Pneumonia is the cause of most pertussis-related deaths.
Other complications, though infrequent, can include neurological complications such as seizures and encephalopathy, otitis media, and conditions resulting from the pressure effects of the severe coughing. Disease rates and risk of serious complications, including death, are highest among young children, particularly those less than one year of age. Since the introduction of pertussis vaccine in the 1940s, the cases of pertussis decreased from 150 per 100,000 population between 1922 and 1940 to an average of 1.2 per 100,000 in the years 1980-1991. However, since then, reported pertussis cases have increased for unknown reasons with 6,586 cases reported in 1993, and cases were reported from every state. Since 1991, pertussis has emerged as the most frequently reported vaccine-preventable disease among children less than five years of age.