Residual susceptibility to measles among young adults in Victoria, Australia following a national targeted measles-mumps-rubella vaccination campaign

BACKGROUND Past measles immunisation policies in Australia have resulted in a cohort of young adults who have been inadequately vaccinated, but who also have low levels of naturally acquired immunity because immunisation programs have decreased the circulation of wild virus. A measles-mumps-rubella (MMR) immunisation campaign aimed at addressing this susceptibility to measles among young adults was conducted in Australia in 2001-2. By estimating age-specific immunity, we aimed to evaluate the success of this campaign in the state of Victoria. METHODS We conducted serosurveys after the young adult MMR program at state and national levels to estimate immunity among young adults born between 1968-82. We compared results of the Victorian (state) surveys with the Victorian component of the national surveys and compared both surveys with surveys conducted before the campaign. We also reviewed all laboratory confirmed measles cases in Victoria between 2000-4. RESULTS The Victorian state serosurveys indicated no significant change in immunity of the cohort following the young adult MMR campaign (83.9% immune pre and 85.5% immune post campaign) while the Victorian component of the national serosurvey indicated a significant decline in immunity (91.0% to 84.2%; p = 0.006). Both surveys indicated about 15% susceptibility to measles among young Victorian adults after the campaign. Measles outbreaks in Victoria between 2000-4 confirmed the susceptibility of young adults. Outbreaks involved a median of 2.5 cases with a median age of 24.5 years. CONCLUSION In Victoria, the young adult MMR program appears to have had no effect on residual susceptibility to measles among the 1968-82 birth cohort. Young adults in Victoria, as in other countries where past immunisation policies have left a residual susceptible cohort, represent a potential problem for the maintenance of measles elimination.