When discussing the topic of highly contagious infectious diseases such as measles that require a the population to maintain a high vaccination rate to be kept at bay I invoke concepts like herd protection. Herd protection, sometimes also described as herd immunity, refers to the ability of an unvaccinated or vaccine non-responding individual to rely on the fact that others are adequately immunized so the contagion's ability to reach them is severely restricted.
When I was an infectious disease fellow one of my attending physicians would illustrate this point with a graphic of a herd of cows with various degrees of vaccination within the ranks of their herd.
The point he was concretizing is that the more contagious a microbe is the higher the vaccination rate must be. In mathematical and epidemiological terms the level of vaccination must be 1 - (1/Ro). Ro represents the contagiousness of the disease and you can see just by plugging in an approximate number of say 20 for measles, how high the vaccination rate must be (95%).
If that all seems too abstract and the cow example seems too simplistic, I have a solution.
The Graduate School of Public Health (GSPH) at the University of Pittsburgh has a fun tool to help illustrate these points. Affectionately known as FRED, this epidemic stimulator allows one to see a measles outbreak unfold in various locales with disparate vaccination coverage rates. One can see how a measles outbreak is stopped when vaccination rates are at the 95% threshold and how the spread when below.
Such a tool, coupled with the data and graphics of another of GSPH's great projects, is a welcome addition to the armamentarium when trying to educate the public and policymakers (some of who defend vaccines like PA State Rep. Corbin and US Reps. Dent and Marino) of just how important the measles vaccination has been in eliminating this disease from the US--a status that is now severely threatened.