During coughing spells the patient may turn blue, followed by vomiting and exhaustion. Lymphocytosis is usually present. Between attacks the patient usually appears normal. The paroxysmal phase may last 6 weeks or longer. During the convalescent phase the coughing becomes less paroxysmal and disappears over 2 to 3 weeks.
Young infants are at highest risk of acquiring clinical pertussis and complications.
The most common complications are secondary bacterial pneumonia and otitis media. Other complications such as seizures and encephalopathy may also occur. Adults and adolescents with waning immunity, or those never vaccinated, may become infected with B. pertussis . Adult or adolescent disease is usually a milder cough lasting more than 7 days. Mildly ill adults may be a reservoir of pertussis and a source of infection for infants.