A Health Surveillance Update on Canadian Children and Youth

Pertussis

Hospitalization for pertussis is still common in Canada, and several deaths occur each year, particularly in unimmunized infants.

Pertussis (whooping cough) is a highly communicable infection of the respiratory tract.⁽⁴⁾ The disease can affect individuals of any age, but it is most severe in young infants.⁽¹¹⁾ Hospitalization for pertussis is still common in Canada, and several deaths occur each year, particularly in unimmunized infants.⁽⁴⁾ Pertussis cases are reported from the local level to provincial and territorial health authorities and subsequently to the Bureau of Infectious Diseases, LCDC, Health Canada. The pertussis incidence is the number of newly reported pertussis cases for a given time period, expressed per 100,000 persons.

Pertussis data from 1997 indicate a total of 4,439 reported cases in Canada for a rate of 14.7 per 100,000. Of these cases, 3,930 or 88.5% occurred among children and youth less than 20 years of age. The highest age-specific incidence was 121.7 per 100,000, among infants less than one year of age. Other age-specific incidence rates in 1997 were 72.4, 72.3 and 37.4 per 100,000 persons in the 5-9, 1-4 and 10-14 year age groups respectively. Preliminary national data from 1998 indicate more than 7,000 reported pertussis cases, for an incidence of approximately 25 per 100,000.

Before the introduction of the first pertussis whole-cell vaccine in 1943, the mean annual reported incidence was approximately 160 cases per 100,000 persons. Since the introduction of the vaccine, reported pertussis rates have greatly decreased. As depicted in Figure 5, the overall crude rates for pertussis have declined in a cyclical manner from 182 per 100,000 in 1934 to as low as 4 per 100,000 in 1988. The rates have increased during the 1990s, and the mean annual pertussis incidence during this decade has been 23 per 100,000 persons. The introduction of a new acellular vaccine in 1996 may result in greater vaccine coverage and a reduction in the incidence of pertussis over the next decade. However, in addition to increasing pertussis incidence, the 1990s have also seen the emergence of new pertussis strains with antigenic differences. To address this issue, Health Canada (the Bureau of Infectious Diseases and the Bureau of Microbiology) is developing a laboratory-based surveillance program to monitor changes in circulating strains of the Bordetella pertussis organism.

* preliminary data
Source: Bureau of Infectious Diseases, LCDC⁽⁵⁾

Figure 6 compares the pertussis incidence in Canada and the United States from 1980-1998.^(5,12) The higher Canadian rates likely reflect more rigorous surveillance and reporting practices for pertussis in Canada. Nevertheless, it is noteworthy that the rates in both countries demonstate variation over time.

* preliminary data
Source: Bureau of Infectious Diseases, LCDC^(5,12)

Data limitations
It is suspected that the reported pertussis incidence greatly underestimates the true number of pertussis cases. A suspected case of pertussis must be either laboratory-confirmed or linked to a laboratory-confirmed case before it is recorded in the national database. Individuals with pertussis who do not receive appropriate diagnostic tests will not be counted.

Summary
Pertussis remains the least controlled vaccine-preventable disease for which routine childhood immunization is available. Concerns regarding both the effectiveness and the adverse effects of the whole-cell vaccine have likely contributed to sub-optimal vaccine coverage and disease control. Ongoing surveillance of pertussis will be required to assess the impact of the new acellular pertussis vaccine recently added to routine vaccine programs across Canada.

Unless referenced otherwise, pertussis statistics are the product of the Bureau of Infectious Diseases, LCDC⁽⁵⁾

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