PUBLIC HEALTH SIGNIFICANCE
Varicella (chickenpox) is a highly contagious infectious disease caused by the varicella-zoster virus (VZV). Varicella is the primary infection caused by VZV, which, like other herpes viruses maintains latency in the human body and can reactivate to result in the secondary or reactivated form of disease known as herpes zoster or shingles. In temperate climates without a routine vaccination program, varicella is a common, highly communicable, childhood illness characterized by fever and a generalized pruritic vesicular exanthem. In the United States, prior to the availability of a varicella vaccine, this disease affected essentially everyone during their lifetime, with more than 95% of adults demonstrating antibodies to VZV by age 20–29 years.1,2 In tropical climates, varicella may be acquired at older ages with more infections and a higher susceptibility in adults. Varicella may result in serious consequences both in healthy persons and those at higher risk for severe disease including newborn infants, immunocompromised persons, pregnant women, and adults.3–6 Severe complications of varicella include sepsis, pneumonia, encephalitis, coagulation defects, shock, and death.5–9 In the United States before the vaccine era, annually varicella was responsible for an average of 11,000–13,500 hospitalizations and 100–150 deaths.7,8,10–12 Substantial burden of school absenteeism, costs of parental leave, and medical costs were associated with childhood varicella with net benefit to cost estimates for a routine childhood vaccination program.13,14
A live, attenuated, varicella vaccine (VARIVAX) was licensed for use in the United States in 1995. A combination measles, mumps, rubella, and varicella vaccine (MMRV, ProQuad) was licensed in 2005. Recommendations for routine use of varicella vaccine among children at 12–18 months of age, older susceptible children, and priority adult groups including healthcare workers were established in 1996 and further expanded in 1999.15 Widespread use of varicella vaccine has resulted in substantial decline in varicella morbidity, mortality, and related healthcare expenditures. Despite the success of an initial one-dose childhood varicella vaccination program, one-dose vaccine effectiveness of approximately 85% has not been sufficient to prevent varicella outbreaks, which, although less common than in the prevaccine era, have continued to occur in highly vaccinated populations. In 2007, the varicella vaccination policy in the United States was changed to include a routine second dose of varicella vaccine for children.15 Additional declines in incidence and occurrence of outbreaks have occurred since the second dose implementation.16–18
ETIOLOGIC AGENT AND TRANSMISSION
The varicella-zoster virus is a DNA virus of the herpes family (Herpesviridae). Humans are the only natural host. Although the etiological agent responsible for varicella and herpes zoster was not identified and named until the 1950s, herpes zoster was described in the early medical literature. Varicella, however, was frequently confused with another “pox” illness, smallpox (variola), until the end of the nineteenth century. In the early 1900s, the association between varicella and herpes zoster was suggested when von Bokay reported on ...