10 - Epidemiology of varicella

Summary

Although varicella occurs worldwide, the epidemiology of the disease differs in temperate compared with tropical climates. Moreover, as vaccine use increases in countries such as the United States, in which the vaccine is recommended as part of the routine childhood immunization schedule (Committee of Infectious Diseases, 1995; CDC, 1996), the epidemiology of the disease will change. This chapter summarizes methodological issues that must be considered when comparing results of varicella studies; reviews varicella epidemiology in the pre-vaccine era for temperate and tropical regions; and outlines the likely changes in the epidemiology of disease following widespread use of vaccine.

Methodological issues

Reports of varicella incidence differ in the methods used to determine the number of cases; the method used impacts completeness of ascertainment and, therefore, estimates of age-specific incidence and age distribution of cases. Because varicella is easily diagnosed by the lay public, information on disease incidence can be collected from household-based surveys (Guess et al., 1986; Finger et al., 1994; Yawn et al., 1997). Data collected from such surveys with a defined recall period, optimally one year or less, are more complete than data obtained from medical record review or passive reporting (surveillance) systems. In surveillance systems, completeness of ascertainment may vary by age, reflecting age- related differences, such as the proportion of patients seeking health care. Variations in completeness of reporting by age have been documented by comparing varicella cases reported via a reportable communicable disease system with cases detected by a household survey over the same time period (Sydenstricker & Hedrick, 1929). Reporting was more complete for adults (50%) than for school-aged children (25%) and was lowest for preschool children (12%).