These vaccines are apparently very effective; they clearly induce protective antibodies in individuals who receive them. Other evidence of effectiveness is more circumstantial. There are currently less than five cases of diphtheria and less than fifty cases of tetanus in the United States every year, compared to thousands or hundreds of thousands in other parts of the world. Some of this might be related to better medical care and sanitation, but not all. One important point is that the effectiveness of the pertussis vaccine wanes over time. This is why we now recommend the Tdap vaccine, which is a pertussis booster, for adolescents and adults.