The Wall Street Journal ran a great collection of infographics in 2015 superimposing the dates on which vaccination was introduced for various childhood diseases on their incidence: http://graphics.wsj.com/infectious-diseases-and-vaccines/
The note below the heatmap mentions that vaccination was introduced in 1914. A bit of digging online suggests one factor could be the introduction of the (more effective) combined diphtheria-pertussis-tetanus (DTP/DPT) vaccine in 1949 - is this the whole story? Did the original vaccine have any effect? Did anything like coverage, availability, pricing and/or legal requirements for vaccination improve significantly at the same time as the new vaccine was introduced?
And why did the CDC stop collecting data on pertussis cases from 1955-1974 during a period when you'd have expected them to be keen to track the effectiveness of the vaccination programme?
(I realise that the CDC data gathering question might seem like a separate issue, but (a) it's necessary to understand this figure, and (b) the point of lumping these together is because I have a strong suspicion that antivaxxers will point at that gap and come up with a conspiracy theory, and I'd like to be able to point to a full explanation of the data behind this figure in one place. So, to be useful for explaining this figure to sceptics, they need to be answered together.)