This article is from the Childhood Vaccinations FAQ, by Lynn Gazis-Sax email@example.com with numerous contributions by others.
An estimated 3.7 million Americans are affected by chickenpox each
year, with more than 90% of the cases occurring in persons under 15
years of age. 33% of cases are estimated to occur in children ages 1
to 4, and 44% in children ages 5 to 9 (estimates from January 28, 1994
In tropical regions, chicken pox is less common, and many people may
reach adulthood without immunity (adult immigrants from tropical to
temperate regions may therefore be at risk).
I do not have data on how varicella incidence has been affected by the
availability of the vaccine, but vaccine coverage is still (as of
1999) fairly low for this particular vaccine.