A Health Surveillance Update on Canadian Children and Youth

AUTHORS:
A. Bentsi-Enchill, MB, ChB, MSc
M. Litt, BScN, RN, MHSc

Measles

In 1994, Canada joined all other countries in the Americas in adopting a resolution to eliminate measles from the Western Hemisphere by the year 2000.

Measles is a highly contagious viral disease that may result in complications, including pneumonia, otitis media, bronchopneumonia and encephalitis.⁽⁴⁾ Every year, nearly one million deaths worldwide are attributed to measles.⁽⁸⁾ In 1994, Canada joined all other countries in the Americas in adopting a resolution to eliminate measles from the Western hemisphere by the year 2000.⁽⁷⁾ In Canada, confirmed measles cases are reported from the local health units and laboratories to the provincial and territorial health authorities. In an enhanced surveillance procedure, the provinces and territories report on a weekly basis directly to the Bureau of Infectious Diseases, LCDC, Health Canada. The national measles incidence is calculated by dividing the number of newly reported measles cases in a given period over the total population, expressed per 100,000 persons.

In 1998, there was a total of 12 confirmed measles cases in Canada, for an incidence of 0.04 per 100,000. This was the lowest annual number of cases ever reported in Canada. The confirmed cases were distributed across the country and were not related to outbreaks. Five of the confirmed cases had traveled outside of Canada 7-21 days before the onset of the rash. The median age at onset of the rash for the confirmed cases was 5 years, and all but one case occurred among children and youth less than 20 years of age. Vaccine histories were available for 9 of 12 reported cases; two cases had not been vaccinated, six cases had received only their first dose of measles vaccine, and only one of the confirmed cases had reported receiving two doses of the vaccine.

Figure 3 depicts the overall crude measles rates, which have declined in a cyclical manner from 429 per 100,000 in 1924 to 0.04 per 100,000 in 1998.

Source: Bureau of Infectious Diseases, LCDC⁽⁵⁾

In 1995, over 40% of all reported measles cases in the Americas occurred in Canada.⁽⁹⁾ Furthermore, it was estimated that approximately 10% of vaccinated children remained unprotected after a single measles vaccine dose given after 12 months of age.⁽⁴⁾ In 1995, the National Advisory Committee on Immunization (NACI) recommended a second dose of measles vaccine for all Canadian children.⁽¹⁰⁾ By 1997, all provinces and territories had added a second measles dose into their routine immunization schedule,⁽⁴⁾ and by 1998 Canadian measles cases made up only 0.02% of all reported cases in the Americas. Figure 4 depicts the 1997 reported rate of measles in selected countries.

Source: Bureau of Infectious Diseases, LCDC^(5,8,9,12)

Data limitations
The case definition for a confirmed measles case in Canada is very specific. A suspected case must be either laboratory-confirmed or linked to a laboratory-confirmed case before it is reported to the national database. This highly specific case definition may result in an underestimate of the true occurrence of measles in Canada. Similarly, international variations in case definitions, reporting systems and access to laboratory testing limit the conclusions that can be drawn from international comparisons.

Summary
The introduction of a second dose of measles vaccine into the routine childhood immunization schedule was followed by a considerable decline in the incidence of Canadian measles cases. However, the threat of future outbreaks persists as long as susceptible populations remain as a result of incomplete or absent vaccine coverage.

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

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