Visualizing herd immunity

The parents who refuse to vaccinate their children are taking a calculated risk; they're weighting whatever doubts they have about the efficacy of vacItcines against their doubts about vaccine safety and their doubts about the seriousness of infectious diseases, basically betting that either vaccines don't work and/or vaccines aren't safe and/or the diseases just aren't that big a deal.

A key part of this calculus is a kind of folk understanding of "herd immunity," the process by which immunity among a critical mass of members of a population bestows a kind of immunity on the whole group because the population-scale immunity levels make it much harder for the disease to spread.

In an outstanding video, the physicist Robert Rohde models the incredible power of herd immunity, as well as its dreadful fragility.

It's a short, sharp lesson in the game-theoretical limits of counting on other peoples' kids to keep your kid safe -- and on the risk that your choice about your kid's vaccination imposes on everyone else.

(via JWZ)