To the Editor: Varicella vaccine was licensed in Australia in 2000. It is safe and efficacious and can prevent significant acute morbidity, significant out-of-pocket expenses for parents of affected children and, in Australia, prevents an estimated 450 admissions to hospital and one death per year.1 In September 2003, the National Health and Medical Research Council (NHMRC) recommended giving varicella vaccine to all children at 18 months of age.2

The net effect of childhood immunisation on varicella morbidity will depend on vaccination coverage. Modelling of Australian (unpublished National Centre for Immunisation Research and Surveillance of Vaccine Preventable Diseases data) and UK3 seroprevalence data suggests that, for a range of vaccine efficacy estimates, 80% coverage is required before morbidity is reduced in adults. Accurate Australian coverage data are unavailable, as varicella vaccine is not funded under the National Immunisation Program.2 National serosurveillance can provide a practical alternative estimate of vaccine uptake.

The first Australian national serosurvey of vaccine-preventable diseases, for which sera were collected between July 1996 and February 1999, established baseline seroprevalence for future assessment of the effects of changes in the vaccination schedule. It showed that the incidence of varicella was highest in 5–9-year-old children,4 indicating that infant vaccination would provide optimal protection. The second national serosurvey (with sera collected in 2002) is under way, with methods identical to those of the first.4 We compared varicella IgG levels in children aged 1–5 years, testing 459 sera in the first serosurvey and 380 in the second (see Box). The proportion with protective or equivocal antibody levels increased between the two serosurveys. The difference was statistically significant only in 3–4-year-olds, which is consistent with the greatest uptake when children enter childcare. Although there is some variation in varicella incidence over time, the changes are consistent with only modest vaccine uptake in the time that varicella vaccine has been available in Australia.

The only other available estimates of varicella vaccine uptake are from GP consultations5 and reports to the Australian Childhood Immunisation Register (ACIR), which also indicate it is modest. For example, only 6.2% of children aged 4 years were reported to the ACIR to have received varicella vaccine (Brynley Hull, Epidemiologist, National Centre for Immunisation Research and Surveillance of Vaccine Preventable Diseases, personal communication). This is probably an underestimate, as there is no incentive for notifying varicella vaccination. However, a low vaccination uptake is consistent with use only in the private sector, and could lead to an increase in adult morbidity (despite an overall reduction in infection rates) because of a higher average age of infection (unpublished National Centre for Immunisation Research data).

Unless vaccine is provided in the routine immunisation program at no cost to parents, uptake is unlikely to meet the 80% target required to reduce disease burden in all ages.3