ARCHIVED - Immunization coverage by age 2 for five recommended vaccines in the Capital Health Region (Edmonton)

 

Canada Communicable Disease Report

15 May 2006

Volume 32
Number 10

Introduction

Following recommendations from the National Immunization Strategy1, Alberta provides universal childhood immunizations for a variety of diseases. As shown in Table 1, the recommended schedule for children up to 2 years of age for these immunizations includes five vaccines providing protection for 11 different diseases. Scheduling depends on the age of the child when immunization was started and, in the case of chickenpox, whether the child has a history of the disease.

In order to assess the quality of the program, the managers of a public health system need to monitor the timeliness of vaccine delivery. Such information can help public health managers evaluate whether they need to take measures to increase coverage. Because of the increased complexity of the routine childhood immunization schedule following the introduction of two new vaccines (meningococcal conjugate in 2001 and pneumococcal conjugate in 2002), coverage rates need to be carefully monitored.

The 2002 national immunization telephone survey provided an estimate of current coverage in Canada for measles, mumps and rubella; and diphtheria, pertussis, tetanus, polio, and Haemophilus influenzae type B Hib2. It indicated 2-year coverage rates of just < 95% for measles, mumps and rubella, and lower rates for immunization against the other conditions. The survey report indicated that there is a need for immunization registries to validate survey results and to provide regional decision-makers with appropriate planning data. Measures of coverage have been provided in two studies at the provincial or regional level. Using electronic records from the Manitoba Immunization Monitoring System, Roberts estimated provincial coverage rates for children at the age of 1 for three DPT and two polio vaccines as 79%3. Hudson et al.4 conducted a telephone survey in Montreal of parents of a sample of 100 children aged 2 to determine coverage with three vaccines - four doses of DPT-Hib, three of polio, and one of MMR vaccine. They estimated complete coverage to be 83.1%.

In the Capital Health region of Edmonton, as in all of Alberta, routine universal childhood vaccines are delivered through the public health division. Administrative immunization data are recorded and stored in a centralized database called Caseworks. For every child born in the region, individual records are created at birth and updated at each contact with the public health system, including vaccine administration. Records of children known to have moved out of the region are inactivated and are therefore not included in coverage rate calculations. Records of children who move into the region are initiated at the time of first contact with the public health system and include historical information, such as previous vaccination events. The purpose of the present study is to use these administrative data to determine the immunization coverage rate, according to the Alberta childhood immunization schedule, of all children in the Capital Health Region as of their second birthday.

Methods

The study population consisted of all children in the Capital Health region of Alberta (2002 boundaries) with records in Caseworks and dates of birth between 1 July and 31 December, 2002 (n = 4,988). Information on the study population's immunization events dating up to 31 December, 2004, was extracted for cleaning and analysis. Missing demographic information was obtained from the Alberta Wellnet Electronic Health Record database.

Immunization coverage rates were evaluated for each of the routine childhood vaccines (DTaP-IPV-Hib, meningococcal conjugate, pneumococcal conjugate, MMR, and varicella). The DTaP-IPV and Hib components of the Pentacel™ vaccine were evaluated separately, as this is the way in which they are entered into the database. Coverage rates were assessed by determining the percentage complete, partially complete, and not vaccinated for each vaccine, according to Alberta Health and Wellness (AH&W) recommendations for minimum ages and intervals between doses (see Table 1). Alternative schedules were taken into account. A child was considered complete for an immunization if he/she received the correct number of doses with adequate spacing between doses, as specified by the AH&W schedule. If a child had started a series of vaccines but did not complete the recommended number of doses or had inadequate spacing between doses, that child was considered partially complete. Finally, a child is considered not vaccinated if he or she has not received any doses of the specific vaccine. Immunization status for each child was assessed on the date of their second birthday. Applicable only to varicella immunization status, the category immune by disease is assigned to those children who did not receive the vaccine but who are considered immune because of a recorded history of disease. These children are not eligible for the vaccine under the AH&W guidelines.

This study received ethical approval through the University of Alberta Medical Faculty Ethics Review Committee.

Table 1. Schedule of childhood immunizations in Alberta

Vaccine

Primary series

Schedule alterations

DTaP-IPV-Hib*

2 months
4 months
6 months
18 months

  • Can be started as early as 6 weeks.
  • Spacing can be shortened to 4 weeks.
  • Fourth dose can be given as early as 15 months provided there are ≥ 6 months between doses 3 and 4.
  • If DTaP-IPV and Hib are given separately, fourth dose of DTaP-IPV can be given as early as 12 months provided there are ≥ 6 months between doses 3 and 4.

For Hib

  • If series starts at 7 to 11 months, two doses spaced 8 weeks apart with a third dose at 18 months (can be given as early as 15 months).
  • If series starts at 12 to 14 months, one dose with a second dose at 18 months (can be given as early as 15 months).
  • If series starts ≥ 15 months, one dose.
Pneumococcal
conjugate

2 months
4 months
6 months
18 months

  • Can be started as early as 6 weeks.
  • Spacing can be shortened to 4 weeks (except when series is started at 12 to 23 months).
  • If series starts at 7 to 11 months, two doses spaced 8 weeks apart with a third dose at 18 months.
  • If series starts at 12 to 23 months, two doses 8 weeks apart.
  • If series starts at ≥ 12 months, one dose.
  • Third and fourth dose can be given any time after 12 months provided there are at least 8 weeks between doses 3 and 4, and doses 2 and 3.
Meningococcal
conjugate

2 months
4 months
6 months
18 months

  • If series starts at 4 to < 12 months, two doses spaced 8 weeks apart.
  • If series starts at ≥ 12 months, one dose.
  • Spacing can be shortened to 4 weeks.
MMR†

12 months

  • If dose administered prior to 1 year, consider
    invalid and give another dose after
    12 months
Varicella‡
(chickenpox)

12 months

 

* Diphtheria, tetanus, acellular pertussis, polio, Haemophilus influenzae type b.
Measles,mumps, rubella.
If no history of disease or not previously immunized

Results

Coverage rates for each immunization are presented in Table 2. Partial completion rates for MMR and varicella vaccines are not applicable since only one dose is required by 2 years of age. Our results indicate that coverage rates vary widely, according to the vaccine. The percentage not vaccinated was highest for varicella (10.9%) and lowest for DTaP-IPV (2.7%). Even though some vaccines (DTaP-IPV-Hib, pneumoccocal conjugate, meningococcal conjugate) could be given at the same visit, the coverage rates were different.

Table 2. Capital Health immunization coverage rates (at 2 years of age, n = 4,988)

  DTaP-IPV
(or equivalent)
n (%)
Hib
n (%)
Meningococcal
conjugate
n
(%)
Pneumococcal
conjugate
n
(%)
MMR
n (%)
Varicella
n (%)

Complete

4,265
(85.5%)

4,278
(85.8%)

4,701
(94.2%)

4,181
(83.8%)

4,641
(93.0%)

4,328
(86.8%)

Partial

590
(11.8%)

570
(11.4%)

107
(2.1%)

560 (11.2%)

N/A - S/O

N/A -
S/O

Not vac-cinated

133
(2.7%)

140
(2.8%)

180
(3.6%)

247
(5.0%)

347
(7.0%)

546
(10.9%)

Immune by
disease

N/A -
S/O

N/A - S/O

N/A -
S/O

N/A - S/O

N/A - S/O

114
(2.3%)


In addition to coverage rates at 2 years of age, immunization status was calculated at the end of the follow-up period (as of 31 December, 2004). A considerable number of children were found to have completed their vaccine series later than age 2. This was especially true for those vaccines that require a fourth dose at 18 months of age. The DTaP-IPV and pneumococcal conjugate completion rates each increased by 3% (to 88.5% and 86.8% respectively) when measured at the end of the follow-up period. Because of unequal follow-up duration we did not compare coverage rates at the end of the follow-up period. It is note-worthy, however, that some of those children who were partially covered at 2 years of age eventually completed their immunization series.

Discussion

We calculated the immunization completion rates for five routine childhood vaccines for children at age 2 in the Capital Health Region of Edmonton. Our results indicate that there is a wide variation in coverage among the recommended vaccines. Complete coverage rates for DTaP-IPV-Hib are higher than for MMR, but there were 11.3% who were partially covered for DTaP-IPV-Hib at 2 years, and who may become covered late.

The national telephone survey2 indicated that there was much higher coverage for MMR than for DTaP-IPV. The MMR coverage rate in the national survey is similar to that reported in this study, but DTaP-IPV coverage is much higher in our survey. However the national survey did not break out partial - from non - coverage, and so but it did not break out partial from non - coverage, and so the results are difficult to compare to those from the present study. Since vaccine regimens have changed over time, and coverage may vary by province/territory. These results support the need for registries as a tool for regional planning.

Coverage for DTaP-IPV-Hib is not representative of immunization coverage for all the vaccines made available by the Alberta government. Although the statistic of DPT coverage is used as a national and international standard5, it is not indicative of the complete childhood immunization picture. As well, the 2-year measure is not entirely indicative of the degree of immunization. Many children who are partially covered at age 2 do eventually complete their immunization series, although they are potentially susceptible to the vaccine-preventable disease for longer. It would be important, therefore, to follow up this analysis as the cohort increases with age.

There are two limitations of this study. First, although the Caseworks database captures the vast majority of children living in the Capital Health region, there is the possibility that some children in this 6-month birth cohort were not included in the sample. In particular, children who have moved into the region whose families have not made contact with the public health system would not be included. Second, the possibility of a cohort effect exists, although there is no reason to suspect that the cohort sampled in this study differs significantly from other children in the region.

Using our data, we could not determine the reason why immunization coverage varied among vaccines. Nevertheless, it would appear that assumptions cannot be made about vaccine uptake for all vaccines on the basis of the indicator vaccine's uptake. Vaccine schedules are becoming more complex, and new vaccines continue to be added to routine immunization schedules. Our data suggest that there are differences in uptake that need to be carefully monitored, and the reasons for underimmunization should be determined so that appropriate interventions can be implemented.

Acknowledgements

Funding for this project was provided by the Institute of Health Economics, Research Coordination Committee.

References

  1. Advisory Committee on Population Health and Health Security. National Immunization Strategy: Final report 2003. Ottawa: Minister of Health, 2004.

  2. McWha L, MacArthur A, Badiani T et al. Measuring up: Results from the National Immunization Coverage Survey, 2002. CCDR 2004;30(5):37-50.

  3. Roberts JD, Poffenroth LA, Roos LL et al. Monitoring childhood immunizations: A Canadian approach. Am J Public Health 1994;84:1666-8.

  4. Hudson P, Allard R, Joseph L et al. Vaccine coverage of 2-year-old children in Montreal - 2003. CCDR 2005;31(5):166-7.

  5. World Health Organization. Make every mother and child count:World Health Report 2005. Annex Table 7. Geneva: World Health Organization, 2005.

Source : A Brown-Ogrodnick, MSc, A Hanrahan, MN, J Loewen, BSN, Communicable Disease Control, Capital Health, Edmonton; T Nguyen, BS, Health Surveillance, Alberta Health and Wellness; P Jacobs, PhD, A Ohinmaa, PhD, W Vaudry, MD, Faculty of Medicine, University of Alberta; R Richardson, RN, First Nations and Inuit Health Branch, Health Canada.

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