A vaccine may not be completely effective in preventing an infection. Failure of the vaccine to protect can often be traced to one or more problems. Often a vaccine does not need to be completely effective to be beneficial, provided it interrupts the transmission of disease in a population at risk.
Design of the vaccine:
(1) selection of a poorly immunogenic antigen
(2) choice of a poorly protective antigen
(3) failure to cover a particular strain (if multiple strains possible, as for Neisseria meningitidis)
Deterioration of the vaccine:
(1) outdate during storage
(2) improper storage temperature (either too hot or too cold)
(3) improper reconstitution, including use of wrong diluent
(4) contamination
(5) exposure to heat or light
Administration of the vaccine:
(1) leakage out after injection, resulting in too low a dose
(2) too low a dose or an incomplete series
(3) placement in scar tissue or other site where inaccessible to immune system
(4) administration while the organism is already incubating
(5) vaccine never administered (error in records)
Host response:
(1) inadequate antibody formation (immunodeficiency, malnutrition, immature immune system, etc)
(2) loss of immune status over time (decline in antibody levels)
(3) presence of low level passive antibody (from maternal serum, blood transfusion, immune globulin)
Organism:
(1) mutation in the organism with loss of target antigen
(2) infection with a totally different organism (different genus, species, etc)
A key question is whether the patient has formed the expected level of antibodies within the expected period of time.
