As a mathematician, my immediate reaction was: duh! Of course! Why is this interesting?

The go-to example would be a person having 10 friends, being the only friend for each of them. It's not too hard to generalize from there, or at least to have an intuition.

The interesting part for me was this:

> If there are only enough vaccines for a small fraction of the population, an effective strategy is to vaccinate a random friend of each randomly chosen person.

Oh. Makes sense!

Somehow, something like that didn't occur to me even when the math felt intuitive.