ARCHIVED - Canadian National Report on Immunization, 2006

Volume: 32S3 - November 2006

2. Vaccine Coverage

2.1 Background and objectives

Immunization is considered to be among the most cost-effective public health interventions available ¹. The NIS works with the jurisdictions to set national goals for vaccine coverage in order to promote the high levels of coverage that are required to prevent and control vaccine-preventable diseases, and it supports National Immunization Coverage Surveys (NICS). The measurement of immunization coverage rates is required to monitor the effectiveness of immunization programs and progress towards national goals, and these rates are a sensitive indicator of the health of a population and the capacity of a health system to deliver essential services².

NICS are implemented every 2 years by IRID in order to assess national coverage rates for routine childhood immunizations and for select adult immunizations during alternating years. Questions on knowledge, attitudes and beliefs (KAB) towards immunization were added to the NICS from 1998 onwards. The purpose of these surveys is to monitor immunization coverage levels over a number of years; to assess up-to-date and on-time immunization coverage levels; to evaluate changes in KAB; and to monitor progress towards national immunization coverage goals.

In 1994 and 1996, surveys were mailed to households to assess national coverage rates for routine childhood immunizations among 2-year-olds. In 1997 this method was repeated with the addition of a cohort of 7-year-olds, and parental KAB towards immunization were assessed. In 2002, the methodology of the NICS was re-designed to employ a telephone survey of households with children 2 years of age (24 to 36 months) and 7 years of age^3,4.

2.2 Methodology

In 2004, the telephone survey methodology was modified slightly from the 2002 survey: the range of the cohort of children aged 2 was broadened from 24 to 36 months to 20 to 40 months to facilitate data capture; a 17-year-old cohort was added; four new publicly funded vaccines under the NIS were added to the list of routine childhood immunizations; and the KAB section was expanded.

The primary objective of the 2004 NICS was to estimate routinely recommended childhood immunization coverage rates of children by the second birthday (i.e. on or before the child's second birthday), by the seventh birthday and by the seventeenth birthday⁵. This differs from the method used in the 2002 NICS, which assessed coverage for children aged 2 (between the second and third birthdays), 7 (between the seventh and eight birthdays) and 17 (between the seventeenth and eighteenth birthdays). The coverage assessment methodology was changed to reflect current coverage assessment standards for age⁵ and to ascertain whether children had been immunized in accordance with NACIrecommended immunization schedules. The results are compared with the 1997 and 2002 NICS results.

Secondary objectives, such as assessing the circumstances surrounding immunization and parental knowledge and attitudes with respect to certain immunization issues, were also measured and the results will be published in a future Canada Communicable Disease Report (CCDR) publication.

Sample selection and data collection

Respondents were selected from the Ipsos-Reid's Canadian Household Panel and supplemented using random digit dialing. Eligible households were those that included a child between 20 and 40 months of age, 7 to 8 years or 17 to 18 years of age as of the date of survey administration. Respondents were selected from seven regions (British Columbia, Alberta, Manitoba and Saskatchewan, Ontario, Quebec, the Atlantic Provinces, and the territories).

Computer-assisted telephone interviews were performed by trained interviewers in both English and French from 22 September to 18 October, 2004. Questions were asked of the member of the household who was reported to be most familiar with the child's immunization history.

Data analysis

Data extraction and preliminary data analysis were performed by Ipsos-Reid, with further analysis done by IRID using SPSS (Statistical Package for the Social Sciences). For respondents who did not have a copy of their child's immunization record at the time of interview and who answered questions from recall, the results were excluded from the analysis. With the respondent's permission, IRID validated the information collected during the survey with records from physicians or local public health authorities. All national coverage estimates have been assessed according to the NACI-recommended schedule at the time of survey implementation. The results may vary when assessed according to provincial and territorial schedules.

2.3 Results

In total, interviews were conducted with 499 parents of children aged between 20 and 40 months, 546 parents of children 7 years of age and 552 parents of children 17 years of age. The sample was weighted using Canadian Census population proportions for each region.

The following results are based on the analysis of responses from parents reporting from immunization records only for 2004 (N2-year= 431; N7-year = 441; N17-year= 381). The results are considered accurate to within 4.2%-4.4%. The margin of error will be larger within regions and for other sub-groupings of the survey population.

Table 2 compares immunization coverage results from national coverage surveys from 1997, 2002 and 2004. Caution must be taken when comparing results, as methodologies have changed over time.

Measles, mumps and rubella (MMR)

Coverage estimates for a single dose of the MMR vaccine by the 2nd birthday is 94%, which is close to the national goal of 97%. This is similar to the 2002 estimate of 93%. Coverage for the second dose of measles by the 7th birthday is 79% and 93% for one dose of mumps and rubella. Coverage for this age group is similar to the 2002 levels but falls short of the recommended national goal of 97%. Coverage for a second dose of measles is lowest in the 17-year-old group, at 62%. Rubella and mumps coverage remains relatively high for one or more doses, at 93% for both by the 17th birthday.

Diphtheria, pertussis, tetanus, polio and DTaP-polio-Haemophilus influenzae type b (Hib)

This family of pertussis-containing vaccines can be given as a single pentavalent vaccine (Pentacel^TM) or in combination as a quadrivalent (Quadracel^TM) vaccine with Hib vaccine. The variability of reported doses across the five antigens is significant, and coverage estimates for these five antigens will be presented individually.

Compared with 2002, coverage estimates by the 2nd birthday cohort in the 2004 NICS have remained approximately the same for diphtheria (2002: 77%; 2004: 78%), pertussis (2002: 75%; 2004: 74%), tetanus (2002: 74%; 2004: 73%) and polio (2002: 88%; 2004: 89%). Coverage estimates for Hib have increased the most in this group (2002: 64%; 2004: 73%); however, they still remain well below the nationally recommended target of 97% for this age group.

Coverage estimates for the sixth dose or booster by the 17th birthday of age for diphtheria and tetanus were both well below nationally recommended targets, at 47% and 44%, respectively. Coverage estimates for the acellular pertussis sixth dose booster was also low, at 23%. This result was expected, as the majority of jurisdictions introduced their adolescent programs for acellular pertussis in late 2003 and 2004, so the survey captured only those immunizations that began before 2003 through the initial programs in Nunavut, Newfoundland and the Yukon.

2.4 Discussion

Overall the results of the NICS survey are encouraging. Preliminary results using age range instead of up-todate status by the second or seventh birthday demonstrated improvement across all antigens for the 2-year and 7-year cohorts ⁶. Applying the national standard for up-to-date status by second, seventh or seventeenth birthday reduced coverage levels for most of the antigen, as children past their eligible birthday were excluded from the numerator. Despite this change in methodology, the majority of antigens still showed modest increases in coverage.

Ninety percent of parents in all age cohorts believed that their children were up to date for immunizations for their age group according to their provincial/territorial records, the most commonly cited reason/challenge for keeping their children up-to-date being “remembering to have it done/to make an appointment” (11% overall). However, further analysis shows that only 61% of 2-year-olds and 41% of 7-year-olds were up to date for the NACI recommended number of doses for the combination of DTaP-IPV-Hib and MMR vaccinations by their second and seventh birthdays, respectively.

Other new vaccines, including those recently introduced through NIS funding, were added to the NICS questionnaire in 2004. Considering the variability of jurisdictional schedules and the time required to roll out new programs, 2004 coverage estimates for influenza, meningococcal C conjugate, pneumococcal conjugate and varicella vaccines, shown in Table 2, should be considered as a baseline for comparison with future coverage estimates.

Table2. Routine childhood immunization coverage, National Immunization Coverage Survey, 1997, 2002 and 2004

There are two potential limitations of the NICS. First, responses may be subject to errors of reporting or recording on immunization records. The validation component of the 2004 NICS will assess accuracy of parental reporting from immunization records compared with medical records. Second, the household panel used in both the 2002 and 2004 NICS surveys is a convenience sample. Although it is nationally representative, the sample may underrepresent special populations, including First Nations persons living on reserves, single-parent families, households in which the first language is not an official language, those without telephones or those with only cellular phones. Immunization coverage surveys, with their limitations, will continue to be used to assess national and jurisdictional coverage rates until electronic immunization registries (the “gold standard” for coverage assessment) are consistently used to record and store immunization records across the country.

A full report of the 2004 NICS, including the validation component and analysis of parental KAB, will be published in CCDR in 2006. The adult NICS has been implemented in the spring of 2006.