-HDV alone during the early acute period of illness or IgG anti-HDV alone during convalescence. Anti-HDV generally declines to subdetectable levels after the infection resolves and there is no serologic marker that persists to indicate that the patient was ever infected with HDV. Hepatitis Delta antigen (HDAg) can be detected in serum in only about 25% of patients with HBV-HDV coinfection. When HDAg is detectable it generally disappears as HBsAg disappears and most patients do not develop chronic infection. Tests for IgG anti-HDV are commercially available in the United States. Tests for IgM anti-HDV, HDAg and HDV RNA by PCR are only available in research laboratories.
HBV-HDV Superinfection: Typical Serologic Course
In patients with chronic HBV infection who are superinfected with HDV several characteristic serologic features generally occur, including: 1) the titer of HBsAg declines at the time HDAg appears in the serum, 2) HDAg and HDV RNA remain detectable in the serum because chronic HDV infection generally occurs in most patients with HDV superinfection, unlike the case with coinfection, 3) high titers of both
IgM and IgG anti-HDV are detectable, which persist indefinitely.
Geographic Distribution of HDV Infection
*
In general, the global pattern of HDV infection corresponds to the prevalence of chronic
HBV infection; however, several distinct features of the distribution of HDV infection have been identified. In countries with a low prevalence of chronic HBV infection, HDV prevalence is generally low among both asymptomatic HBV carriers (<10%) and among patients with chronic HBV-related liver disease (<25%). HDV infection in these countries occurs most commonly among injecting drug users and persons with hemophilia. In countries with moderate and high levels of chronic HBV prevalence, the prevalence of
HDV infection is highly variable. In southern Italy and in parts of Russia and Romania, the prevalence of HDV infection is very high among both asymptomatic HBV carriers
(>20%) and among patients with HBV-related chronic liver disease HBV (>60%). Other countries, including northern Italy, Spain, Turkey, and Egypt, have a moderate prevalence of HDV infection among asymptomatic HBV carriers (10%-19%) and among patients with chronic HBV-related liver disease (30%-50%). However, in most of
Southeast Asia and China, where the prevalence of chronic HBV infection is very high,
HDV infection is uncommon. In some South American countries in the Amazon River
Basin, periodic epidemics of HDV infection have occurred among chronic HBV carriers in relatively isolated regions. Disease related to HDV infection in these outbreaks has been very severe, with rapid progression to fulminant hepatitis and case-fatality rates of
10%-20%. The cause of the atypical course of HDV infection in these populations is unknown.
Because HDV is dependent on HBV for replication, HBV-HDV coinfection can be prevented with either pre- or postexposure prophylaxis for HBV. However, no products exist to prevent HDV superinfection of persons with chronic HBV infection. Thus, prevention of HDV superinfection depends primarily on education to reduce risk behaviors.
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Hepatitis E Virus
Hepatitis E virus (HEV), the major etiologic agent of enterically transmitted non-A, non-B hepatitis worldwide, is a spherical, nonenveloped, single stranded RNA virus that is approximately 32 to 34 nm in diameter. Based on similar physicochemical and biologic properties, HEV has been provisionally classified in the Caliciviridae family; however, the organization of the HEV genome is substantially different from that of other caliciviruses and HEV may eventually be classified in a separate family.
Hepatitis E: Clinical Features
The incubation period following exposure to HEV ranges from 15 to 60 days (mean, 40 days). Typical clinical signs and symptoms of acute hepatitis E are similar to those of other types of viral hepatitis and include abdominal pain anorexia, dark urine, fever, hepatomegaly, jaundice, malaise, nausea, and vomiting. Other less common symptoms include arthralgia, diarrhea, pruritus, and urticarial rash. The period of infectivity following acute infection has not been determined but virus excretion in stools has been demonstrated up to 14 days after illness onset. In most hepatitis E outbreaks, the highest rates of clinically evident disease have been in young to middle-age adults; lower disease rates in younger age groups may be the result of anicteric and/or subclinical HEV infection. No evidence of chronic infection has been detected in longterm follow-up of patients with hepatitis E.
Hepatitis E Virus Infection: Typical Serologic Course
The typical serologic course following HEV infection has been characterized using experimental models of infection in nonhuman primates and human volunteer studies.
In two human volunteer studies, liver enzyme elevations occurred 4-5 weeks after oral ingestion and persisted for 20-90 days. Virus excretion in stools occurred approximately
4 weeks after oral ingestion and persisted for about 2 weeks. Both IgM and IgG antibody to HEV (anti-HEV) are elicited following HEV infection. The titer of IgM anti-
HEV declines rapidly during early convalescence; IgG anti-HEV persists and appears to provide at least short-term protection against disease. No serologic tests to diagnose
HEV infection are commercially available in the United States. However, several diagnostic tests are available in research laboratories, including enzyme immunoassays and Western blot assays to detect IgM and IgG anti-HEV in serum, polymerase chain reaction tests to detect HEV RNA in serum and stool, and immunofluorescent antibody blocking assays to detect antibody to HEV antigen in serum and liver.
Hepatitis E: Epidemiologic Features
HEV is transmitted primarily by the fecal-oral route and fecally contaminated drinking water is the most commonly documented vehicle of transmission. Although hepatitis E is most commonly recognized to occur in large outbreaks, HEV infection accounts for
>50% of acute sporadic hepatitis in both children and adults in some high endemic areas. Risk factors for infection among persons with sporadic cases of hepatitis E have not been defined. Unlike hepatitis A virus, which is also transmitted by the fecal-oral route, person-to-person transmission of HEV appears to be uncommon. However, nosocomial transmission, presumably by person-to-person contact, has been reported to occur. Virtually all cases of acute hepatitis E in the United States have been reported among travelers returning from high HEV-endemic areas.
Geographic Distribution of Hepatitis E
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Outbreaks of hepatitis E have occurred over a wide geographic area, primarily in developing countries with inadequate environmental sanitation. The reservoir of HEV in these areas is unknown. The occurrence of sporadic HEV infections in humans may maintain transmission during interepidemic periods, but a nonhuman reservoir for HEV is also possible. In the United States and other nonendemic areas, where outbreaks of hepatitis E have not been documented to occur, a low prevalence of anti-HEV (<2%) has been found in healthy populations. The source of infection for these persons is unknown.
Prevention and Control Measures for Travelers to HEV-Endemic Regions
Prevention of hepatitis E relies primarily on the provision of clean water supplies.
Prudent hygienic practices that may prevent hepatitis E and other enterically transmitted diseases among travelers to developing countries include avoiding drinking water (and beverages with ice) of unknown purity, uncooked shellfish, and uncooked fruits or vegetables that are not peeled or prepared by the traveler. No products are available to prevent hepatitis E. IG prepared from plasma collected in non-HEV-endemic areas is not effective in preventing clinical disease during hepatitis E outbreaks and the efficacy of IG prepared from plasma collected in HEV-endemic areas is unclear. In studies conducted to date with prototype vaccines in animals, vaccine-induced antibody attenuated HEV infection, but did not prevent virus excretion in stools. If a vaccine is developed, the epidemiology of hepatitis E needs to be further defined in order to determine whether vaccination strategies could be effectively used to prevent this disease.
Alter MJ, Mast EE. The epidemiology of viral hepatitis in the United States.
Gastroenterol Clin North Am 1994;23:437-455.
Mast EE, Alter MJ. Epidemiology of viral hepatitis: an overview. Seminars in Virology
1992;4:273-283.
Shapiro CN, Coleman PJ, McQuillan GM, Alter MJ, Margolis HS. Epidemiology of hepatitis A: seroepidemiology and risk groups in the USA. Vaccine 1992 (Suppl
1);10:S59-S62.
Shapiro CN, Margolis HS. Worldwide epidemiology of hepatitis A virus infection. J
Hepatol 1993;18(Suppl 2):S11-S14.
Margolis HS, Shapiro CN. Considerations for the development of recommendations for the use of hepatitis A vaccine. J Hepatol 1993;18(Suppl 2):S56-S60.
Margolis HS, Alter MJ. Will hepatitis A become a vaccine-preventable disease? Ann
Intern Med 1995;122:464-465.
Centers for Disease Control. Hepatitis B virus: a comprehensive strategy for eliminating transmission in the United States through universal childhood vaccination: recommendations of the Immunization Practices Advisory Committee (ACIP). MMWR
1991;40(No. RR-13):1-25.
Hadler SC, Margolis HS. Hepatitis B immunization: vaccine types, efficacy, and indications for immunization. In: Current Topics in Infectious Diseases: 12, Remington and Swartz, eds. Blackwell Scientific Publications, Boston, pp. 282-308.
Margolis HS, Alter MJ, Hadler SC. Hepatitis B: evolving epidemiology and implications for control. Sem Liv Dis 1991;11:84-92.
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Moyer LA, Mast EE. Hepatitis B: Virology, epidemiology, disease and an overview of viral hepatitis. Am J Prev Med 1994 (Suppl);10:45-55.
Centers for Disease Control. Public Health Service inter-agency guidelines for screening donors of blood, plasma, organs, tissues and semen for evidence of hepatitis
B and hepatitis C. MMWR 1991;40 (RR-4):1-17
Alter MJ. The detection, transmission, and outcome of hepatitis C virus infection.
Infectious Agents and Disease 1993;2:155-166.
Alter MJ, Margolis HS, Krawczynski K, et al. The natural history of community-acquired hepatitis C in the United States. N Engl J Med 1992;327:1899-1905.
Alter MJ. Occupational exposure to hepatitis C virus: A dilemma. Infect Control Hosp
Epidemiol 1994;15:742-744.
Alter MJ. Epidemiology of hepatitis C in the West. Sem Liv Dis 1995;15:5-14.
Polish LB, Gallagher M, Fields HA, Hadler SC. Delta hepatitis: Molecular biology and clinical and epidemiological features. Clin Microbiol Rev 1993;6:211-229.
Fields HA, Hadler SC. Delta Hepatitis: A review. J Clin Immunoassay 1986:128-142.
Bradley DW. Hepatitis E: Epidemiology, aetiology and molecular biology. Reviews in
Medical Virology 1992;2:19-28.
Purdy MA, Krawczynski K. Hepatitis E. Gastroenterol Clin North Am 1994;23:537-546.
Fields HA, Favorov MO, Margolis HS. Hepatitis E virus: A review. J Clin Immunoassay
1993;16:215-23.
Mast EE, Krawczynski K. Hepatitis E: An overview. Ann Rev Med 1995; in press.
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1. During the past several decades, the incidence of hepatitis A in the United States has been _____. a.
Declining b.
Increasing c.
Cyclic d.
Level
2. The most frequently reported source of infection for reported hepatitis A cases is
_____, which is consistently reported by about 25% of persons with acute hepatitis A. a.
Personal contact with an infected person b.
Sexual contact with an infected person c.
Transmission through saliva d.
Shared needles
3. In general, HAV transmission patterns correlate with socioeconomic and hygienic conditions. In many developing countries where environmental sanitation generally is poor, nearly ___ children <9 years of age have evidence of prior HAV infection. a.
Half b.
One in three c.
Three in four d.
All
4. In the United States, hepatitis B is largely a disease of _____. a.
Young children b.
Adolescents c.
Young adults d.
The elderly
5. HBV is transmitted by percutaneous or permucosal exposure to infectious blood or body fluids from persons who have either acute or chronic HBV infection. The highest concentrations of virus are in ______. a.
Blood and saliva b.
Blood and serous fluids c.
Semen and vaginal fluids d.
Sweat and breast milk
6. In the United States, the most important route of HBV transmission is by ______. a.
Sex contact with an infected person b.
Casual contact with an infected person
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21 c.
Saliva of an infected person d.
Feces of an infected person
7. Chronic liver disease develops in about 70% of persons who become infected with ____ and nearly all (85%-100%) persons with acute ____ infection become persistently infected; these persons are at risk for developing cirrhosis and liver cancer. a.
HAV b.
HBV c.
HCV d.
All of the above
8. ________, including transfusion and transplantation from an infectious donor and injecting drug use, are the most efficient modes of HCV transmission. a.
Percutaneous exposure b.
Blood exposure c.
Mucous membrane exposure d.
Direct contact
9. There are currently no vaccine or immune globulin products available to prevent transmission of ___ and developing such products is proving to be difficult because no effective protective antibody response has been demonstrated following infection with ____. a.
HAV b.
HBV c.
HCV d.
None of the above
10. HDV is a defective single-stranded RNA virus that requires the helper function of
___ to replicate. a.
HAV b.
HBV c.
HCV d.
A and B
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