Terry M. Bella
The true mark of a successful vaccine is a successful vaccination protocol. Though barriers related to the adoption of the protocol by the public will be discussed later, this section discusses the reason that a vaccine is deemed successful based on the community level immunological barrier created by individual immunizations. If a sufficient, that which protects vulnerable members, proportion of a population has immunity this is considered herd immunity. Thus achieving herd immunity indicates that the vaccine is a success because the threat of the disease becoming epidemic has been mitigated. In some cases diseases can be eliminated from a population when the herd immunity threshold is reached.
Surely it is important that individuals do not contract diseases and fall ill, but it is arguable that it is more important that there are not outbreaks. Considering communicable diseases the immunity of one person actually protects other people. If a disease can be passed from person to person, the fact that one person cannot contract the disease will reduce the instance of the transmission of the disease, thus reducing the likelihood of others contracting the disease despite their immunity. This is the premise that herd immunity relies upon. If enough people in a community are vaccinated the likelihood of those that are not vaccinated becoming ill significantly decreases.
Within a population there are people that cannot be vaccinated. The case may be that of age, too young or too old. Another reason is that the individual is immunocompromised because of disease and cannot be vaccinated as it stimulates the immune system. There are also instances wherein people are allergic to the vaccinations. The instance of increased immunity for those that have not been vaccinated because of those that have within the same population can be substantiated by a reduction of infections within the non-vaccinated portion of the population. Herd immunity helps to protect those that cannot be vaccinated, hence the phenomenon that a single individuals immunity increases the immunity of others within the community.
In general if 80% or more of the population is immune to a disease then the herd immunity threshold has been reached. For the purposes of the classroom one can use the 80% or greater rule. The threshold for diseases does vary from disease to disease based on how infectious the disease is, namely how many people are likely to be infected by an infected individual. The herd immunity threshold for small pox is 80-85%, for polio 80-86%, and rubella 83-85% while measles calls for 83-94% and pertussis 92-94%.1 One can see that diseases with higher propensity to spread warrant a higher threshold. The metrics for determining these thresholds are beyond the scope of this unit.