How successful is the tdap vaccine for preventing diphtheria?

Very successful. This is a vaccine that most people should get at 11-12 years old and than every 10 years. It also prevents whooping cough.
Immunizations. Tetnus immunity diminish over time and needs to be replenished with vaccinations at least every ten years. Whooping cough immunity does the same however no long term recommendations have been made. If immunized and in direct contact with active pertussis a child may still get the disease. Immunizations work best if EVERYONE is immunized. As long as pools of infection exist we can get it ! .
Very Successful. Dpt given to children to complte the Primary Immunisation gaist Diptheria/Tatanus/Whooping Cough and booster of tdap to adolescnts and adults is very successful in preventing all 3 diseases.
Pretty darn good. No reported diptheria cases in the usa for years. So basically 100%.