The whooping cough vaccine is about 90% effective at best, so 1 in 10 children who are fully immunised are still able to contract the disease. It usually starts with cough and cold type symptoms, which progresses to the classic coughing paroxysms. Immunised children sometimes develop milder symptoms, and babies might not have the "whoop" at the end of the cough, but they will often have a post-coughing vomit.