Chickenpox epidemiology and demographics

Epidemiology
Primary varicella is an endemic disease. Cases of varicella are seen throughout the year but, like other viruses spread by the respiratory route eg. measles and rubella, they are seen more commonly in the winter and early spring. This is unlike that for enteroviruses and lends some support to the view that varicella is spread mainly by the respiratory route. Herpes zoster, in contrast, occurs sporadically and evenly throughout the year. Varicella is one of the classic diseases of childhood, with the highest prevalence occurring in the 4 - 10 years old age group. Like rubella, infection is uncommon in preschool children. Varicella is highly communicable, with an attack rate of 90% in close contacts. Most people become infected before adulthood but 10% of young adults remain susceptible. However, this pattern of infection is not universal, eg. in rural India, varicella is predominantly a disease of adults, the mean age of infection being 23.4 years. It was suggested that this could be due to interference by other respiratory viruses that the children are exposed to at an early age.

Historically, varicella has been a disease predominantly affecting preschool and school-aged children. Although mostly noted in preschool and primary levels, the said disease has also been noticed among adults, with the pocks being darker and the scars more prominent than their younger counterparts.

Prevalence
Since the chickenpox vaccine was developed in 1995, the major data of chickenpox in the environment is before 1995. In the prevaccine era, varicella was endemic in the United States, and virtually all persons acquired varicella by adulthood. As a result, the number of cases occurring annually was estimated to approximate the birth cohort, or approximately 4 million per year. Varicella was removed from the list of nationally notifiable conditions in 1981, but some states continued to report cases to CDC. The majority of cases (approximately 85%) occurred among children younger than 15 years of age. The highest age-specific incidence of varicella was among children 1–4 years of age, who accounted for 39% of all cases. This age distribution was probably a result of earlier exposure to VZV in preschool and child care settings. Children 5–9 years of age accounted for 38% of cases. Adults 20 years of age and older accounted for only 7% of cases (National Health Interview Survey data, 1990–1994).

Data from three active varicella surveillance areas indicate that the incidence of varicella, as well as varicella-related hospitalizations, has decreased significantly since licensure of vaccine in 1995. In 2004, varicella vaccination coverage among children 19–35 months in two of the active surveillance areas was estimated to be 89% and 90%. Compared with 1995, varicella cases declined 83%–93% by 2004. Cases declined most among children aged 1–4 and 5–9 years, but a decline occurred in all age groups including infants and adults, indicating reduced transmission of the virus in 308 Varicella 21 these groups. The reduction of varicella cases is the result of the increasing use of varicella vaccine. Varicella vaccine coverage among 19–35-month-old children was estimated by the National Immunization Survey to be 90% in 2007.

Despite high one-dose vaccination coverage and success of the vaccination program in reducing varicella morbidity and mortality, varicella surveillance indicates that the number of reported varicella cases appears to have plateaued. An increasing proportion of cases represent breakthrough infection (chickenpox occurring in a previously vaccinated person). In 2001–2005, outbreaks were reported in schools with high varicella vaccination coverage (96%–100%). These outbreaks had many similarities: all occurred in elementary schools; vaccine effectiveness was within the expected range (72%–85%); the highest attack rates occurred among the younger students; each outbreak lasted about 2 months; and persons with breakthrough infection transmitted the virus although the breakthrough disease was mild. Overall attack rates among vaccinated children were 11%–17%, with attack rates in some classrooms as high as 40%. These data indicate that even in settings where almost everyone was vaccinated and vaccine performed as expected, varicella outbreaks could not be prevented with the current one-dose vaccination policy. These observations led to the recommendation in 2006 for a second routine dose of varicella vaccine.