The percentage of the population that needs to be vaccinated to prevent ANY outbreak it about 85%, depending a bit on the disease in question and how isolated the community is. The percentage of the population that is actually immune at any given moment varies based on overall health, so even people who have been vaccinated may not be fully immune if they, for example, have a really bad cold and haven't slept for a week. The estimate is that about one in five vaccinated people isn't really immune at any given time. The level of immunity in the population needs to be greater than 60% to prevent most outbreaks, which leads to a vaccination level between 85% and 90%.

So, how old are her kids? Is she putting them in public school? My school is in the process of sending out suspension letters to kids who haven't had their most recent round of vaccinations. Have you mentioned to her that even if she has some kind of exemption from the school board, if there is an outbreak, her kids won't be allowed to go to school?