THE JOURNAL OF THE CANADIAN PAEDIATRIC SOCIETY

FEATURE ISSUES

Vaccine coverage

Since 1994, the Division of Immunization, LCDC has collected national population-based estimates of vaccination coverage among Canadian children through annual mail surveys (1). Coverage is estimated for routinely recommended childhood vaccines against eight diseases: diphtheria, pertussis, tetanus, poliomyelitis, measles, mumps, rubella and Hib infections. This report provides results from the 1998 survey of two-year olds (July 1995 to June 1996 birth cohort) and seven-year olds (July 1990 to June 1991 birth cohort). Up-to-date coverage is defined as the receipt of the number of doses recommended by NACI by the age of assessment (2). Two sets of coverage estimates are provided for two-year olds surveyed in 1997 and 1998: coverage at two years of age (based on all vaccine doses received) for comparison with previously surveyed cohorts, and coverage by the second birthday (based on vaccine doses received before or on the date of the child’s second birthday) for evaluation of national targets for vaccine coverage (3). Coverage estimates for seven-year olds are only based on vaccinations received by the seventh birthday (based on vaccine doses received before or on the date of the child’s seventh birthday) for evaluation of the national targets.  

COVERAGE AMONG TWO-YEAR OLDS

Results of the 1998 survey show stable coverage levels for MMR vaccines compared with previous surveys (Table 8). The coverage levels for one dose of measles (96.0%), mumps (95.4%) and rubella (95.3%) vaccines are the highest for any of the routinely recommended vaccines and the closest to national targets for 97% coverage by the second birthday (3). However, as reported for previous cohorts, a small proportion of children (2.7% by the second birthday) continue to receive their first dose measles vaccination before their first birthday, contrary to the NACI recommendations.

TABLE 8: National estimates of vaccination coverage among two-year olds, Canada, 1994 to 1998
	Percentage of coverage* at two years of age by birth cohort surveyed†						Percentage of coverage* by the second birthday by birth cohort surveyed‡
Vaccine (number of doses)	1990- 1991	1991- 1992	1992- 1993	1993- 1994	1994- 1995	1995- 1996	1994- 1995	1995- 1996
Diphtheria (4)	84.7	84.0	84.4	87.1	86.8	84.2	84.2	80.4
Pertussis (4)	80.1	81.6	82.9	84.8	85.2	83.0	83.0	79.2
Tetanus (4)	82.0	82.5	83.9	85.9	85.1	83.8	82.9	80.4
Polio (>=3)	89.7	89.0	87.4	89.9	85.8	90.1	85.3	89.8
Measles (>=1)§	96.1	97.2	96.2	97.0	96.0	96.2	95.1	96.0
Measles (>=1)¶	–	–	91.4	93.3	94.1	93.6	93.0	93.3
Mumps (>=1)	92.8	93.6	96.0	96.8	95.9	95.6	95.1	95.4
Rubella (>=1)	93.0	94.4	96.0	96.7	95.9	95.5	95.0	95.3
Haemophilus influenzae type b (4)	–	–	54.6	69.3	73.7	74.9	71.5	72.6
*95% confidence limits range between ± 1 and 5; †Coverage at two years of age is based on all vaccine doses received by children aged 24 to 35 months; ‡Coverage by the second birthday is based on ONLY those vaccine doses received before or on the date of the child’s second birthday; §Coverage based on measles vaccine dose(s) received at any time; ¶Coverage based on measles vaccine dose(s) received on or after the first birthday, as recommended

Coverage for three or more doses of polio vaccine was estimated as 89.8% by the second birthday, approximately 7% below the national target. Although a slight decrease in polio coverage was reported in the 1997 survey (1), results from the 1998 survey do not show a significant drop in polio coverage and the overall trend is fairly stable. Coverage levels for diphtheria, pertussis and tetanus in the 1998 survey were reported as approximately 84% at two years of age and 80% by the second birthday; these levels are slightly lower than those found in previous surveys.

The estimated coverage of 72.6% for the Hib conjugate vaccine by the second birthday in the 1998 survey is noticeably lower than levels for the other childhood vaccines. This suggests that inaccurate record keeping of Hib vaccination continues to be a problem given that the majority of provinces and territories have routinely used the five-component PENTA (diphtheria, tetanus, pertussis, inactivated polio and Hib) (Pasteur Mérieux Connaught, North York, Ontario) vaccine since the end of 1995; all provinces and territories have used PENTACEL (diphtheria, tetanus, acellular pertussis, inactivated polio and Hib) (Pasteur Mérieux Connaught) since 1998.

COVERAGE AMONG SEVEN-YEAR-OLDS

The highest coverage levels by the seventh birthday are for first dose MMR vaccination (Table 9). Two-dose measles coverage by the seventh birthday was estimated in the 1998 survey at only 66.5% (62.7% for both doses received after the first birthday) despite recent catch-up programs and a two-dose vaccination schedule for which most of the children surveyed (1990 to 1991 birth cohort) were eligible. Available information suggests that the national two-dose measles coverage is likely an underestimate due to a combination of factors: an average of 90% coverage of targeted children in the nine provinces and territories (93% of Canadian population) where catch-up programs were implemented; limited catch-up of preschoolers in some jurisdictions; and, most important, poor recording of catch-up doses in regular immunization records during school-based programs and/or a lack of immunization monitoring and updating at school entry in some jurisdictions. Up-to-date coverage levels for diphtheria, pertussis and tetanus by the seventh birthday remain low and range from 16% to 19% below the national targets. As reported in previous surveys, despite improved primary immunization levels (four doses), the evidence remains strong that school-entry booster vaccinations are considerably delayed. This potentially leaves selected children at risk of remaining underimmunized. Up-to-date coverage for polio is slightly higher than coverage for diphtheria, pertussis and tetanus but is still about 9% below the national target. Hib coverage for the 1990 to 1991 birth cohort was 87.7% by the seventh birthday. This birth cohort was eligible for the previously used Hib polysaccharide vaccine administered as a single dose.

The national guidelines for childhood immunization practices provide recommendations for improving vaccination rates among children (4). Sustained efforts are required to maintain the high levels of vaccine coverage achieved for selected vaccines and to improve the age-specific coverage levels that remain below our national targets. This requires identification and targeted catch-up immunization of individual children as well as of groups of children who remain underimmunized.

TABLE 9: National estimates of vaccination coverage by the seventh birthday, Canada, 1997 to 1998
Vaccine	Number of doses	Percentage of coverage* by the seventh birthday by birth cohort surveyed†		Number of doses	Percentage of coverage* by the seventh birthday by birth cohort surveyed†
		1989-1990	1990-1991		1989-1990	1990-1991
Diphtheria	4	94.5	96.9	5	78.7	81.0
Pertussis	4	90.9	95.3	5	74.9	78.9
Tetanus	4	93.1	96.0	5	76.8	79.6
Polio	>=3	95.4	96.9	>=4	85.1	90.8
Measles‡	>=1	98.8	99.4	2	55.9	66.5
Measles§	>=1	97.6	98.3	2	50.2	62.7
Mumps	–	–	–	>=1	96.7	96.4
Rubella	–	–	–	>=1	97.2	96.8
Haemophilus influenzae type b	–	–	–	>=1	86.2	87.7
*95% confidence limits range between ±1 and 5; †Coverage by the seventh birthday is based on only those vaccine doses received before or on the date of the child’s seventh birthday; ‡Coverage based on measles vaccine dose(s) received at any time; §Coverage based on measles vaccine dose(s) received on or after the first birthday, as recommended

REFERENCES

1. LCDC. Canadian National Report on Immunization, 1997. Paediatr Child Health 1998;3(Suppl B).

2. National Advisory Committee on Immunization. Canadian Immunization Guide, 5th edn [cat. no. H49-8/1998E]. Ottawa: Supply and Services Canada, 1998.

3. Laboratory Centre for Disease Control. National goals and objectives for the control of vaccine-preventable diseases of infants and children. Can Commun Dis Rep
1995;21:49-54.

4. National Advisory Committee on Immunization. National guidelines for childhood immunization practices. Can Commun Dis Rep 1997;23:1-12.

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