The Australian Immunisation Handbook 10th Edition 2013

4.22.3 Epidemiology
In an unimmunised population in temperate climates, the annual number
of cases of varicella approximates the birth cohort. 8 Tropical regions have
a higher proportion of cases in adults. Approximately 5% of cases are
subclinical. A serosurvey conducted in 1997–1999 found that 83% of the
Australian population were seropositive by 10–14 years of age. 9 Prior to the
introduction of a varicella vaccination program in Australia, there were about
240 000 cases, 1500 hospitalisations and an average of 7 to 8 deaths each year
from varicella in Australia. 10-12 The highest rates of hospitalisation occur in
children