I'll just quote Science-Based Medicine (http://www.sciencebasedmedicine.org/?p=516 http://www.sciencebasedmedicine.org/?p=516): "The decrease in the number of

I'll just quote Science-Based Medicine (http://www.sciencebasedmedicine.org/?p=516 http://www.sciencebasedmedicine.org/?p=516): "The decrease in the number of disease carriers is vital to the prevention of bacterial infections. Vaccines are never 100% effective. Some people are genetically unable to respond to the vaccine, some have immunodeficiencies that preclude receiving vaccines or developing a response to the vaccine, some haven’t gotten around to vaccination or are too young to receive a vaccine. If you vaccinate a large number of people, besides preventing disease in an individual, it helps protect the vulnerable in a population. Vaccines prevent disease propagation." "The vaccine [for whooping cough] is good, but not perfect. Vaccine efficacy is 64% for cases defined by mild cough, 81% for paroxysmal cough, and 95% for severe clinical illness (11). Note the vaccine is good for attenuating the disease, not preventing it entirely."