As the dynamics of disease transmission depend upon the interactions and proportions of susceptible, infected (cases) and immune people, it follows that epidemics could be prevented by increasing the proportion of immune individuals.
Conversely, if the critical mass (density per some area) of susceptibles crosses a certain threshold, it will result in an explosive increase in incidence (new cases) of an introduced infection. This is termed the epidemic threshold.
When the proportion of immune is above a threshold value, the number of susceptibles will be less than the epidemic threshold. This reduces the possibility of susceptibles acquiring infection. Therefore, the incidence (new cases) will decrease. The proportion of immunes that must be exceeded for the incidence to decrease is called the Herd immunity threshold.
In the case of vaccine-preventable diseases, if the proportion immunized at birth is maintained at or above the herd immunity threshold, the cumulative incidence will be reduced to zero, and the disease eliminated from the population.
Assuming that vaccination takes place in a random manner, the proportion of the population that must be vaccinated to achieve herd immunity threshold is called the critical vaccination level.
Vaccines stimulate an immune response in recipients that protects them from natural infection. Some vaccines also decrease infectiousness- the ability to transmit infection. For the purpose of herd immunity, it is a vaccine’s effect on transmission that is more important. If the only effect of a vaccine were to prevent disease but not to alter either the risk of infection or infectiousness, then there would be neither indirect effect, nor herd immunity.
The magnitude of the indirect effect of vaccine-derived immunity is determined by
- The transmissibility of the infectious agent
- The nature of immunity induced by the vaccine
- The pattern of mixing and infection transmission in populations
- The distribution of the vaccine and immunity in the population