The effectiveness of a vaccine depends on the immune system reaction which the patient develops. This involves the generation of long term memory B cells and T cells via adaptive immunity following innate immune responses. Sometimes the antigen contained in the vaccine doesn't trigger an immune response. In the latter case there is need for new and stronger vaccines.
Vaccination acts as a sort of "firebreak" in the spread of the disease, slowing or preventing further transmission of the disease to others. For example, if Person A had a disease and exposed Person B who was immune because of vaccination, Person B would not get ill and could not pass on the disease to Person C when he comes into contact with him. So even if Person C is not vaccinated, he indirectly gets protection from the disease. Hence herd immunity may be used to reduce spread of an illness and to protect a vulnerable, un-vaccinated subgroup. However because only a small fraction of the population (or herd) can be left un-vaccinated for this method to be effective, it is considered best left for those who cannot safely receive vaccines due a medical condition such as an immune disorder or for organ transplant recipients.
|Disease||Transmission||R0||Herd immunity threshold|
|Measles||Airborne||12-18||83 - 94%|
|Mumps||Airborne droplet||4-7||75 - 86%|
|Pertussis||Airborne droplet||12-17||92 - 94%|
|Polio||Fecal-oral route||5-7||80 - 86%|
|Rubella||Airborne droplet||5-7||80 - 85%|
|Smallpox||Social contact||6-7||83 - 85%|
|- R0 is the basic reproduction number, or the average number of secondary infectious cases that are produced by a single index case in completely susceptible population.|
Although no vaccine offers 100% protection, the spread of disease from person to person is much higher in those who remain un-vaccinated. Virologists have found that when a certain percentage of a population is vaccinated, the spread of the disease is effectively stopped. This critical percentage, called the herd immunity threshold, depends on the disease, the vaccine, and the contact parameter for the population. It is the general aim of those involved in public health to establish herd immunity in most populations. However complications arise when wide spread vaccination is not possible, and when vaccines fail (See the MMR vaccine controversy in the UK.) Additionally, some vaccines, such as the tetanus vaccine, protect only vaccinated persons from pathogens, and do not generate herd immunity.
Herd immunity should not be confused with contact immunity, a related concept wherein a vaccinated individual can 'pass-on' the vaccine to another individual through contact.