Theoretical Epidemiologic and Morbidity Effects of Routine Varicella Immunization of Preschool Children in the United States

Abstract

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.

Epidemiology of varicella zoster virus infection in Canada and the United Kingdom

“…The shape of the force of infection estimated for Canada and the United Kingdom (increasing in the pre-school years, peaking during primary school and declining thereafter) is consistent with other countries and with that of other childhood infections such as measles, mumps and rubella [40,41]. This suggests that preschool and school-age mixing is an important determinant of varicella transmission as with other childhood infections such as measles and rubella.…”

Burden of adult varicella hospitalizations in Spain (2001-2007)

“…6 The implementation of a childhood universal vaccination program has always been questioned due to a possible shift in the age distribution of varicella cases that would lead to increases in varicella incidence among adults and in overall morbidity. 7,8 In Spain, publicly funded varicella vaccination programmes were first introduced for the high-risk population in 1997; in 2005, the vaccine was included in the vaccination schedule for adolescents (age 11-14 y) who had not yet suffered from the illness. Currently, 2 of the 17 Autonomous Regions and the two Autonomous Cities (Ceuta and Melilla) of Spain have implemented varicella vaccination programmes for ages when most varicella cases can be prevented (age 12-15 mo).…”

Impact of methodology on the results of economic evaluations of varicella vaccination programs: is it important for decision-making?

“…Most of the dynamic models 29,36,42,45,51,52 were based on the initial dynamic model developed by Halloran et al 54 , and the others 30,40,43,48 were based on similar approaches. All the dynamic models included the indirect effects of vaccination on non-vaccinated individuals (herd immunity) in the analysis.…”

“…Four studies used the rate reported by Halloran et al 54 , where 15% of the Table 2 Characteristics of indirect costs included in studies from society's perspective. successfully vaccinated individuals had lost their immunity by the end of life.…”

“…All evaluations that included indirect costs in their calculation favored the vaccination program, and the higher the value attributed to in- Table 5 Impact of methodology and parameters on the results of economic analyses for varicella. 54 to evaluate the cost-effectiveness of their vaccination programs.…”