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Current Trends Measles -- North America, 1984
Since the introduction of measles vaccine in North America, Canada, Mexico, and the United States have made great strides in reducing morbidity and mortality associated with measles illness.
In Canada,* live attenuated measles vaccine came into limited use in 1964, and more broadly based vaccination programs were instituted in each province over the next 4-5 years. The incidence rate fell from 358 cases per 100,000 population during 1948-1958 to 51/100,000 during 1969-1978, an 86% reduction (1). In 1981 and 1982, two of the 10 provinces (Ontario and New Brunswick, representing 39% of Canada's population) adopted school immunization laws requiring immunization against measles and five other vaccine-preventable diseases for school entry. In 1985, a third province (Manitoba) has also made measles vaccination a condition of school entry. Other provinces maintain a voluntary approach. All have achieved a vaccination coverage in excess of 90% at school entry (2). In 1981, the Canadian National Advisory Committee on Immunization has recommended a compulsory school measles vaccination policy for all provinces (3).
In Mexico,** measles vaccine became available in 1970, and the country's National Program of Vaccination against the disease formally began in 1973. By the early 1980s, the program's routine immunization in health centers and clinics and intensive house-to-house campaigns had afforded a coverage of approximately 70% of susceptible preschool-aged children. The reported incidence rate of measles has fallen from an average of 90 cases/100,000 population in the early 1970s to 10-20 cases/100,000 in the 1980s. Reported measles mortality has also fallen, from 2,609 deaths in 1973 (4.8/100,000 population) to 824 in 1981 (1.1/100,000) (4).
Measles vaccine was licensed in the United States*** in 1963, and in October 1978, a goal was established to eliminate indigenous measles from the United States. The annual number of reported measles cases has declined from about 500,000 cases in the prevaccine era to fewer than 5,000 cases since 1981. The reported incidence rate has also fallen from 315 cases/100,000 population in the prevaccine era to less than 1.5/100,000 since 1981. By the 1981-1982 school year, 96% of school enterers were vaccinated against measles (5).
In 1984, the number of reported measles cases increased for the first time in several years in Canada, Mexico, and the United States.
In Canada, provisional data indicate that 4,125 cases were reported during 1984, an incidence rate of approximately 16/100,000 population. This was the first significant increase in measles activity since the epidemic of 1980 (Figure 4), and represents a fourfold increase compared to 1983. Three of the 10 provinces accounted for almost all cases: Ontario (36%), Quebec (32%), and British Columbia (28%). More than 75% of cases occurred among children 5-19 years of age (Table 3). From 1983 to 1984, the greatest rise in the age-specific incidence rates was also in the school-aged population, with approximately fivefold, tenfold, and eightfold increases in the age groups 5-9 years, 10-14 years, and 15-19 years, respectively (2).
In Mexico, 5,158 cases were reported during 1984, an incidence rate of 6.6/100,000 population. This compares with the 3,368 reported cases for 1983 (Figure 5) and the incidence rate of 4.6/100,000. Five of the 32 districts accounted for 53.7% of cases: Jalisco (684), Nayarit (675), Distrito Federal (602), Michoacan (598), and Oaxaca (212). Fifty-six percent of patients were preschool-aged; 36% were 5-14 years old; and 8% were 15 years of age or older (Table 3). The highest age-specific incidence rate was 27.7/100,000 for children under 1 year of age, who accounted for 16% of total cases. The age distributions for 1983 and 1984 were similar.
Measles data for the United States in 1984 were recently reported in detail (6). Briefly, a provisional total of 2,534 cases was reported. This is a 69% increase from the 1,497 cases reported in 1983 (Figure 6). The incidence rate during this period rose from 0.6/100,000 population to 1.1/100,000. Eighty-two percent of cases were reported from seven states: Texas (602), Michigan (465), California (330), Illinois (182), Washington (172), New York (165), and Hawaii (163). The greatest increase in age-specific incidence rate from 1983 to 1984 occurred in the 10- to 14-year age group, from 1.1/100,000 to 3.8/100,000 (Table 3). Reported by P Varughese, Bureau of Epidemiology, Laboratory Centre for Disease Control, Ottawa, Ontario, Canada; JH Burguete, MD, Director of Epidemiologic Surveillance, Ministry of Health, Mexico; Div of Immunization, Center for Prevention Services, CDC.
Editorial Note: In 1984, the United States, Canada, and Mexico experienced substantial increases in the number of reported measles cases compared to 1983. The increases ranged from 53% in Mexico to a fourfold increase in Canada. The absolute figures for these countries may not be comparable with one another because reporting efficiencies vary. Nevertheless, they demonstrate a common trend toward measles control in the continent in spite of some increase in measles activity throughout North America in 1984.
Canadian health officials have attributed the increase in that country to clusters of measles cases in individuals who are susceptible for a variety of reasons, including the earlier practice of vaccination below the age of 12 months, the use of killed measles virus vaccine in some of the older children, unimmunized populations refusing vaccination for religious reasons, and the importation of measles cases (2). The high proportion of patients 5-9 years of age in Canada (38%), compared to the United States (11%), suggests that lower vaccination levels in the first few grades of school may also be an important factor.
In contrast to Canada and the United States, where the greatest number of measles cases is seen among school-aged children, the measles problem in Mexico primarily involves infants and children under 5 years of age. Therefore, the current measles control strategy in Mexico focuses on young children and is based on routine vaccination throughout the year of children brought to health centers and clinics, reinforced by intensive house-to-house programs of immunization. The large number of small villages with scattered populations and shortages of vaccine have been identified as obstacles to improving coverage (4).
In the United States, 66% of cases are considered nonpreventable,**** reflecting coverage levels in excess of 90%. Most cases classified as preventable are among school-aged children, although the highest proportion of preventable cases occurs among preschool-aged children above the age of 15 months and in young adults. This illustrates the need for additional emphasis on: (1) age-appropriate vaccination of preschool-aged children; (2) continued vigorous enforcement of school immunization laws; and (3) vaccination of susceptible postschool-aged persons (particularly those in congregate settings, such as colleges) (6).
While increases in the measles incidence rate were seen in all three countries in 1984, the reported occurrence was still substantially less than in recent years. As recently as 1979, each of the three countries reported more than 10,000 measles cases. All three countries have made substantial progress in controlling measles by vaccination. Continued application of the current strategies should result in further decreases. Recent concurrent establishment of measles elimination goals in Europe provides reason to hope that Europe and North America will be the first two continents to eliminate indigenous measles transmission.
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