Vaccine immunogenicity is a measure of the immune response to a vaccine and usually involves measuring specific antibodies in the blood. For some, but not all vaccines, a defined threshold of specific antibody levels has been correlated with protection. However, this does not definitively tell us if a person is fully protected against disease. If good immune memory has been established, a large rise in antibody levels occurs following a booster dose of vaccine.
Vaccine efficacy and effectiveness are measures that compare the rates of disease between vaccinated and unvaccinated people. Efficacy is measured in controlled clinical trials, whereas effectiveness is measured once the vaccine is approved for use in the general population. From these we can identify the proportion of vaccinated people we would expect to be protected by the vaccine.
Herd immunity (also called community immunity) is an important mechanism by which the larger community is protected. For some diseases, if enough people are immune then transmission of the disease is reduced or eliminated. This is particularly so for diseases such as rubella and pneumococcal disease. High vaccine coverage must be maintained in order to prevent the disease re-entering the population.
No vaccine is 100% effective, a small percentage of people are not protected after vaccination and for others the protection may wane over time. Also, some people are unable to be vaccinated due to certain conditions such as immune suppression. Maintaining immunity in those around these people protects them from disease.
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Plotkin, S.A., W.A. Orenstein, and P.A. Offit, Vaccines. 5th ed. 2008, Pennsylvania: Elsevier Inc
According to the current NZ Immunisation Schedule1
Plotkin. Vaccines. Fifth Edition. 2
Any medical event occurring after immunisation can be classified as an adverse event following immunisation (AEFI) whether the event has any causal relationship to getting the immunisation or not.
Throughout the development of a new vaccine, from discovery to marketing and beyond, the safety of each candidate vaccine is carefully assessed. At any stage, development is discontinued if the safety risk outweighs the benefit.