A vaccine is a substance that can increase protection from a particular disease. The substance stimulates the body’s immunological defenses to create antibodies against the disease. If the body encounters the microorganism that causes the disease, then the body will be able to fight it and avoid the full-blown disease or even be immune to it. In developing vaccines, there must be a balance between ensuring that the vaccine causes the desired immunological response and reducing the side effects and risk of illness.

Vaccines offer protection in two ways: (1) individual immunity and (2) herd immunity. The concept of herd immunity is that, if a certain [Page 347]percentage of the population is immune to a disease (usually 80 to 95 percent, depending on the disease), the ...