Whooping cough (pertussis) is most easily diagnosed in young children because they develop a characteristic cough - paroxysms of coughing followed by a long inspiration that makes a whooping sound.

However, infants, immunized children, adolescents and adults do not usually exhibit whooping cough. Indeed, adolescents and adults may experience only a persistent cough. As a result, the disease is underdiagnosed, undertreated and can be spread to others.

The introduction of a whole-cell vaccine in the 1940s resulted in a dramatic decrease in the incidence of pertussis. The disease, however, made a comeback in the 1990s, probably because of waning immunity, which occurs 6-12 years after the last episode of illness or booster dose. A new (less toxic) acellular vaccine introduced in the 1990s has had some effect, but again, recently, the incidence of pertussis has increased. The highest increase in rates of the disease in the 1990s was among adolescents and adults. Since 2004, authorities have recommended that adolescents receive a booster dose of pertussis vaccine.

Eleni Galanis and colleagues discuss these trends and their implications for both patients and physicians. The changing epidemiology of pertussis in Canada in the 1990s has led to the emergence of new groups at higher risk of the disease: young infants, adolescents and adults.

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