THEORETICAL EPIDEMIOLOGIC AND MORBIDITY EFFECTS OF ROUTINE VARICELLA IMMUNIZATION OF PRESCHOOL-CHILDREN IN THE UNITED-STATES

The authors studied the effects of routine varicella immunization of U S preschool children and of implementation of a catch-up program in ol der 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 a mong vaccine breakthrough cases. An age-structured theoretical transmi ssion model was used, with values for vaccine efficacy based on a revi ew of the literature by an expert panel. Although implementation of a vaccination program resulted in a shift in the age distribution of rem aining varicella cases toward older ages with higher complication rate s, the overall reduction in cases resulted in decreased morbidity as m easured by overall number of hospitalizations and number of primary ca ses. Routine immunization with live-virus varicella vaccine would prob ably result in a substantial reduction in the number of uncomplicated primary cases of chickenpox, as well as a decreased number of complica ted cases requiring hospitalization. The number and age distribution o f 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 infectiou sness, duration of protection, and effects of boosting by wildtype rei nfection.