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Varicella
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Varicella-zoster is a virus that causes chickenpox, a highly contagious disease that is usually mild, but can causes severe problems in infants, adults, and persons with impaired immune systems. The virus is passed from person to person through direct contact, or through the air. Chickenpox causes fever and an itchy rash that forms blisters, which dry in 4-5 days. Chickenpox is contagious 1-2 days before the rash appears and until all blisters have formed scabs. Adults are more likely to develop a more severe case of chickenpox with a higher rate of complications and death. The varicella virus remains dormant in the body after the appearance of chickenpox and may reappear decades later in the form of herpes zoster (shingles).
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Varicella is a highly contagious disease. After continuing household exposure, as would occur in a family, almost all susceptible persons are infected. The subclinical attack rate is believed to be no more than 4%. The results of nonhousehold exposure are less predictable. Chickenpox may be most contagious the day before the onset of rash. The period of contagiousness lasts for no more than 5 days after the appearance of the first lesion.
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Varicella vaccine contains live, attenuated VZV. It is available as a monovalent formulation and in combination formulation, as measles-mumps-rubella-varicella (MMRV) vaccine, which is licensed in the U.S for children 1–12 years only. In June 2005 and June 2006, the Advisory Committee on Immunization Practices (ACIP) approved changes to the 1996 and 1999 recommendations for use of varicella-containing vaccines and approved new criteria for evidence of immunity to varicella (4-6). Two doses of varicella-containing vaccine are now recommended for routine immunization of all children younger than 13 years of age who are without contraindications. The first dose should be administered at 12 – 15 months of age and the second dose at 4 – 6 years of age. A second dose of catch-up varicella vaccination is recommended for children, adolescents, and adults who previously had received one dose.
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Varicella (chickenpox) is mainly a childhood disease: 50% of children will have had the infection by 5 years of age and 90% by 12 years of age. Recurrences of varicella-like rash have been reported by 4% to 13% of individuals who had previous varicella infection. The risk factors identified for these recurrences were young age (< 12 months) at first infection and having a milder first infection(3). The lifetime risk of having at least one reactivation to herpes zoster (shingles) is 15% to 20%. People who grew up in the tropics are less likely to have acquired immunity to varicella during their childhood and have higher rates of susceptibility as adults after migrating to Canada(4,5).
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Varicella is more common than other childhood diseases during the early months of life. After the first 2 weeks of life, the disease is generally mild. Maternal antibody transferred across the placenta may not be as effective in protecting infants against this disease as are antibodies against other viruses. However, nursery outbreaks have been rare. Children who have varicella during the early months of life or are exposed in utero have a greater risk of herpes zoster in childhood.
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Varicella (chickenpox) was not reliably distinguished from smallpox until the end of the 19th century. Herpes zoster (shingles) has been recognized since ancient times and was described in the early medical literature. Clinical observations of the relationship between varicella and herpes zoster were made in 1888 by Von Bokay, when susceptible children acquired varicella after contact with herpes zoster. Varicella virus (VZV) was isolated from vesicular fluid of both chickenpox and zoster lesions in cell culture by Weller in 1954. Subsequent laboratory studies of the virus have led to the development of a live attenuated varicella vaccine, Oka strain, in Japan in the 1970s. The vaccine has been shown to be safe and effective in healthy and immunocompromised children, and healthy adults.
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Varicella