Vaccines stimulate the immune system to produce memory cells that “remember” their exposure to the antigens in the vaccine. These memory cells can last for decades, triggering the production of antibodies when the real pathogen comes along. Yet most childhood vaccines require a series of shots, spaced out over months or years. And the tetanus and diphtheria vaccines require booster shots at least every 10 years. Why isn’t one dose enough?
The answer has to do with the number of memory cells that the body produces after exposure to the vaccine. After just one dose, the number of memory cells may be too small to launch an effective immune response against a pathogen. Repeated doses ensure a stronger immune response because they trigger the formation of more memory cells.
The vaccine’s formulation therefore helps determine the need for booster shots. A vaccine that consists of active viruses, such as the one Edward Jenner used against smallpox, causes a mild infection that lasts for about a week. This lengthy exposure to viral antigens stimulates a robust immune response, so booster shots are not usually necessary. On the other hand, when a vaccine contains toxins or inactivated viruses, no infection occurs. The body’s exposure to the antigens is therefore relatively brief. In that case, repeated shots help boost the number of memory cells over time.