Most ten-year-old children with negative or unknown histories of chickenpox are immune.

To evaluate the proportion of children to vaccinate against varicella in a catch-up program targeting 9- to 10-year-old children, a study was conducted among children age 10 years to assess the age-specific incidence of varicella and document the immunity against varicella in those with negative or unknown chickenpox history. Of the latter 62% were seropositive for varicella.