As there are individual differences among animals (and humans) with respect to immunity, populations also differ in immunity. Resistance in populations or a community is referred to as Herd Immunity. This represents the proportion of resistance animals in the population.
Either due to natural exposure to an infectious agent or immunization, acquired herd immunity could result. In cases of mass immunizations, the level of herd immunity depends upon:
1. The efficacy of the vaccine
2. The proportion of animals actually vaccinated
3. The proportion of immunologically competent animals vaccinated
4. Proportion of vaccines maintained under optimal storage conditions
5. Proportion of vaccines properly administered.
To establish and maintain an adequate herd immunity, it is not necessary to have every member of the population protected.
Whether an epidemic or the spread of disease in the population is a function of:
1. Proportion of resistant animals in the population (herd immunity)
2. The probability of contact (rate of contact) between an infected and an uninfected individual in the population.
The level of herd immunity may change depending on whether new susceptible individuals are introduced into the population (via birth, purchase, immigration, etc.). Such changes could gradually lead to lowering of herd immunity. In general, the dynamics of the infectious process is such that if only 70-80% the population can be made resistant, epidemic outbreaks are minimized or prevented.