To add some data points about why ongoing vaccination is necessary, DC has seen outbreaks of both measles and pertussis (whooping cough) in the last year. Whooping cough is on the rise again, largely-- according to the CDC-- thanks to the reduction of herd immunity caused by unvaccinated children.

Whooping cough is not pretty. It's long and painful and terrible to watch, and it's easily prevented with a common vaccination-- the tetanus shot.