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Guidelines for measles control in Canada.

M easles control is a high priority throughout Canada. Provincial and territorial immunization programs are in place to ensure high vaccination coverage among children. The effectiveness of the vaccine has generally been shown to be greater than 90%, and in recent years large areas in Canada and the United States either have remained measles free or have had only a few cases. Nevertheless, the occurrence of confirmed cases demonstrates the continuing presence of the measles virus. Sustained outbreaks have occurred, and a major resurgence of measles was documented in Canada and the United States in 1989.1,2 Primary and secondary vaccine failures have been documented. Primary failure is due to (a) vaccination in the presence of maternal antibody (i.e., in infants before 1 year of age), (b) improper handling or administration of the vaccine or (c) unknown causes. New stabilizers, introduced in 1980, and better attention to proper refrigeration likely have reduced the number of problems associated with improper handling. Secondary vaccine failure is due to waning immunity,3 but the proportion of people in whom this occurs appears to be small. If waning immunity were a significant problem one would expect the attack rate to increase steadily with age. This has not been observed.2 Even though vaccine coverage has been documented to be as high as 100% transmission of the measles virus can still occur. This may be because of transmission within the 2% to 10% of vaccinated people who have primary vaccine failure, occasional cases of secondary vaccine failure or poor immunization records and follow-up procedures. The following guidelines are intended to assist provincial and territorial epidemiologists and local health departments in developing strategies to control measles within their jurisdictions. The three main areas are immunization, surveillance (of cases and nonvaccinated people) and outbreak control.