whooping cough epidemic, page-87

Jantimot

Given the research you have down I would have thought you know the answer. Wiki has a good summary:

Vaccines do not guarantee complete protection from a disease. Sometimes, this is because the host's immune system simply does not respond adequately or at all. This may be due to a lowered immunity in general (diabetes, steroid use, HIV infection, age) or because the host's immune system does not have a B cell capable of generating antibodies to that antigen.

Even if the host develops antibodies, the human immune system is not perfect and in any case the immune system might still not be able to defeat the infection immediately. In this case, the infection will be less severe and heal faster.

The efficacy or performance of the vaccine is dependent on a number of factors:

the disease itself (for some diseases vaccination performs better than for other diseases)
the strain of vaccine (some vaccinations are for different strains of the disease)
whether one kept to the timetable for the vaccinations (see Vaccination schedule)
some individuals are "non-responders" to certain vaccines, meaning that they do not generate antibodies even after being vaccinated correctly
other factors such as ethnicity, age, or genetic predisposition
When a vaccinated individual does develop the disease vaccinated against, the disease is likely to be milder than without vaccination

http://en.m.wikipedia.org/wiki/Vaccine