This article is from the Childhood Vaccinations FAQ, by Lynn Gazis-Sax firstname.lastname@example.org with numerous contributions by others.
Pertussis, or whooping cough, is a very contagious disease of the
respiratory tract. Its attack rate in unvaccinated household contacts
is over 90% (PDR) or up to 90 and in some cases 100% (Harrison).
Pertussis is very serious in children under 2, with a mortality rate
on about 1 to 2%. (Merck) "Prior to the availability of a vaccine,
pertussis caused as many deaths as all other contagious disease
_combined_." (Harrison, p. 607) Complications include various lung
complications (The Merck Manual has a long list of these), cerebral
complications, hemorrage into the brain, eyes, skin, and mucuous
membranes. In addition to killing, it can leave surviving infants with
lasting lung damage and neurological diseases.
The mortality rate is higher in developing countries, partly because
children in these countries contract pertussis at a younger age (and
mortality is higher at younger ages), and partly due to an association
with protein-energy malnutrition (Galazka). This same article
estimated that in the industrialized world, 0.04% of infected children
die from pertussis and complications, usually pneumonia "Among
vaccine-preventable diseases, pertussis rivals measles and neonatal
tentanus in importance and severity in young children in developing
countries and is third only to measles and neonatal tetanus as a cause
of death. It is estimated that pertussis is still causing some 340,000
deaths of children in the world each year." (Galazka)