A Health Surveillance Update on Canadian Children and Youth

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

Vaccine Coverage

Vaccine coverage at two years of age for DPT and polio has shown small but steady increases in the successive birth cohorts surveyed to date.

Accurate assessment of vaccine coverage is important in the monitoring and evaluation of immunization programs at the national, provincial and local levels. The annual National Vaccine Coverage Survey has provided vaccine coverage estimates for two-year-old children (aged 24-35 months) since 1994 and for seven-year-old children since 1997. Vaccine coverage is assessed for routinely recommended childhood vaccines against eight diseases: diphtheria, pertussis and tetanus (DPT); poliomyelitis; measles, mumps and rubella (MMR); and Haemophilus influenzae type b (Hib) infections.^(2,3) Coverage for each vaccine is calculated as the percentage of children in the age category who have received the age-appropriate number of doses of that vaccine (as recommended by the National Advisory Committee on Immunization).⁽⁴⁾

In 1998, coverage at two years of age (1995-1996 birth cohort) was 83%, 90%, 94% and 75% for DPT, polio, MMR and Hib respectively.⁽⁵⁾ Coverage at seven years of age was 79%, 91% and 88% for DPT, polio and Hib vaccines respectively. The Hib vaccine coverage for seven-year-olds is based on the single-dose polysaccharide vaccine used before 1992, whereas that for two-year-olds is based on the 4-dose Hib conjugate vaccine introduced in 1992. One-dose coverage with MMR at seven years of age was 98%.

Figure 1 depicts the national coverage for routine childhood vaccines at two years of age by birth cohort from 1994 until 1998. Coverage for DPT and polio at two years of age has shown small but steady increases in the successive birth cohorts surveyed to date. MMR coverage, defined as at least one dose of a measles vaccine received on or after the first birthday, ranges from 92-94%.

* 95% confidence limits range between +/- 1% - 5%
** based on measles coverage with first dose administered at any time
*** based on measles coverage with at least one dose administered on or after the first birthday as recommended
Source: Bureau of Infectious Diseases, LCDC^(2,3,5)

International comparisons for vaccine coverage are limited. Figure 2 compares Canadian vaccine coverage for two-year-old children with vaccine coverage reported from the United States National Immunization Survey for children aged 19-35 months.⁽⁶⁾ Coverage is similar for DPT, polio and a measles-containing vaccine. However, the reported 3-dose coverage for Hib is slightly higher for children in the United States. These comparisons must be interpreted with caution, as the United States household survey includes data verification from vaccine providers, resulting in more complete data.

Source: Bureau of Infectious Diseases, LCDC^(5,6)

Data limitations
The data from the National Vaccine Coverage Survey do not provide reliable provincial, territorial or regional estimates for the identification of sub-populations with low vaccine coverage. As well, the survey data do not allow a detailed assessment of age-appropriate immunization, such as the correct spacing of multiple-dose vaccinations. Finally, collected data are not verified with vaccine providers and rely on the accuracy and completeness of immunization records provided to parents.

Summary
While national coverage for MMR remains high, coverage for DPT, polio and Hib remains below the national targets for the control of these vaccine-preventable diseases.⁽⁷⁾ Ongoing surveillance of vaccine coverage at the national level is required to track the progress made in these areas. As well,  the impact of new vaccines, such as the acellular pertussis vaccine, on overall vaccine coverage should be monitored.

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

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