Why incidence falls faster than coverage rises
As vaccination coverage climbs, disease incidence usually drops more steeply than the coverage curve alone would suggest. This is because each vaccinated person not only avoids infection themselves but also removes a link in the chain of transmission, indirectly protecting unvaccinated contacts. The relationship is non-linear: early gains in coverage reduce cases modestly, while approaching a critical threshold can collapse transmission.
The threshold at which sustained spread becomes unlikely is the herd immunity threshold, and it is higher for more transmissible pathogens. Beyond that point even people who are not immune are partly shielded because the pathogen struggles to find a continuous path of susceptible hosts.
Why the threshold is not a single fixed number
The herd immunity threshold depends on the pathogen's transmissibility, on how immunity is distributed across a population rather than just its average, and on whether immunity wanes over time. Pockets of low coverage can sustain local outbreaks even when overall coverage looks adequate, so uniform high uptake matters as much as the headline percentage.
This is general educational information about epidemiology, not medical advice. Vaccination decisions should be discussed with a qualified healthcare professional.