Trying my best,
You are thinking of the mumps cases, not the measles cases. The mumps vaccine does not provide 100% immunity (neither does actually contracting mumps). I met a Centers for Disease Control investigator who was researching the outbreak at Kiryas Joel; he reported that the Rebbe personally took his team to all the schools and clinics in order that they receive total cooperation. And indeed virtually the entire village had been vaccinated pretty much according to protocol.