rologic complications including cerebellar ataxia, encephalitis, Guillain-Barré syndrome (GBS), meningitis, and transverse myelitis (Ey et al., 1981; Fleisher et al., 1981; Guess et al., 1986; Jackson et al., 1992; Liu and Urion, 1992; Preblud, 1986). Immunocompromised individuals such as those treated for cancer or with congenital defects in cellular immunity often experience more severe varicella infection and are at greater risk of fatal infection (Whitley, 2010).

Following the acute phase of the infection, the primary VZV infection is resolved, and the virus begins a dormant phase in the sensory nerve ganglia of the individual. The individual usually has lifetime immunity against reinfection, and will not again have an illness that resembles primary chickenpox; however, the latent VZV may be reactivated and cause shingles (also called herpes zoster [HZ]). Shingles (or HZ) is a painful, unilateral, pruritic rash appearing on dermatomal areas of one or more sensory-nerve roots (Arvin, 1996). Risk factors for shingles include aging, immunosuppression, and VZV infection prior to 12 months of age (Arvin, 1996). An estimated 15 to 30 percent of the population develops shingles, a percentage that is expected to increase with increasing life expectancies (CDC, 2007). Postherpetic neuralgia (PHN) is the most common complication of herpes zoster, especially in older individuals (CDC, 2007). The pain of PHN can last from 4 weeks to 10 years, and in one study, it lasted more than 1 year in 22 percent of study participants (Arvin, 1996; Ragozzino et al., 1982). Additional complications of herpes zoster include herpes ophthalmicus, dissemination, and central nervous system, pulmonary, and hepatic disease (CDC, 2007).

Prior to the development and dissemination of the varicella vaccine in 1995, varicella was a common childhood disease in the United States. The Centers for Disease Control and Prevention (CDC) estimates that from 1980 through 1990, 4 million cases of varicella occurred annually with approximately 77 percent of cases in children 9 years old and younger, and more than 90 percent in children less than 15 years of age (CDC, 2007). Furthermore, national seroprevalence data from 1988–1994 showed that 95.5 percent of adults aged 20–29 years, 98.9 percent of adults aged 30–39 years, and 99.6 percent of adults aged 40 years and older were immune to varicella (Kilgore et al., 2003).

From 1988 through 1995, hospitalizations due to varicella ranged from 2.3 to 7.0 per 100,000 cases (CDC, 2007). Among those most often hospitalized were adults 20 years of age and older, and children 4 years and younger, respectively representing 31.9 and 44.4 percent of varicella-related hospitalizations (Galil et al., 2002). Despite adults being less likely to require hospitalization due to varicella infection, from 1990 to 1994 adults were 25 times more likely to experience fatal varicella infections than children between the ages of 1 and 4 years (Meyer et al., 2000). Secondary



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