Varicella. Active immunization

In individuals who developed natural varicella after vaccination, the clinical course was much less severe than seen in unimmunized controls. Significant reductions in the number of skin lesions and extent of fever and an increased rate of recovery were seen in the vaccinated group. The investigators commented that disease frequently was so mild that it was not recognizable as varicella, resembling instead insect bites. The rate of natural varicella disease after vaccination averaged 1% to 3% per year. This is in contrast to an annual rate of 7% to 8% in unvaccinated children and has not changed with time after vaccination. In adults, complete protection was demonstrated after a household exposure to varicella in 70% of vaccinees. Those who acquired disease after exposure had a greatly attenuated clinical course, with fewer than 100 lesions and few systemic signs. According to the CDC, “Based on all available data, the Advisory Committee for Immunization Practices (ACIP) concludes that the varicella virus vaccine provides 70% to 90% protection against infection and 95% protection against severe disease for 7 to 10 years after vaccination.”

Varicella vaccine has been well tolerated by persons of all ages in clinical trials. Within 1 month of vaccination, 7% of vaccinees develop a mild maculopapular or vesicular rash either around the injection site or systemically, with a median of two to five lesions. Rarely has vaccine virus been isolated from these lesions. Twenty percent of children and 30% of adults complain of injection site pain, tenderness, or redness. In children, fever was not seen in significantly more vaccinees than in placebo recipients, but low-grade fever was noted in 15% of adult vaccinees. No increase in the incidence of reactions was noted in vaccinees who were seropositive prior to vaccine administration.

The estimated incidence of later occurrence of zoster-like eruptions is 18 per 100,000 person-years. (Zoster after natural infection has an incidence of 77 per 100,000.) All cases were mild and without complications.

Spread of vaccine virus from the normal healthy vaccinee to susceptible contacts is theoretically possible but appears to be rare. To our knowledge, only one clinical case of disease has been reported after exposure to an immunized immunocompetent child. Seroconversion was noted in 3 of 446 vaccine contacts in one study. Children who had leukemia and received vaccine have been reported to spread vaccine virus to contacts, who developed mild illness or asymptomatic seroconversion.

The major concern about routine use of vaccine has been the potential loss of immunity over time, with the possibility of acquiring natural disease in adulthood when the natural history of disease is more severe. No evidence for waning immunity has been documented in follow-up studies of vaccinees for up to 10 years. However, natural virus is circulating and may boost antibody levels. Extensive serologic followup of vaccinated individuals is ongoing after vaccine licensure to ensure that if antibody levels wane with time, routine booster immunizations can be given.

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