Measles immunity among young adults in Victoria

Measles outbreaks in Victoria in 1999 and 2001 have suggested that a substantial proportion of young Victorian adults may be susceptible to measles infection. We performed a serosurvey of 300 18-30-year-old healthy blood donors and 312 sera retrieved after diagnostic testing for a non-rash illness in patients of the same age group, with the aim of estimating the proportion of young adults in Victoria immune to measles. We also aimed to define more precisely the birth cohorts at risk of measles infection, with cohorts reflecting the measles immunisation policies of previous years. There was no significant difference in measles immunity between the 300 blood donors (79.0%, 95% confidence interval 73.9-83.5) and the 312 patients whose sera had been stored (84.0%, 95% CI 79.4-87.9, p=0.11). There was, however, a significant difference in immunity by birth cohort. In the combined results from both samples, the proportion of people born between 1968 and 1974 who were immune to measles was 88.4 per cent (95% CI 84.1-91.6) while the proportion of those born between 1975 and 1981 was 74.1 per cent (95% CI 68.7-79.1). This study confirms that a substantial proportion of young Victorian adults are susceptible to measles, but also demonstrates that those born between 1975 and 1981 are more likely to be non-immune than those born before 1975. A review of published Australian data supports this conclusion and confirms the need for a measles control program aimed at young adults.