If you count all the cases that occur, (including the ones that go undiagnosed), the number that get complications is small. About 1% in the developed world in my experience. Of course, if you only count hospital cases or laboratory proven cases, (which are going to be the more severe cases,) then the proportion of those with complications is greater. But you can only get a true perspective if ALL cases that occur are counted.
This is where many published figures are misleading and may lead you to believe that whooping cough has a high rate of complications. Because whooping cough goes unrecognised much of the time official figures tend to exaggerate the severity and underestimate the incidence (the number of cases).
In my published study of 500 consecutive cases in an English village over 20 years, only 1 in 100 developed significant complications (always pneumonia).
The worst complication is death. This is rare except in young babies for whom it is a more exhausting illness than some can stand. In babies it can lead to respiratory failure, convulsions and coma from encephalopathy. It is thought that some very young babies who get it, do not cough at all, but simply get the 'stopping breathing' bit that usually comes after a bout of coughing, with possibly very serious consequences. In the United Kingdom one child in 50 who gets it under the age of one, dies from it. In older children death is very rare; 1 in 20,000 cases. In the underdeveloped world, the mortality is vastly greater.
There are minor complications that are often described but usually occur only in the most severe. These are; bleeding over the white of the eye (subconjunctival haemorrhage), blood spots in the skin (petechiae), tearing of the ligament at the base of the tongue and umbilical hernia. All these are caused by congestion of blood or the strain of coughing, retching and vomiting.
All these things are described in textbooks and reading them gives the impression they are very common.
I have to tell you that although I know these things do occur. None of my patients (in my family practice) has yet, to my knowledge, developed any of these congestion caused complications.
Some people faint with paroxysms and may make involuntary jerking movements resembling a fit. They may have no recollection of the faint, but unlike a true fit, they will usually remember the events leading up to it.
Long term complications
More people who have whooping cough have asthma than those who have not had whooping cough. Whooping cough DOES NOT CAUSE ASTHMA. It just so happens that people with asthma seem more susceptible to it.