Varicella Epidemiology Among Children Aged 1, 2, and 6 Years, American Samoa, 2008-2009

Background: Routine varicella vaccination has significantly reduced the incidence and severity of varicella in the United States.  Varicella epidemiology in tropical island locations, where routine varicella vaccination has been delayed for logistical reasons, is poorly understood.  The American Samoa Immunization Program is preparing to implement varicella vaccination, and will need baseline data to measure the impact locally.  Information from the first island-wide household immunization coverage survey conducted from 2008-2009 was used to describe varicella epidemiology in American Samoa.

Objectives: To describe the epidemiology of varicella disease in young children in American Samoa, prior to the introduction of varicella vaccine. 

Methods: We analyzed household interview data on children aged 1, 2, or 6 years.  Caregivers reported whether children had a history of varicella infection and the child’s age at the time of infection.

Results: We conducted a weighted analysis of 2,241 households visited. Among the 647 children of eligible age, 21% reported a history of varicella. Of those children with a history of varicella, 49% were 6 years old.  History of varicella was reported for 35% of 6 year-olds, 17% of 2 year-olds, and 14% of 1 year-olds.  The mean age at infection among 6 year-olds was 4.1 years.  Nearly ¾ of all infections were estimated to occur in 2007-2008. Infection before age 2 was reported by 1% among 6 year-olds, 8% for 2 year-olds and 14% of 1 year-olds. Prevalence did not vary by household size or income. Of children attending school or daycare, 34% had a history of varicella compared with 15% of children who did not (p<.0001). 

Conclusions: Varicella prevalence among children in American Samoa appears to be lower than the >80% seen among 1-4 year olds in the United States prior to vaccination. Vaccination could protect this population, which may remain susceptible to varicella into adulthood, when disease can be more severe.