Varicella-zoster virus (VZV) infection causes two clinically distinct forms of disease: varicella (chickenpox) and herpes zoster (shingles). Primary VZV infection results in the diffuse vesicular rash of varicella, or chickenpox.
The epidemiology of varicella has changed dramatically since the introduction of the varicella vaccine in 1995. In the United States, routine childhood immunization has reduced disease incidence, complications, hospital admissions, and deaths in children and in the general population, indicating strong herd immunity. Similar immunization programs have been adopted by several other countries, including Uruguay, Germany, Taiwan, Canada, and Australia .
This topic will address the morbidity and mortality of varicella infection prior to and after immunization. The clinical manifestations and management of varicella are discussed elsewhere. (See "Clinical features of varicella-zoster virus infection: Chickenpox" and "Treatment of varicella-zoster virus infection: Chickenpox" and "Vaccination for the prevention of varicella-zoster virus infection: Chickenpox" and "Prevention and control of varicella-zoster virus in hospitals" and "Varicella-zoster virus infection in pregnancy".)
EPIDEMIOLOGY OF VARICELLA PRIOR TO VACCINE
Incidence of varicella — Varicella occurs throughout the year in temperate regions, but the incidence typically peaks in the months of March through May . According to national seroprevalence data from the prevaccine era, greater than 95 percent of persons in the United States acquired varicella before 20 years of age, and fewer than 2 percent of adults were susceptible to infection [3-6]. Prior to 1995 the Centers for Disease Control and Prevention (CDC) estimated the yearly incidence of chickenpox in the United States at approximately four million cases, with nearly 11,000 admissions and 100 deaths .
A chart review of over 250,000 members of a health maintenance organization (HMO) was conducted from 1990 to 1992 to assess the epidemiology of varicella and its complications . The following results were noted: