The asymptomatic incubation period is 7-10 days. The catarrhal stage follows and lasts about 2-7 days. The paroxysmal stage follows, lasting about 1-8 weeks. These are less spontaneous than in typical respiratory infections. Infants younger than 6 months often vomit in association with the cough, which leads to dehydration.
Hypoxia tends to be more severe than the child's clinical appearance suggests.
A significant number present with cyanosis and apneic spells. Vaccinated adults usually only develop a prolonged bronchitis without a whoop. Unvaccinated adults are more likely to have whooping and posttussive emesis. On average, they wait a median of 3 weeks before seeking treatment. Hypoxia should be considered.