Epidemiology of pertussis.
The World Health Organization recommended that a pertussis incidence of <1 case per 100,000 population be achieved in Europe by 2000. Available data indicate that this goal has generally not been achieved, and the incidence is actually rising in some countries. Understanding the reasons for this increased incidence may lead to better global control of pertussis. In the majority of countries where pertussis is a notifiable disease, a case-based national surveillance system is in place. However, different case definitions, methods of diagnosis and reporting and surveillance systems make direct intercountry comparisons difficult, and pertussis is not a statutory notifiable disease in every country. Nevertheless the general consensus is that reported incidences are probably considerably lower than the actual incidence of pertussis; underreporting is common. Prolonged cough may be the only clinical feature in adolescents or adults, who may present for diagnosis late (precluding laboratory confirmation) or not at all. When they do present, their condition is often misdiagnosed because, in part, clinicians continue to perceive pertussis as a childhood disease. Despite underreporting, an increased incidence of infant, adolescent and adult pertussis has been observed worldwide since the introduction of widespread vaccination. This is of concern because adolescents and adults have been identified as a source of transmission of pertussis to very young infants who are unimmunized or partially immunized and thus more vulnerable to disease-related complications and higher mortality. In recent years, acellular pertussis vaccines have been incorporated into the immunization schedules of many developed countries, gradually replacing whole cell vaccines. Dosing schedules vary between countries, although primary immunization with 3 doses of the pertussis vaccine within the first 6 months of life exists in most countries. Only Australia, Austria, Canada, France and Germany have incorporated an adolescent booster dose into their current immunization schedules, in recognition of the rising incidence of pertussis in adolescents and adults. Despite high coverage rates for primary immunization in infants and children, pertussis continues to be a global concern, with increased incidence widely noted. This global epidemiologic summary highlights differences worldwide in pertussis reporting, incidence and approaches to prevention. It underscores a general shift in the age distribution of pertussis toward older groups. Understanding the link between these observations may lead to better informed global control strategies, especially those pertaining to immunization schedules and use of pertussis vaccine.