The authors studied the effects of routine varicella immunization of US preschool children and of implementation of a catch-up program in older children on the age distribution of cases and on overall morbidity, with emphasis on the sensitivity of the results to level of vaccine coverage, duration of protection, responsiveness to boosting, relative residual susceptibility and infectiousness, and degree of morbidity among vaccine breakthrough cases. An age-structured theoretical transmission model was used, with values for vaccine efficacy based on a review of the literature by an expert panel. Although implementation of a vaccination program resulted in a shift in the age distribution of remaining varicella cases toward older ages with higher complication rates, the overall reduction in cases resulted in decreased morbidity as measured by overall number of hospitalizations and number of primary cases. Routine immunization with live-virus varicella vaccine would probably result in a substantial reduction in the number of uncomplicated primary cases of chickenpox, as well as a decreased number of complicated cases requiring hospitalization. The number and age distribution of vaccinated cases would depend strongly on the characteristics of the vaccine. Vaccine efficacy studies in the field should be designed to obtain better estimates of residual susceptibility, residual infectiousness, duration of protection, and effects of boosting by wild-type reinfection.

At the time of varicella vaccine introduction in the United States, an estimated 4 million episodes of varicella occurred annually. This survey of varicella seroprevalence is the first to describe immunity to a vaccine-preventable disease prior to vaccine introduction in the United States population. The objective of this analysis is to describe patterns of naturally-acquired varicella and understand characteristics associated with infection in the varicella vaccine-naive United States population. A nationally representative cross-sectional health examination survey that included venipuncture was conducted among 21,288 U.S. participants aged 6 years and older from 1988 through 1994. Serologic evidence of varicella-zoster virus infection was measured by enzyme immunoassay of varicella-zoster virus-specific IgG antibody. The seroprevalence of IgG antibody to varicella-zoster virus increased from 86.0% in children aged 6 through 11 years to 99.6% in adults aged 40 through 49 years. Immunity to varicella remained at 99% or higher in Americans aged 50 years and older. Among persons aged 6 through 19 years, non-Hispanic black children were 40% less likely to be seropositive compared with white children (odds ratio [OR], 0.6; 95% confidence interval [CI], 0.4-0.8). Among young adults aged 20 through 39 years, women with a history of live birth (OR, 4.3; 95% CI, 2.1-8.7) and married men (OR, 2.7; 95% CI, 1.2-5.7) were more likely to have naturally-acquired immunity to varicella. This study found that, prior to use of varicella vaccine in the United States, age, race, and marital characteristics were independently associated with naturally acquired varicella. Future varicella serosurveys in Americans will provide essential information to interpret the population impact of varicella vaccine.

From January to July 1991, an outbreak of mumps occurred in Maury County, Tennessee. At the primarily affected high school, where 98% of students and all but 1 student with mumps had been vaccinated before the outbreak, 68 mumps cases occurred among 1116 students (attack rate, 6.1%). Students vaccinated before 1988 (the first year mumps vaccination was required for school attendance in Tennessee) may have been at greater risk of mumps than those vaccinated later (65[6.1%] of 1001 vs. 2[2.2%] of 89; risk ratio, 2.9; 95% confidence interval, 0.7-11.6). Of 13 persons with confirmed mumps who underwent serologic testing, 3 lacked IgM antibody in well-timed acute- and convalescent-phase serum specimens. Vaccine failure accounted for a sustained mumps outbreak in a highly vaccinated population. Most mumps cases were attributable to primary vaccine failure. It is possible that waning vaccine-induced immunity also played a role.

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