The graph shows levels of primary and secondary immune responses versus time. The secondary response is greater and more rapid than the primary response. What does the graph BEST indicate about primary and secondary immune response? A) A vaccine elicits a primary response, after which the subsequent infection is fought strongly and more rapidly. B) An infection elicits a primary response, after which subsequent vaccination causes a heightened secondary response. C) A vaccine triggers the immune system twice, but does not affect the elicited immune response on subsequent infection. D) A vaccine causes a primary immune response, but does not affect the elicited immune response on subsequent infection.

The main purpose of vaccines is to induce the production by the body of antibodies, natural biological agents of defense of the body vis-à-vis identified pathogenic elements. A vaccine is therefore specific to one disease but not to another. This production of antibodies gradually decreases in a more or less long time, thus fixing the duration of effectiveness of the vaccine. It is measurable and this measure can be used in some cases to know if the subject is vaccinated effectively (hepatitis B vaccine and anti-tetanus in particular).

The immune defenses thus "stimulated" by the vaccine prevent an attack of the pathogen for a period that may vary from one vaccine to another. This avoids the development of an infectious disease at the level of the individual and, in the case of contagious disease and mass vaccination, at the level of a population.