Background: High national vaccination coverage may conceal age-specific and spatially concentrated measles susceptibility. Objective: To assess whether published age-specific seropositivity results can be converted into a time-updated susceptibility profile that corresponds with subsequent measles incidence, while distinguishing susceptibility from infectious introductions and transmission conditions. Methods: For Israel, published 2015 age-specific seropositivity estimates were mapped to monthly birth cohorts and projected to 1 March 2018, accounting for births, aging, maternal antibody, routine vaccination, vaccine effectiveness, and uncertainty in assay interpretation. The primary outcome was reported age-specific incidence during the 2018-2019 outbreak; national and Jerusalem District case burdens were secondary outcomes. Published evidence from the Netherlands, Czechia, and Australia was compared using a common framework covering age distribution, assay classification, vaccination, importation, spatial concentration, and transmission context. Results: The estimated number susceptible in Israel on 1 March 2018 ranged from approximately 0.55 million (6.3% of the modelled population) to 1.92 million (22.4%), with a central estimate of 1.22 million (14.2%). Children aged