Several hepatitis viruses (A, B, C, D, and E) are known to cause a systemic infection resulting in major pathologic changes in the liver. Features of hepatitis A and hepatitis B are described below.

Hepatitis A: The hepatitis A virus (HAV) belongs to the picornavirus family.

Hepatitis A is a highly contagious disease with the predominant mode of transmission being person-to-person via the fecal-oral route. Infection has been shown to be spread (1) by contaminated water or food; (2) by infected food handlers¹; (3) after breakdown in usual sanitary conditions or after floods or natural disasters; (4) by ingestion of raw or undercooked shellfish (oysters, clams, mussels) from contaminated waters²; (5) during travel to areas of the world with poor hygienic conditions³; (6) among institutionalized children and adults⁴; (7) in day-care centers⁵; (8) by parenteral transmission, either blood transfusions or sharing needles with infected people⁶; and (9) sexually, especially among men who have sex with men.⁷

The incubation period for hepatitis A averages 28 days (range: 15 to 50 days).7 The course of hepatitis A infection is extremely variable, ranging from asymptomatic infection to icteric hepatitis and death.⁸

Chronic shedding of HAV in feces has not been demonstrated, but relapses of hepatitis A can occur in as many as 20% of patients^9,10 and fecal shedding of HAV may recur at this time.9 Approximately 70% of pediatric patients less than 6 years of age infected with hepatitis A are asymptomatic, and serve as a reservoir for infection among adults.7

The presence of antibodies to HAV (anti-HAV) confers protection against hepatitis A disease. However, the lowest titer needed to confer protection has not been determined. Natural infection provides lifelong immunity even when antibodies to hepatitis A are undetectable. At present, studies show the duration of protection afforded by TWINRIX against hepatitis A lasts at least 4 years.¹¹

Hepatitis B: The hepatitis B virus (HBV) belongs to a family of genetically related DNA-containing animal viruses, which are hepatotropic. The incubation period of hepatitis B ranges between 30 and 180 days.¹²

HBV infection occurs throughout the world with highly variable prevalences. A human reservoir of persistently infected persons is present in nearly all communities of the world. In the United States, parenteral drug abuse, unprotected sexual activity, occupationally acquired infection, or travelers returning from high prevalence countries may be the principal mechanisms of HBV transmission.

Modes of transmission of hepatitis B virus include sexual contact with an infected person, percutaneous or mucosal exposure to infectious blood, and perinatal exposure to an infected mother. Antibody concentrations ≥10 mIU/mL against HBsAg are recognized as conferring protection against hepatitis B.¹³

Clinical infection with hepatitis B may occur in 2 major forms: Asymptomatic or symptomatic hepatitis. Asymptomatic HBV infection can be subclinical or inapparent. In subclinical infection, patients have abnormal liver enzymes without jaundice, while inapparent asymptomatic infection is identified only by serological testing. One in 4 adults who has symptomatic disease has jaundice (anicteric/icteric hepatitis).

HBV infection can have serious consequences including acute massive hepatic necrosis, chronic active hepatitis, and cirrhosis of the liver. As many as 90% of infants and approximately 5%of adults who are infected with HBV will become HBV carriers.7 More than 350 million people are chronic carriers of HBV worldwide.7 The Centers for Disease Control and Prevention (CDC) estimates that there are approximately 1 million to 1.25 million chronic carriers of HBV in the United States.7 The annual number of unreported infections may be 10 times greater than the number of reported cases.7 Close contact (sexual contact or household contact) or exposure to blood from infected individuals is associated with increased risk of infection.7 Those patients who become chronic carriers can infect others and are at increased risk of developing primary hepatocellular carcinoma. Among other factors, infection with HBV may be the single most important factor for development of this carcinoma.^7,14

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