'Measles cases continued to pop up here and there in the densely populated city in 1989 and 1990 — as is common with highly contagious infectious diseases. But measles never managed to get a firm grip on any particular neighborhood or community in that two-year period, in part because the vaccination rate among the overall population was reasonably good. That all changed once the illness made its way to the Faith Tabernacle school.'

https://www.phillymag.com/news/2021/03/20/philadelphia-measles-outbreak-1991/