Estimating current susceptibility to measles in Australia following sustained measles elimination.
OBJECTIVE This study estimates Australia's measles susceptibility across all age groups. To our knowledge, this study is the first internationally to combine multiple consecutive serosurvey and population immunisation register data. METHODS We estimated age-specific measles susceptibility in 2019 using data from nationally representative seroprevalence surveys for people born between 1920 and 1960 (infection-acquired immunity) and those born 1961-1999 (infection/vaccine-acquired immunity). For people born 2000-2018 (vaccine-acquired immunity only), we estimated measles susceptibility from coverage of one or two doses of measles-containing vaccine from the Australian Immunisation Register and presumed vaccine effectiveness, adjusting for 0·04% waning of protection per year. RESULTS Susceptibility increased from less than 1% across those born 1920-1960 to 11% for the 1993-1995 cohort, and ranged from 7-14% for those born 2000-2015. We estimated that 1.7 million Australians (6·7%) were susceptible to measles in 2019 (95%UI: 1·3-3·2 million). Among notifications of measles infection between 2008 and 2022, people aged 15-35 years at time of notification accounted for 50%. CONCLUSIONS Despite sustained high two-dose measles vaccine coverage, Australia's overall population-level measles immunity is at the lower end of the World Health Organization estimated herd protection threshold range (93-95%). IMPLICATIONS FOR PUBLIC HEALTH Our estimates provide baseline data to inform vaccination activities to address immunity gaps in older age groups, particularly those likely to travel internationally.