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Volume: 23S4 - May 1997

Canadian National Report on Immunization, 1996

5. Measles Elimination in Canada

Measles is a severe respiratory tract infection frequently complicated by pneumonia, croup, sinusitis, otitis media, and febrile convulsions. It is the most contagious infection of humans. Every year, nearly one million children die from it worldwide. Available vaccines are safe and effective. About 90% of children vaccinated after the first birthday develop protective immunity.

The routine one-dose measles immunization program, introduced in Canada in the mid-60s, has had a very positive effect on the incidence of measles; > 95% reduction from the pre-vaccine era. Before immunization, an estimated 300,000 to 400,000 cases of measles occurred annually in Canada. Several measles outbreaks have made the limitations of Canada's one-dose program apparent: one in Quebec in 1989 with 10,184 reported cases, and one in Ontario in 1991 with 5,283 reported cases. The actual number of cases was likely much higher as the reporting rate was estimated to be only 25% to 30%. Despite stable vaccination coverage levels of around 97% for 2-year-olds, these outbreaks continued to occur mostly in school-aged children even in populations with virtually 100% documented immunization. The spread of the virus was likely caused by the small proportion of children who failed to respond to primary vaccination or, less commonly, by those who lost protection over time after vaccination. It was increasingly clear that a routine one-dose program administered after 12 months of age would not achieve the goal of eliminating indigenous measles because of its extreme contagiosity. This was strongly supported by the international experience. The typical pattern of measles in highly-vaccinated populations is one of outbreaks at extended intervals involving 1% to 5% of school children, with a spillover into pre-school children. Control measures such as exclusion from school and emergency mass revaccination are extremely disruptive, costly, and of limited effectiveness. The administration of a second dose of measles-containing vaccine has been shown to diminish the proportion of susceptible children, thus decreasing the potential for outbreaks.

Despite the fact that participants of a National Conference on Measles Control in 1992 endorsed the goal of eliminating indigenous measles in Canada by 2005⁽⁶⁾, little progress had been made. Competing developments in childhood vaccination programs pre-empted the formal introduction of a two-dose measles program in Canada. In 1995, with only 3.6% of the population in the Americas, Canada accounted for 40% of all reported cases and nearly 80% of all confirmed cases. Other countries in the Americas have recently conducted highly effective mass measles-vaccination campaigns or previously implemented routine two-dose programs for many years. The approach taken in Canada was the least effective. Compared with 1993, when the lowest level of measles activity ever recorded in Canada was 204 reported cases, the number of cases rose steadily to 512 in 1994 and 2,362 in 1995.

Analysis of the situation in Canada suggested that sufficient numbers of unprotected children existed in every province to fuel outbreaks at any time. It was estimated that, without action, an outbreak with more than 20,000 cases, 2,000 complications, and several deaths could occur as early as April 1996. Mathematical modelling and a Delphi study predicted that there were enough susceptibles in the population to have an average of 12,800 cases of measles per year. Mathematical modelling also predicted that giving a second dose only to young children would not eliminate measles for 10 to 15 years and would be inconsistent with the elimination targets. A national catch-up campaign would be the only way to avoid forecasted outbreaks as well as to prevent up to an additional 58,530 cases per year and several deaths. Cost-benefit analysis indicated that these programs would save in excess of $2.5 per dollar invested. This situation prompted the endorsement in December 1995 of a formal, politically-endorsed, national goal of eliminating measles.

In August 1995, NACI reaffirmed its commitment to the goal of eliminating measles⁽⁷⁾: this goal is shared by all countries of the Americas. The committee also confirmed its recommendation that a second dose of measles vaccine should be offered, at least 1 month after the first dose, on a routine basis to raise protection rates as high as possible. This dose could be conveniently linked with other routinely scheduled vaccinations. Options included giving it with next-scheduled vaccinations at 18 months of age, or with school-entry vaccinations at 4 to 6 years of age, or at any practicable intermediate age. NACI also recommended that, for the earliest elimination of measles, a second dose of measles vaccine should be provided as part of special catch-up programs to all children and adolescents previously immunized under the one-dose schedule. The principal target group for a catch-up campaign is school children because they have had the highest rates of measles in recent Canadian outbreaks, and are most readily identified and served.

Following NACI's recommendation, Health Canada encouraged a massive catch-up vaccine campaign over a short period to be followed by routine two-dose immunization. All provinces (except New Brunswick) and territories, which represent 97% of the Canadian population, have since introduced a routine second measles, mumps, and rubella (MMR) vaccination (measles and rubella [MR] in Saskatchewan to be replaced by MMR after its catch-up program is over) at either 18 months or 4 to 6 years of age, depending on the province. New Brunswick will implement such a program starting in April 1997. Six provinces and territories (Ontario, Quebec, British Columbia, Prince Edward Island, Yukon, and Northwest Territories), which represent 80% of the Canadian population, have already completed mass school catch-up programs for all school-aged children. In Quebec and British Columbia, catch-up programs have been extended for children down to 18 months of age. However, these programs have a lower priority than those for older age groups and catch-up will likely not be completed until these children enter school. A more limited catch-up program was also started in Manitoba to include all primary-school students and in Saskatchewan to include all children > 18 months of age up to the end of school age, and to be completed over a 3-year period in a staggered manner.

Public-health nurses conducted catch-up campaigns in schools, after careful planning and public awareness campaigns. Coverage levels have reached around 90% on average in targeted school-age groups in provinces that have had catch-up campaigns. Nearly 4 million children have been immunized. Although some mass immunization programs had been implemented in the past for invasive meningococcal disease, they were more limited, mostly on regional and provincial levels. This is the first national campaign of such magnitude in Canada. It has had an immediate effect; three potential outbreaks that were developing in early 1996 were stopped. Only 327 cases of measles have been reported to date for 1996; these mainly occurred before the catch-up campaigns in the largest provinces. Very few cases have occurred since May and transmission seems to have been interrupted. A total of 12 imported cases have been identified, mostly from European countries. In provinces that have not yet implemented catch-up campaigns, school-aged susceptible populations still remain in sufficient numbers to fuel outbreaks through importations.

Retrospectively, the heavy measles activity that occurred early in 1996, with 2.5 times the number of cases than that reported for the corresponding period in 1995, and the number of outbreaks indicate that the prediction of a large outbreak occurring after April 1996 was likely true; the provincial campaigns were very timely (Figures 2 and 3).

Surveillance is extremely important and must continue in a very active manner. Several evaluation and surveillance activities related to the catch-up campaigns have been implemented; these include disease surveillance, surveillance of vaccine-associated adverse events, monitoring of coverage target achievement, assessment of the process and cost (the overall estimated cost was around $8.30 per targeted child), and evaluation of promotion activities.

A survey of promotional activities in Ontario and British Columbia, conducted by the Division of Immunization, showed that the distribution of a leaflet in schools appears to have been the most common and useful source of information. The knowledge of attitudes toward, and practices of, measles immunization were strikingly similar in both provinces; notable differences were in the sources of information and those that were the most useful. Promotional materials in British Columbia were more varied because its campaign covered school-aged children and pre-schoolers; the leaflet was only 60% effective in British Columbia as compared to 80% in Ontario. During the campaign, a high proportion of parents changed their opinions about immunization and recognized the importance of measles immunization, presumably as a result of the promotional materials. When parents in both provinces were asked to rank five infectious diseases (including measles) in order of decreasing severity, most replied in the following order: hepatitis B, measles, pertussis, varicella, and influenza.

Health Canada provided technical assistance, and helped to plan provincial activities and evaluate the mass catch-up campaigns. Health Canada also produced turn-key material for public awareness campaigns, and helped to reduce vaccine costs through competitive soliciting and speedy licensing of products needed for the catch-up campaigns.

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