Vaccination isnt 100% effective. When people talk about herd immunity, the "free riders" it tolerates should be are those that were vaccinated yet it didn't take. If one of them gets sick, the disease wont be able to transmit to the others because of those that are immune. Adding other non-vaccinated people to the mix increases the odds that these vaccinated-but-vulnerable people will be at risk as the disease is able to find enough transmission to stay viable.
Herd immunity[1]. There is a threshold however, and if the vaccination rates get too low then herd immunity is less effective as the free-riders become a risk vector. In other words herd immunity works when the great majority is immunized and can sustain some free-riders but it can't sustain many free-riders.
ergothus|7 years ago
mc32|7 years ago
[1]https://en.wikipedia.org/wiki/Herd_immunity