Vaccination patterns in pregnant women during the 2009 H1N1 influenza pandemic: a population-based study in Ontario, Canada.

Liu, Ning ; Sprague, Ann E. ; Yasseen, Abdool S., III 等

Pregnant women are at an increased risk of influenza infection, and once infected also face an elevated risk of illness, complications and death. (1) Evidence from seasonal influenza epidemics and influenza pandemics consistently demonstrate disproportionally high morbidity and mortality among pregnant women. (2-4) In the 2009 H1N1 pandemic, despite their composition of only 1% of the population of the United States, pregnant women accounted for 5% of influenza A (H1N1)-related death in the country. (2) In the Canadian population, hospital admission rates for influenza illness among healthy pregnant women in non-pandemic influenza seasons were about five times higher than those among non-pregnant women. (3)

Routine influenza vaccination in pregnant women of all trimesters has been recommended by the World Health Organization (WHO), the United States Advisory Committee on Immunization Practices (ACIP), and the National Health and Medical Research Council of Canada. (5,6) These recommendations were based on a lack of evidence demonstrating any harmful effects of inactivated influenza vaccination on maternal or fetal health in vaccine safety studies, (7-9) and the growing evidence showing that maternal vaccination against influenza could potentially benefit not just the mother but also the infant during the latter's first few months of life. (7,10)

Despite these recommendations, the vaccination rate in pregnant women has typically been low. In the United States, the vaccination rate during pregnancy was estimated to be from less than 1% to 12.8% before 2003, (11) and from 12-24% during the 2005-2008 influenza seasons. (12) No comparable estimates of pre-2009 H1N1 pandemic immunization rates are currently available for Canada.

In the 2009 H1N1 influenza pandemic, health authorities unanimously identified pregnant women as one of the priority groups to be vaccinated against H1N1, and all pregnant women were encouraged to get influenza vaccine, regardless of their stage of pregnancy. (13,14) The H1N1 influenza vaccination rate was reported to be 46.6% among women with recent live births in 10 states in the United States, 37.1% in a French population randomly recruited from 3 maternity hospitals, 6.9% to 10.3% in antenatal clinics in Western Australia, 37.4% to 39.9% in the 2010 Canadian Community Health Survey, and 72% among 402 women in the province of Alberta. (15-19) In Ontario, Canada, the 2009 H1N1 vaccination campaign started on October 26, 2009 for high-priority groups, including pregnant women, and soon after was offered free of charge to every resident in the province.

Understanding the underlying barriers to maternal vaccination and identifying characteristics of pregnant women with low vaccination rates can aid in the development of targeted public health strategies for future influenza vaccination programs. Although several studies have tried to identify barriers existing at the patient, provider and organizational levels, (16,18-27) to date, there is little population-based information on vaccination uptake in pregnant women of different characteristics.

By using a population-based cohort of women who gave birth during the 2009 H1N1 pandemic season, this study aims to evaluate the influenza vaccination rate among pregnant women of different characteristics, and to identify predictors that can cause low vaccination uptake.

METHODS

Study design and population

We conducted a population-based retrospective cohort study among women who gave birth to a live born or stillborn infant ([greater than or equal to]20 weeks' gestation and [greater than or equal to]500 grams) in all hospitals in the Canadian province of Ontario between November 2, 2009 and April 30, 2010, using data from Better Outcomes Registry & Network (BORN) Ontario's birth record database.

The BORN database is a province-wide, Internet-based timely data collection system. It collects labour, birth and early postpartum information from both hospitals and midwifery groups in Ontario, and includes clinical, demographic and health behavioural information of all women who come to an Ontario hospital to give birth, through chart abstraction and patient interview. In 2009, more than 97% of all Ontario hospital births were included in the system.

Measures

The primary outcome was whether a woman received influenza vaccine (H1N1, seasonal or both) during pregnancy. Vaccination status and type of vaccine received (H1N1, seasonal or both) was confirmed on admission for birth from either the documentation in the patient's chart, the antenatal record or by asking the woman whether she had received influenza vaccine at any time during the current pregnancy. Women who received any kind of influenza vaccine were considered vaccinated.

Other information extracted from the database included maternal demographics (age, area of residence, and rural or urban status denoted by postal codes), obstetrical characteristics (month of delivery, parity, multifetal pregnancy, high-risk medical co-morbidity, history of preterm birth, pre-existing health problems), prenatal health behaviours (antenatal visit in the first trimester, smoking during pregnancy) and type of primary prenatal health care provider.

We linked individual birth records to the 2006 Canadian Census data by geocoding maternal postal codes into dissemination areas (DAs--the smallest unit of census geography), and obtained neighbourhood-level information on highest level of attained education, median family income, employment level, proportion of recent immigrants, and proportion of Aboriginal residents for each woman based on the DA in which she lived. All neighbourhood-level variables were converted into quintiles prior to analysis with the exception of proportion of Aboriginal residents, which used 10% of the population of the DA being Aboriginal residents as the cut-off (>10% versus [less than or equal to]10%).

Analysis

Characteristics of women with missing information on vaccination were compared with women who had complete information using the chi-square test statistics to determine if there were any differences between the two groups.

We calculated the influenza vaccination rates and their 95% confidence intervals (CI) for women of different demographic, behavioural and clinical characteristics, and compared the vaccination rates among different groups by calculating unadjusted relative risks (uRR) along with their 95% CIs.

To examine the association between influenza vaccination and each independent predictor while controlling the potential influence from other predictors, we employed a log binomial regression model, which can produce unbiased risk estimates for common outcomes ([greater than or equal to]10%), (28) for multivariate analysis. The model was constructed by using influenza vaccination status as the dependent variable, and all identified characteristics as the independent variables.

RESULTS

A total of 64,293 pregnant women presented at Ontario hospitals for delivery during the six-month study period. Information on whether they had been immunized against influenza was unavailable for 7,638 of them (11.9%). The percentage of women who delivered in November was higher in the group with missing information on influenza vaccination than in the one with complete vaccination information (21.1% vs. 15.9%, p<0.001). Other differences (age, rurality, smoking status, parity, high-risk co-morbidity, neighbourhood variables) between women with missing information and those with complete information were minor and not directly meaningful.

Of the remaining 56,654 (88.1%) women in our dataset, 21,773 (38.4%) received only H1N1 vaccine, 283 (0.5%) received only seasonal influenza vaccine, 2,033 (3.6%) received both vaccines, and 45 (0.1%) reported having received influenza vaccination but were unsure which type of vaccine was administered. Altogether, 24,134 (42.6%) of women received at least one type of influenza vaccine (H1N1, seasonal, or both).

The vaccination rate varied substantially across women with different demographic and obstetric characteristics: 29.8% among pregnant women <20 years old and 47.8% among those aged 35-39 years; 35.9% among women with a history of preterm birth compared to 43.1% among those without; 42.0% among women without any medical co-morbidity in comparison to 49.0% among those with one or more; 32.9% among women who did not initiate antenatal care in the first trimester in contrast to 43.7% among those who did; and 36.4% among smokers compared to 43.4% among non-smokers. The vaccination rates among women with no antenatal care provider (18.9%) or with a midwife as antenatal care provider (33.1%) were lower than those among women with an obstetrician/gynecologist (41.4%), a family physician (47.5%), or a nurse practitioner (42.1%) as their antenatal care provider. A gradient in vaccination uptake was observed among women of different neighbourhood income quintiles (37.2%, 40.3%, 42.3%, 44.5% and 52.6%, respectively, from the lowest to the highest neighbourhood income groups), and among women from neighbourhoods with different proportions of recent immigrants (35.5%, 43.8%, 46.7%, 47.2% and 47.1%, respectively, for neighbourhoods ranking from the highest to the lowest proportion of recent immigrants). There was also almost a 10% absolute difference in the vaccination rate between women from the lowest versus those from the highest neighbourhood education quintile (41.6% vs. 50.9%).

After adjusting for all the covariates, vaccine uptake was lower among women: of age <20 (aRR=0.80, 95% CI: 0.76-0.84); without an antenatal care provider (aRR=0.72, 95% CI: 0.59-0.88); who did not initiate antenatal care within the first trimester (aRR=0.93, 95% CI: 0.91-0.96); who smoked during pregnancy (aRR=0.92, 95% CI: 0.89-0.95); with a history of preterm birth (aRR=0.97, 95% CI: 0.94-1.00); and in a neighbourhood of higher proportion of recent immigrants (aRR=0.83, 95% CI: 0.81-0.86), lower education quintile (aRR=0.85, 95% CI: 0.83-0.88), or lower income quintile (aRR=0.93, 95% CI: 0.90-0.96). An increased vaccination rate was observed among women with medical co-morbidities (aRR=1.10, 95% CI: 1.07-1.13) and those with family physicians (vs. obstetricians) as antenatal care providers (aRR=1.08, 95% CI: 1.06-1.10).

DISCUSSION

To our knowledge, this is the largest population-based cohort study to date investigating multiple determinants associated with influenza vaccine uptake among pregnant women. The large sample size and the high vaccination rate enable us to explore the association between vaccination status and multiple predicting factors of different domains and produce stable risk estimates. The observed vaccination rate of 42.6% is comparable to the Statistics Canada estimation for pregnant women all over the country (37.4% to 39.9%) using the 2010 Canadian Community Health Survey data, (17) and the United States monthly survey data in 10 states for the same period (46.6%). (15)

Our finding that women from a neighbourhood of lower income, lower educational level, and higher concentration of recent immigrants had lower vaccination rates is consistent with previous surveys using individual-level SES measures. (16,18,21,23-26) Since the influenza vaccine was provided to all Ontarians free of charge, cost of vaccine would not have had influence on vaccine uptake. Thus the difference in vaccination rates among women of different SES likely reflects differences in access to medical information, medical care and personal beliefs regarding vaccine benefits and risks. (16) We presume that women from neighbourhoods of higher income and higher education level had a better knowledge of the potential benefits of influenza vaccine and a better understanding of the possible high risk of being infected during pregnancy, which is consistent with other studies. (16,18,24-26)

We found that women's vaccine uptake was lower in neighbourhoods with a higher proportion of recent immigrants. Race/ethnicity was consistently reported in studies conducted in other countries (16,26) to be correlated with influenza vaccine uptake during pregnancy. Other investigators hypothesized that the disparity in vaccination rate between different ethnic groups was a result of low accessibility to information among foreign populations. (16) Considering the multicultural setting of Canada, we add that the language barrier among new immigrants may be the reason for insufficient access to information on the vaccine campaign and influenza-related knowledge. A previous study indicated that the lack of available language translation may lead to low acceptance of physicians' offer of influenza vaccination among pregnant women. (29)

Although the Society of Obstetricians and Gynaecologists of Canada encouraged obstetricians to offer vaccines for all pregnant women during the 2009 H1N1 influenza pandemic, (30) our results showed that women who had antenatal care with family physicians had greater likelihood of getting vaccine than those with obstetricians. This disparity may stem from the practice pattern of health providers. A Canadian survey (27) in 2003-2004 influenza season showed that 41% of the obstetricians did not provide influenza vaccination at their office; and that obstetricians were less likely than family physicians to consider it their responsibility to discuss, recommend or offer influenza vaccination, but more likely to say it was the local public health unit's responsibility to vaccinate pregnant women. In Canada, obstetricians attend more than 60% of all births, and are often the only health care provider pregnant women contact later in pregnancy. It is important to add influenza vaccination and influenza prevention activities into obstetricians' routine preventive care practice.

The significantly lower vaccination rates among women without prenatal care providers and women who did not start antenatal visit in the first trimester can be explained from two perspectives. Women who did not have a first trimester antenatal care visit or did not have antenatal care at all could have lost the opportunity of getting useful information on vaccination from providers. This also reflects women's health-related behaviours in general, as the variable "smoking during pregnancy" did in our analyses. It is highly possible that women who were slow in taking antenatal care and those who smoked during pregnancy had a greater tendency to forgo beneficial health behaviours such as getting a vaccine.

Vaccine uptake can also be affected by women's medical and obstetrical condition. Our finding that women with obstetrical co-morbidities were more likely to have had vaccine is in contrast to a study conducted by Freund et al., which indicated a lower vaccination rate in this group. (16) During previous pandemics, (4) pregnant women with underlying co-morbidities were found to be at significantly greater risk of influenza-associated morbidity and mortality than their healthy counterparts. There is a large body of literature documenting the safety of administering influenza vaccination to pregnant women; none has shown that influenza vaccine uptake could increase adverse birth outcomes. (7-10) Such messages need to be clearly delivered to maternal care providers and pregnant women to reduce unnecessary concerns.

Our study has several limitations. Only pregnant women with a hospital delivery were included in our study. Although less than 2% of Ontario births take place at home under midwifery care, it is still possible these mothers would have different characteristics from our cohort. Also, women with pregnancy loss (miscarriage, termination of pregnancy) during the first 20 weeks of pregnancy were not included by the data source used for this study. The higher proportion of missing vaccination information among women who gave birth in November is another limitation. Although it is unlikely the characteristics of women giving birth in different months would be different, we acknowledge our cohort slightly under-represents women who gave birth in November. Moreover, our study population is a birth cohort rather than a cohort of pregnant women. Considering the time when the Ontario H1N1 vaccination program started and the accrual window of our study population, almost all women in the vaccination group were immunized in their 2nd or 3rd trimester. Therefore the vaccination rate observed in our study may not represent the vaccination situation in the entire population of pregnant women. A further limitation of the current study lies in the inability of a large administrative database to capture all important variables that can influence vaccine uptake. These variables include pregnant women's safety concerns and doubts regarding the effectiveness of vaccine, their perception of the risk of getting influenza during pregnancy, and physician recommendation to receive vaccine. (21-24,26,27)

The influenza vaccination rate we observed is suboptimal as more than half of the pregnant women have not received any kind of influenza vaccine. Many factors are associated with influenza vaccination during pregnancy, including potentially modifiable behavioural factors among both pregnant women and practitioners, and static demographic and clinical characteristics of pregnant women. These demographic and clinical factors indicate subgroups of the prenatal population that should be prioritized in future public health intervention strategies to increase vaccination uptake. Patient-level behavioural factors--such as an early start of antenatal visits--can be changed through health education programs that increase women's awareness, and health care providers' practice patterns can be changed through training and removing organizational barriers.

Acknowledgements: Dr. Mark Walker is supported by a University of Ottawa tier 1 chair in Perinatal Epidemiology. The authors thank the health care providers and hospital staff throughout Ontario hospitals who collected these data. We also acknowledge Barbara Chapman, Monica Prince and the Better Outcomes Registry & Network (BORN) Ontario Regional Coordinators (Tammy Budhwa, Laurie Doxtator, Sandra Dunn, Glenda Hicks, Vivian Holmberg, Susan Jewell, Pam Robertson) for their dedication to ensuring that the data collection was a success. Financial assistance for the H1N1 data collection in the BORN Ontario database was provided by the Public Health Agency of Canada as part of the public health response to the H1N1 influenza pandemic. This study was funded by the Canadian Institutes of Health Research (Grant # 218563).

Conflict of Interest: None to declare.

REFERENCES

(1.) Jamieson DJ, Theiler RN, Rasmussen SA. Emerging infections and pregnancy. Emerg Infect Dis 2006;12(11):1638-43.

(2.) Siston AM, Rasmussen SA, Honein MA, Fry AM, Seib K, Callaghan WM, et al. Pandemic 2009 influenza A(H1N1) virus illness among pregnant women in the United States. JAMA 2010;303(15):1517-25.

(3.) Dodds L, McNeil SA, Fell DB, Allen VM, Coombs A, Scott J, et al. Impact of influenza exposure on rates of hospital admissions and physician visits because of respiratory illness among pregnant women. CMAJ 2007; 176(4):463-68.

(4.) Freeman DW, Barno A. Deaths from Asian influenza associated with pregnancy. Am J Obstet Gynecol 1959;78:1172-75.

(5.) Statement on influenza vaccination for the 2007-2008 season. An Advisory Committee Statement (ACS). Can Commun Dis Rep 2007;33(ACS-7):1-38.

(6.) Harper SA, Fukuda K, Uyeki TM, Cox NJ, Bridges CB. Prevention and control of influenza: Recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR Recomm Rep 2004;53(RR-6):1-40.

(7.) Englund JA, Mbawuike IN, Hammill H, Holleman MC, Baxter BD, Glezen WP. Maternal immunization with influenza or tetanus toxoid vaccine for passive antibody protection in young infants. J Infect Dis 1993;168(3):647-56.

(8.) Munoz FM, Greisinger AJ, Wehmanen OA, Mouzoon ME, Hoyle JC, Smith FA, et al. Safety of influenza vaccination during pregnancy. Am J Obstet Gynecol 2005;192(4):1098-106.

(9.) Fell DB, Sprague AE, Liu N, Yasseen AS 3rd, Wen SW, Smith G, et al. H1N1 influenza vaccination during pregnancy and fetal and neonatal outcomes. Am J Public Health 2012;102(6):e33-e40.

(10.) Zaman K, Roy E, Arifeen SE, Rahman M, Raqib R, Wilson E, et al. Effectiveness of maternal influenza immunization in mothers and infants. N Engl J Med 2008;359(15):1555-64.

(11.) Naleway AL, Smith WJ, Mullooly JP. Delivering influenza vaccine to pregnant women. Epidemiol Rev 2006;28:47-53.

(12.) Fiore AE, Shay DK, Broder K, Iskander JK, Uyeki TM, Mootrey G, et al. Prevention and control of seasonal influenza with vaccines: Recommendations of the Advisory Committee on Immunization Practices (ACIP), 2009. MMWR Recomm Rep 2009;58(RR-8):1-52.

(13.) Strategic Advisory Group of Experts on Immunization--Report of the extraordinary meeting on the influenza A (H1N1) 2009 pandemic, 7 July 2009. Wkly Epidemiol Rec 2009;84(30):301-4.

(14.) Public Health Agency of Canada. Guidance document on the use of pandemic influenza A (H1N1) 2009 Inactivated Monovalent Vaccine, 2009. Available at: http://www.centralhealth.nl.ca/assets/Pandemic-Influenza/Guidance-Document-on-the-Use-of-Pandemic-Influenza-A-H1N1-2009-Inactivated-Monovalent-Vaccine.pdf (Accessed October 21, 2009).

(15.) Seasonal influenza and 2009 H1N1 influenza vaccination coverage among pregnant women--10 states, 2009-10 influenza season. MMWR Morb Mortal Wkly Rep 2010;59(47):1541-45.

(16.) Freund R, Le RC, Charlier C, Avenell C, Truster V, Treluyer JM, et al. Determinants of non-vaccination against pandemic 2009 H1N1 influenza in pregnant women: A prospective cohort study. PLoS One 2011;6(6):e20900.

(17.) Gilmour H, Hofmann N. H1N1 vaccination. Health Rep 2010;21(4):63-69.

(18.) Gracie S, Metcalfe A, Dolan SM, Kehler H, Siever J, Tough S. Utilization of the 2009 H1N1 vaccine by pregnant women in a pandemic year. J Obstet Gynaecol Can 2011;33(2):127-33.

(19.) White SW, Petersen RW, Quinlivan JA. Pandemic (H1N1) 2009 influenza vaccine uptake in pregnant women entering the 2010 influenza season in Western Australia. Med J Aust 2010;193(7):405-7.

(20.) Broughton DE, Beigi RH, Switzer GE, Raker CA, Anderson BL. Obstetric health care workers' attitudes and beliefs regarding influenza vaccination in pregnancy. Obstet Gynecol 2009;114(5):981-87.

(21.) Ding H, Santibanez TA, Jamieson DJ, Weinbaum CM, Euler GL, Grohskopf LA, et al. Influenza vaccination coverage among pregnant women--National 2009 H1N1 Flu Survey (NHFS). Am J Obstet Gynecol 2011;204(6 Suppl 1):S96 S106.

(22.) Fabry P, Gagneur A, Pasquier JC. Determinants of A (H1N1) vaccination: Cross-sectional study in a population of pregnant women in Quebec. Vaccine 2011;29(9):1824-29.

(23.) Fisher BM, Scott J, Hart J, Winn VD, Gibbs RS, Lynch AM. Behaviors and perceptions regarding seasonal and H1N1 influenza vaccination during pregnancy. Am J Obstet Gynecol 2011;204(6 Suppl 1):S107-S111.

(24.) Fridman D, Steinberg E, Azhar E, Weedon J, Wilson TE, Minkoff H. Predictors of H1N1 vaccination in pregnancy. Am J Obstet Gynecol 2011;204(6 Suppl 1):S124-S127.

(25.) Ozer A, Arikan DC, Kirecci E, Ekerbicer HC. Status of pandemic influenza vaccination and factors affecting it in pregnant women in Kahramanmaras, an eastern Mediterranean city of Turkey. PLoS One 2010;5(12):e14177.

(26.) Steelfisher GK, Blendon RJ, Bekheit MM, Mitchell EW, Williams J, Lubell K, et al. Novel pandemic A (H1N1) influenza vaccination among pregnant women: Motivators and barriers. Am J Obstet Gynecol 2011;204(6 Suppl 1):S116-S123.

(27.) Tong A, Biringer A, Ofner-Agostini M, Upshur R, McGeer A. A cross-sectional study of maternity care providers' and women's knowledge, attitudes, and behaviours towards influenza vaccination during pregnancy. J Obstet Gynaecol Can 2008;30(5):404-10.

(28.) McNutt L-A, Wu C, Xue X, Hafner JP. Estimating the relative risk in cohort studies and clinical trials of common outcomes. Am J Epidemiol 2003;157(10):940-43.

(29.) Yeager DP, Toy EC, Baker B, III. Influenza vaccination in pregnancy. Am J Peri natol 1999;16(6):283-86.

(30.) Society of Obstetricians and Gynaecologists of Canada. Experts recommend that all pregnant women over 20 weeks gestation get H1N1 vaccination immediately. 2009. Available at: http://www.sogc.org/media/advisories20091022a_e.asp (Accessed April 18, 2011).

Received: January 18, 2012

Accepted: July 18, 2012

Ning Liu, MB, MSc, [1] Ann E. Sprague, RN, PhD, [2,3] Abdool S. Yasseen III, MSc, [3] Deshayne B. Fell, MSc, [2,3] Shi-Wu Wen, MD, PhD, [3,4] Graeme N. Smith, MD, PhD, [5] Mark C. Walker, MD, MSc [2-4]

Author Affiliations

[1.] Institute for Clinical Evaluative Sciences, Toronto, ON

[2.] Better Outcomes Registry & Network (BORN) Ontario, Ottawa, ON

[3.] Ottawa Hospital Research Institute, Ottawa, ON

[4.] Department of Obstetrics and Gynecology, University of Ottawa, Ottawa, ON

[5.] Department of Obstetrics and Gynecology, Queen's University, Kingston, ON Correspondence: Dr. Ann Sprague, Better Outcomes Registry & Network (BORN) Ontario--Scientific Office, Box 241, The Ottawa Hospital--General Campus, 501 Smyth Rd., Ottawa, ON K1H 8L6, Tel: 613-737-7600, ext. 6011, Fax: 613-737-8402, E-mail: asprague@bornontario.ca

Table 1. Vaccination Rates and Relative Risks of Influenza
Vaccination Among Pregnant Women of Different Characteristics

Characteristics                N (%)       Vaccination Rate
                                            per 100 Women
                                              (95% CI) *

Maternal age (years)

  <20                       2079 (3.7)     29.8 (27.9-31.8)
  20-24                     7496 (13.2)    32.5 (31.5-33.6)

25-34                    34,857 (61.5)   43.9 (43.4-44.4)
  35-39                     9969 (17.6)    47.8 (46.8-48.8)

[greater than             2252 (4.0)     44.8 (42.7-46.8)
  or equal to]40

Public health region
of residence

  North West                1036 (1.9)     41.2 (38.2-44.3)

North East                2158 (3.9)     48.5 (46.3-50.6)
  Eastern                   6814 (12.2)    56.6 (55.4-57.8)

Central East             17,207 (30.9)   37.8 (37.1-38.6)
  Toronto                  12,372 (22.2)   39.3 (38.4-40.1)

South West                7207 (12.9)    43.5 (42.4-44.7)
  Central West              8896 (16.0)    44.7 (43.6-45.7)

Parity

  0                        24,785 (43.9)   42.4 (41.7-43.0)
  1                        19,985 (35.4)   45.3 (44.6-46.0)

[greater than            11,735 (20.8)   38.4 (37.5-39.3)
  or equal to]2

Month of delivery

  November                  8995 (15.9)    36.5 (35.5-37.5)
  December                  9240 (16.3)    46.8 (45.8-47.8)

January                   9667 (17.1)    48.0 (47.0-49.0)
  February                  8909 (15.7)    46.3 (45.3-47.4)

March                    10,176 (18.0)   41.2 (40.2-42.1)
  April                     9667 (17.1)    36.9 (35.9-37.9)

Type of antenatal
care provider

  None                       222 (0.4)     18.9 (14.0-24.7)

Family physician         15,641 (28.5)   47.5 (46.7-48.2)
  Midwife                   4116 (7.5)     33.1 (31.7-34.6)

Nurse practitioner         330 (0.6)     42.1 (36.7-47.7)
  Obstetrician/            34,412 (62.7)   41.4 (40.9-41.9)

Gynecologist
  Other                      188 (0.3)     45.7 (38.5-53.2)

Area of residence

  Urban                    49,751 (87.8)   42.1 (41.6-42.5)
  Rural                     6891 (12.2)    46.4 (45.3-47.6)

History of preterm birth

  No                       52,108 (92.7)   43.1 (42.7-43.5)
  Yes                       4132 (7.3)     35.9 (34.4-37.4)

Smoking during
pregnancy

  No                       48,138 (88.4)   43.4 (42.9-43.8)
  Yes                       6303 (11.6)    36.4 (35.2-37.6)

Multifetal pregnancy

  No                       55,572 (98.1)   42.5 (42.1-43.0)
  Yes                       1082 (1.9)     45.5 (42.5-48.5)

Antenatal visit in
first trimester

    No                      6177 (13.5)    32.9 (31.7-34.1)

Yes                    39,706 (86.5)   43.7 (43.2-44.2)
    Missing                   10,771       44.0 (43.1-45.0)

Maternal medical
co-morbidity
([double dagger])

    No                     51,191 (92.6)   42.0 (41.6-42.4)
    Yes                     4062 (7.4)     49.0 (47.5-50.6)

Neighbourhood
education quintiles

  1 (Lowest)               11,538 (20.9)   41.6 (40.7-42.5)

2                        10,684 (19.3)   40.5 (39.6-41.5)
  3                        11,289 (20.4)   40.2 (39.3-41.1)

4                        11,276 (20.4)   40.7 (39.8-41.6)
  5 (Highest)              10,494 (19.0)   50.9 (50.0-51.9)

Neighbourhood
employment level

  1 (Lowest)               11,578 (20.9)   37.5 (36.6-38.4)

2                        11,400 (20.6)   42.2 (41.3-43.1)
  3                        10,660 (19.3)   43.1 (42.1-44.0)

4                        11,308 (20.5)   45.7 (44.9-46.7)
  5 (Highest)              10,335 (18.7)   45.3 (44.3-46.2)

Proportion of recent
immigrants in the
neighbourhood

  1 (Lowest)                9773 (17.8)    47.1 (46.1-48.1)

2                         9358 (17.0)    47.2 (46.2-48.3)
  3                         8358 (15.2)    46.7 (45.7-47.8)

4                         9714 (17.7)    43.8 (42.8-44.8)
  5 (Highest)              17,760 (32.3)   35.3 (34.6-36.0)

Neighbourhood
median family
income quintiles

  1 (Lowest)               13,589 (24.6)   37.2 (36.4-38.0)

2                        10,646 (19.3)   40.3 (39.4-41.3)
  3                        11,325 (20.5)   42.3 (41.4-43.2)

4                        11,235 (20.3)   44.5 (43.6-45.4)
  5 (Highest)               8486 (15.4)    52.6 (51.5-53.7)

Proportion of
Aboriginal
residents in the
neighbourhood

  Low (<10%)               53,494 (96.8)   42.7 (42.3-43.1)
  High (>10%)               1762 (3.2)     43.0 (40.7-45.4)

Characteristics             Unadjusted RR       Adjusted RR
                               (95% CI)           (95% CI)
                                                 ([dagger])

Maternal age (years)

  <20                      0.68 (0.64-0.73)   0.80 (0.76-0.84)
  20-24                    0.74 (0.72-0.77)   0.85 (0.82-0.87)

25-34                           1                  1
  35-39                    1.09 (1.06-1.12)   1.06 (1.03-1.08)

[greater than            1.02 (0.97-1.07)   1.02 (0.98-1.06)
  or equal to]40

Public health region
of residence

  North West               1.05 (0.97-1.13)   1.02 (0.95-1.10)
  North East               1.23 (1.18-1.30)   1.11 (1.06-1.17)

Eastern                  1.44 (1.40-1.49)   1.16 (1.12-1.19)
  Central East             0.96 (0.94-1.00)   0.95 (0.92-0.97)

Toronto                         1                  1
  South West               1.11 (1.07-1.15)   1.05 (1.02-1.09)
  Central West             1.14 (1.10-1.17)   1.03 (1.00-1.06)

Parity
  0
  1                        1.07 (1.05-1.09)   1.03 (1.01-1.05)

[greater than            0.91 (0.88-0.93)   0.92 (0.90-0.95)
  or equal to]2

Month of delivery
  November                        1                  1
  December                 1.28 (1.24-1.33)   1.16 (1.13-1.20)

January                  1.31 (1.27-1.36)   1.18 (1.15-1.21)
  February                 1.27 (1.22-1.31)   1.16 (1.12-1.19)

March                    1.13 (1.09-1.17)   1.06 (1.03-1.10)
  April                    1.01 (0.97-1.05)   1.00 (0.97-1.03)

Type of antenatal
care provider

  None                     0.46 (0.35-0.60)   0.72 (0.59-0.88)
  Family physician         1.15 (1.12-1.17)   1.08 (1.06-1.10)

Midwife                  0.80 (0.76-0.84)   0.86 (0.83-0.89)
  Nurse practitioner       1.02 (0.90-1.15)   1.04 (0.94-1.15)

Obstetrician/                   1                  1
  Gynecologist
  Other                    1.10 (0.95-1.29)   1.05 (0.91-1.21)

Area of residence
  Urban                           1                  1
  Rural                    1.10 (1.07-1.13)   1.00 (0.97-1.03)

History of preterm birth
  No                              1                  1
  Yes                      0.83 (0.80-0.87)   0.97 (0.94-1.00)

Smoking during
pregnancy
  No                              1                  1
  Yes                      0.84 (0.81-0.87)   0.92 (0.89-0.95)

Multifetal pregnancy
  No                              1                  1
  Yes                      1.07 (1.00-1.14)   1.00 (0.95-1.06)

Antenatal visit in
first trimester

    No                     0.75 (0.72-0.78)   0.93 (0.91-0.96)

Yes                           1                  1
    Missing                1.01 (0.98-1.03)   1.03 (1.01-1.06)

Maternal medical
co-morbidity
([double dagger])

No                            1                  1
    Yes                    1.17 (1.13-1.21)   1.10 (1.07-1.13)

Neighbourhood
education quintiles

  1 (Lowest)               0.82 (0.79-0.84)   0.85 (0.83-0.88)

2                        0.80 (0.77-0.82)   0.85 (0.83-0.88)
  3                        0.79 (0.77-0.81)   0.87 (0.85-0.89)

4                        0.80 (0.78-0.82)   0.88 (0.86-0.90)
  5 (Highest)                     1                  1

Neighbourhood
employment level

  1 (Lowest)               0.83 (0.80-0.86)   1.00 (0.97-1.03)

2                        0.93 (0.91-0.96)   1.03 (1.00-1.06)
  3                        0.95 (0.92-0.98)   1.02 (0.99-1.04)

4                        1.01 (0.98-1.04)   1.01 (0.99-1.04)
  5 (Highest)                     1                  1

Proportion of recent
immigrants in the
neighbourhood
  1 (Lowest)                      1                  1
  2                        1.00 (0.97-1.03)   0.98 (0.95-1.00)
  3                        0.99 (0.96-1.02)   0.96 (0.93-0.99)

4                        0.93 (0.90-0.96)   0.91 (0.88-0.93)
  5 (Highest)              0.75 (0.73-0.77)   0.83 (0.81-0.86)

Neighbourhood
median family
income quintiles

  1 (Lowest)               0.71 (0.69-0.73)   0.93 (0.90-0.96)

2                        0.77 (0.74-0.79)   0.94 (0.92-0.97)
  3                        0.80 (0.78-0.83)   0.96 (0.93-0.99)

4                        0.85 (0.82-0.87)   0.97 (0.95-1.00)
  5 (Highest)                     1                  1

Proportion of
Aboriginal
residents in the
neighbourhood
  Low (<10%)                      1                  1
  High (>10%)              1.01 (0.95-1.06)   1.00 (0.94-1.05)

* Statistically significant differences (p < 0.001) in vaccination
rates were found in all identified characteristics, except in the
proportion of Aboriginal residents in the neighbourhood (p = 0.77).

([dagger]) All independent variables were included in the
multivariate model.

([double dagger]) Maternal medical co-morbidity is defined as having
insulin-dependent diabetes, non-insulin-dependent diabetes, asthma,
heart disease or chronic hypertension.