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.