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Vaccine-Preventable DiseasesPertussisPertussis (whooping cough) is a highly communicable infection of the respiratory tract caused by Bordetella pertussis. The disease can affect individuals of any age; however, severity is greatest among young infants. One to three deaths occur each year in Canada, particularly in infants too young to have begun their immunization and in partially immunized infants (e.g., one or two doses). The number of affected adolescents and adults has steadily increased, and the morbidity in these cases is not insignificant. The goal of pertussis control is to decrease the morbidity and mortality of pertussis across the entire lifespan. Protection of adolescents and adults is a worthy goal for the benefit of these individuals themselves, notwithstanding the added indirect protection that it may provide to infants. Pertussis has been partially controlled in Canada through immunization, and during the last 50 years its incidence has decreased by > 90% (see Figure 9), although outbreaks continue to occur.
EpidemiologyThe whole-cell pertussis vaccine was introduced in Canada in the 1940s. It was replaced by the adsorbed whole-cell vaccine in the 1980s and by acellular vaccine in 1997-98. Since the introduction of pertussis vaccination, the number of reported cases has dropped dramatically, from 160 cases per 100,000 just before the introduction of the vaccine to < 20 cases per 100,000 in the 1980s. The incidence of pertussis in Canada was low during the 1980s but has increased since 1990. Between 1990 and 2004, the annual number of reported cases has ranged from 2,165 to 10,151, although this likely under-represents the true burden because of incomplete diagnosis and reporting. The resurgence of pertussis was likely due to a combination of factors, including the low efficacy of the combined adsorbed diphtheria-tetanus-pertussis whole-cell vaccine used in children in Canada between 1980 and 1997, waning immunity among adolescents and adults, as well as increased physician awareness and improved diagnosis and reporting of pertussis disease. A cohort of children immunized solely with the vaccine used between 1980 and 1997 was poorly protected and constitutes the population that has been most affected by pertussis since 1990. The increasing age of cases parallels that of children belonging to the vulnerable cohort. The proportion of pertussis cases in adolescents (≥ 15 years) and adults increased from 9.6% in 1995 to 16.4%, 21.2%, and 31.3% in 1998, 2001 and 2004 respectively. In addition to a greater incidence, part of this increase may be attributable to better recognition, diagnosis and reporting of pertussis in adolescents and adults, as well as waning immunity. The increased incidence among adolescents has also been observed in the United States, France and other countries. Waning of vaccine-induced protection is a universal phenomenon affecting adolescents and adults worldwide. These persons constitute a major reservoir of the disease and are an important source of transmission to infants. Pertussis is a common cause of prolonged cough illness in adolescents and adults. Active surveillance for pertussis in Canada using a combination of laboratory methods has documented pertussis infection in 10% to 20% of adolescents and adults with a cough illness lasting 7 days or more without improvement. In a placebo-controlled clinical trial of acellular pertussis vaccine conducted in the United States involving 2,781 adults, nine participants from the placebo group developed pertussis in the 2-year follow-up period for an estimated annual rate of 3 per 1,000 person-years. This result was observed during non-epidemic years and would correspond to 60,000 adult cases in Canada annually.
Source: Canadian Immunization Guide, 7th edition, 2006 Links to more informationGuidelines and Recommendations
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Last Updated: 2007-03-16 |  |
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