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Obstetrics & Gynecology:
doi: 10.1097/AOG.0b013e3182878a5a
Original Research

Predictors of Seasonal Influenza Vaccination During Pregnancy

Henninger, Michelle PhD; Naleway, Allison PhD; Crane, Bradley MS; Donahue, James DVM, PhD; Irving, Stephanie MHS

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Author Information

Center for Health Research, Kaiser Permanente Northwest, Portland, Oregon; and the Marshfield Clinic Research Foundation, Marshfield, Wisconsin.

Corresponding author: Michelle Henninger, PhD, Center for Health Research, Kaiser Permanente Northwest, 3800 N. Interstate Avenue, Portland, OR 97227; e-mail: Michelle.L.Henninger@kpchr.org.

Supported by the Centers for Disease Control and Prevention's Vaccine Safety Datalink (200-2002-00732) through America's Health Insurance Plans. The findings and conclusions in this report are those of the authors and do not necessarily represent the official positions of the Centers for Disease Control and Prevention or America's Health Insurance Plans.

Financial Disclosure Ms. Irving and Dr. Donahue have received grants or have grants pending from MedImmune for research support unrelated to this study. Dr. Naleway has received grants or has grants pending from GlaxoSmithKline. Dr. Henninger and Mr. Crane did not report any potential conflicts of interest.

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Abstract

OBJECTIVE: Although pregnant women are a high-priority group for influenza vaccination, vaccination rates in this population remain below recommended levels. This prospective cohort study followed a group of pregnant women during the 2010–2011 influenza season to determine possible predictors of vaccination.

METHODS: Participants were 552 pregnant women who had not already received the influenza vaccine at the time of enrollment. Women completed a survey assessing knowledge, attitudes, and beliefs about vaccination (based on the Health Belief Model) by telephone and were then followed to determine vaccination status by the end of the 2010–2011 influenza season.

RESULTS: Forty-six percent (n=252) of the women were vaccinated, and 54% (n=300) remained unvaccinated after enrollment in the study. Few baseline characteristics, with the exception of study site, month of enrollment, and maternal ethnicity, were predictive of vaccination status. Even after adjusting for significant baseline characteristics, we found that at least one item from each domain of the Health Beliefs Model was predictive of subsequent vaccination. Specifically, women who perceived they were susceptible to influenza, that they were at risk of getting seriously ill from influenza, that they would regret not getting vaccinated, and who trusted recommended guidelines about influenza vaccination during pregnancy were more likely to get vaccinated. Women who were concerned about vaccine side effects were less likely to get vaccinated.

CONCLUSION: Trust in recommendations, perceived susceptibility to and seriousness of influenza, perceived regret about not getting vaccinated, and vaccine safety concerns predict vaccination in pregnant women.

LEVEL OF EVIDENCE: II

Pregnant women are at increased risk for severe influenza-related complications and hospitalizations1–7 and are a priority group for seasonal influenza vaccination during pregnancy.8,9 Before the 2009 outbreak of novel influenza A (H1N1) virus, vaccination coverage in pregnant women ranged from less than 10% to 33%.10–13 Since 2009, vaccination coverage rates have increased to approximately 50%12,14,15 but still fall well below the recommended target of 80%.16

The Health Belief Model identifies factors predictive of health behavior change and has been shown to predict a diverse range of health behaviors such as contraceptive use,17 breast self-examination,18 cervical cancer screening,19 and receipt of influenza vaccination in children, the elderly, and adults with asthma.20–22 Health Belief Model factors associated with higher vaccination rates during pregnancy include recommendations from a health care provider, perceived effectiveness of influenza vaccinations, and perceived risk of influenza infection.6,12,15,23–27 Other factors associated with increased vaccination rates include higher maternal age, higher education and socioprofessional levels, being in late pregnancy at the time of vaccination, and history of prior seasonal influenza vaccination.15,24–26,28,29

Factors often associated with lower vaccination rates during pregnancy include black or Hispanic race or ethnicity; perception that the vaccine had not been adequately tested; concerns about vaccine effects on maternal or fetal health; and lack of knowledge about influenza risk during pregnancy, benefits of vaccination, or where to get vaccinated.6,24,26,27,30–32

The objective of this prospective cohort study was to follow a group of pregnant women throughout the 2010–2011 influenza season to determine possible predictors of seasonal influenza vaccination. We hypothesized that Health Belief Model factors would predict influenza vaccination rates even after controlling for the influence of demographic and maternal factors such as higher maternal age, later gestational age, and race or ethnicity.

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MATERIALS AND METHODS

The Kaiser Permanente Northwest and Marshfield Clinic institutional review boards reviewed and approved this study protocol. Pregnant women were recruited as part of a larger study investigating the safety of trivalent influenza vaccination during pregnancy. Women were enrolled and surveyed before vaccination decision-making and subsequent vaccination status was confirmed using their medical record. Influenza vaccination is strongly encouraged at prenatal appointments. Most pregnant women at Kaiser Permanente Northwest receive their shots at vaccination clinics, typically offered during October of each year. At Marshfield Clinic, the majority of women receive vaccinations at prenatal visits. However, a small number of women at either study site may have been vaccinated at a nurse treatment room or outside the health care organization (ie, a pharmacy or state health department).

Kaiser Permanente Northwest and Marshfield Clinic are health care organizations serving 470,000 patients in Oregon and Washington and 377,000 patients in Wisconsin, respectively. Both study sites participate in the Centers for Disease Control and Prevention's Vaccine Safety Datalink, a collaborative effort between Center for Disease Control and Prevention's Immunization Safety Office and 10 managed care organizations.33

Pregnant women scheduled for upcoming prenatal visits were identified through their electronic medical record. We recruited potential participants by telephone and asked them to provide verbal consent for study participation. Women completed an 18-item survey by telephone and were then followed through the influenza season to determine actual vaccination status.

Surveys were administered by telephone (as opposed to in prenatal clinics) to avoid any potential interference with provision of medical services. Published studies regarding the differences in survey results between telephone compared with in-person surveys have demonstrated that either there are no significant differences between administration methods34–38 or that participants who are interviewed by telephone are actually more forthcoming than when interviewed in person, possibly as a result of increased anonymity or fewer social desirability cues when completing interviews by telephone.39

Participants included 552 pregnant women from Kaiser Permanente Northwest and Marshfield Clinic. Eligibility criteria for participation were as follows: 18 years or older, English-speaking, currently pregnant, less than 36 weeks of gestation, and had not already received influenza vaccination. We limited the study period to October through December 2010 because this is the period of time when influenza vaccinations are traditionally offered in the two participating health care organizations and thus when the majority of women in the sample would have been offered or received vaccination.

Demographics, baseline characteristics, and vaccination data were obtained from electronic data extraction of health plan databases and state immunization registries as well as manual chart review.

We defined “high-risk for influenza complications” status using electronic medical record data from the year before each influenza season. We categorized women with International Classification of Diseases codes for the following diseases as high risk: chronic cardiac disease, chronic pulmonary disease, chronic renal disease, diabetes mellitus, hemoglobinopathies, immunosuppressive disorders, malignancies, metabolic diseases, liver diseases, and selected neurologic or musculoskeletal conditions. Using chart abstraction data, we also calculated the number of pregnancy risk factors for each participant. These risk factors included: age 35 years or older; history of prior pregnancy complications; history of sexually transmitted disease or human immunodeficiency virus; prepregnancy body mass index (calculated as weight (kg)/[height (m)]2) of 30.0 or above; current multiple gestation pregnancy; smoking, alcohol, or other drug abuse during current pregnancy; and use of specific medications during current pregnancy. Detailed code lists are available by request from authors.

We defined gravidity as the cumulative number of pregnancies as assessed and recorded in the electronic medical record by health care providers. This count included the pregnancy during the study period and prior pregnancies of any outcome.

The Health Belief Model identifies five factors predictive of health behavior change: perceived susceptibility (personal assessment of the risk of getting ill), perceived severity (assessment of the seriousness or consequences of getting ill), perceived barriers (assessment of negative influences related to implementation of the health behavior), perceived benefits (assessment of positive consequences of implementing the health behavior), and cues to action (external influences promoting health behavior). Survey items were developed to assess the major domains of the Health Belief Model and were consistent with questions used in other studies assessing predictors of influenza vaccination.20,24,25 We included items for each of the following domains: perceived susceptibility to influenza (four items), perceived seriousness of getting ill with influenza (two items), potential benefits of influenza vaccination (three items), perceived barriers to vaccination (five items), and cues to action (four items). Each item was read verbatim by the interviewer and participants rated each item on a 5-point scale in which 1=“strongly disagree” and 5=“strongly agree.”

Both univariable and multivariable analyses were performed to estimate the association between vaccination status and baseline characteristics as well as survey items endorsed by the participants. Baseline characteristics consisted of sociodemographic (eg, race, ethnicity) and maternal factors (eg, maternal age, gravidity). Records with missing data for any covariates, with the exception of race and ethnicity, were excluded from both univariable and multivariable analyses. In all analyses, less than 5% of records were dropped as a result of missing values. Unknown race and ethnicity were grouped with “other race” and “non-Hispanic ethnicity,” respectively, and kept in analyses.

We built a multivariable logistic regression model by incorporating all baseline characteristics with univariable χ2 P values <.05. We refer to this as “step 1” of the multivariable model. Then, individual survey items with P values <.01 in the univariable analysis were entered into the multivariable model to estimate their association with vaccination while adjusting for the baseline factors. We evaluated each statistically significant survey item in separate models, each time adjusting for baseline characteristics but not the other survey items. We will refer to this group of logistic regression analyses as “step 2” of the model.

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RESULTS

Of 875 pregnant women who were contacted and screened eligible for participation in this prospective cohort study, 649 (74%) enrolled and completed the telephone survey. Ninety-seven women (11%) were excluded from the analyses presented in this article because they completed the survey after December 2010, leaving a sample of 552 pregnant women for analyses.

Table 1 shows baseline sociodemographic and maternal characteristics. The majority of women (62%) fell in the 26- to 35-year age range (mean age 29 years), 88% were white, and 6% were Hispanic. Eighty-seven percent of women were categorized as having “normal” risk for influenza complications and 46% had no pregnancy risk factors. Most women in our study (61%) initiated prenatal care before 10 weeks of gestation and 55% were classified as having a history of two or fewer pregnancies.

Table 1
Table 1
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Of 552 total participants, 252 women (46%) were vaccinated after enrollment and 300 (54%) remained unvaccinated at the end of the study. We found significant differences between vaccinated and unvaccinated participants by study site, month of enrollment, and ethnicity. Vaccination rates were significantly higher at study site 1 (179 of 362 [49%]) than at study site 2 (73 of 190 [38%]). Women who enrolled in the study in October were more likely to be vaccinated (101 of 150 [67%]) than women who enrolled in November or December (41% and 30%, respectively). Finally, although representing a relatively small proportion (6%) of our total population, women of Hispanic ethnicity were more likely to be vaccinated (22 of 34 [65%]) than non-Hispanic women or women of unknown or other ethnicity (44%).

Table 2 summarizes the univariable analysis of survey items. Responses were collapsed from five to three response categories both to simplify the table and to avoid reporting statistical differences that were attributable only to difference in the finer gradation (ie, differences only in “agree” compared with “strongly agree”).

Table 2-a Univariabl...
Table 2-a Univariabl...
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At least one item from each Health Belief Model domain was predictive of subsequent vaccination. All four “perceived susceptibility” items predicted vaccination status with unvaccinated women being significantly less likely to perceive themselves as susceptible to influenza infection. Only one of the “perceived seriousness” items predicted vaccination status: vaccinated women were more likely to perceive themselves at risk of getting seriously ill with the flu. All three of the “perceived benefits” items predicted vaccination status with unvaccinated women less likely to worry or regret their decision not to get vaccinated. Three of the five “perceived barriers” items predicted vaccination status. Unvaccinated women expressed more concern about vaccine side effects and were more likely to believe that the flu shot could give them the flu. Only one of the four “cues to action” items predicted vaccination status. Vaccinated women were significantly more likely to trust guidelines recommending that pregnant women be vaccinated. The cue to action item related to health care provider recommendation for vaccination was marginally significant but did not meet our threshold (P<.01) for inclusion in the multivariable analysis.

Table 2-b Univariabl...
Table 2-b Univariabl...
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Table 3 shows results of multivariable analyses. We entered baseline characteristics with significant univariable results (study site, appointment date used for recruitment, and Hispanic ethnicity) in step 1 of the model. Study site was no longer significant in the multivariable model. We entered survey items with significant univariable results in step 2 of the model (individual models for each item). All but one survey item (“I get sick with the flu more easily than other people my age”) retained significance after controlling for the effects of baseline characteristics.

Table 3
Table 3
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DISCUSSION

This study evaluated possible predictors of seasonal influenza vaccination, including knowledge, attitudes, and beliefs of pregnant women. The vaccination rate in our study population as a whole was 46%, which is consistent with other recently published vaccination rates.12,14,15 We found that women who were surveyed earlier in influenza season were significantly more likely to be vaccinated than women surveyed later in the season. By design, we only included women who were unvaccinated at the time of enrollment; women who enrolled later in the flu season may have already made a decision not to be vaccinated by the time we recruited them into the study.

Previous studies have generally concluded that black and Hispanic women are less likely to get vaccinated than other racial or ethnic groups. However, at least one study (Henninger ML, Crane B, Naleway A. Trends in influenza vaccine coverage in pregnant women, 2008\x{2013}2012. Permanente J 2012) has demonstrated large increases in vaccination coverage in both black and Hispanic pregnant women over several recent influenza seasons. In the current study, we observed that Hispanic women were significantly more likely to be vaccinated than non-Hispanic women or women of unknown or other ethnicity. There were no other significant differences between vaccinated and unvaccinated women in terms of maternal age at enroll>ment, race, influenza risk status, pregnancy risk factors, gestational age at initiation of prenatal care, or gravidity.

Results of multivariable analysis of survey items suggest that pregnant women who decided to get the influenza vaccine during the 2010–2011 season were more likely to perceive themselves as being at risk of serious influenza and were more likely to trust guidelines recommending vaccination for pregnant women. Pregnant women who opted not to vaccinate were less likely to perceive themselves as susceptible to influenza, were less likely to worry about their decision not to get vaccinated, were more likely to worry about vaccine side effects, and were more likely to believe the flu shot could give them the flu.

One potential limitation of studies to date is that they have focused on H1N1 vaccination during the 2009 pandemic and, therefore, may not be generalizable to vaccination behavior during a “typical” influenza season. Furthermore, women in these studies were often interviewed retrospectively after vaccination, during the postpartum period, or both, and in most cases vaccination status was defined by participant self-report. Compared with previous research in this area, our study is unique because of its prospective cohort design, because we were able to evaluate predictors of seasonal influenza vaccination compared with evaluating only pandemic H1N1 influenza vaccination and because vaccination status was confirmed by electronic medical record compared with only self-report.

One potential limitation of the current study is that women who are members of integrated care organizations may not be representative of the general population of pregnant women. Furthermore, our population was predominantly white and non-Hispanic, suggesting that findings related to race and ethnicity in particular should be interpreted with caution. Finally, because we limited the study period to October through December, we may have excluded minority women who opted to receive the vaccine before October.

Our findings suggest that the Health Belief Model may offer a useful framework for educating health care providers to help them better understand predictors of vaccination and to help them counsel patients to overcome misperceptions about vaccination. The following clinical strategies may be particularly helpful: educating pregnant women about their increased risk for influenza and potential seriousness of illness, educating women about potential side effects of vaccination and specifically stating that the flu shot does not cause the flu, and clearly stating recommendations for seasonal influenza vaccination for all pregnant women regardless of trimester.

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© 2013 The American College of Obstetricians and Gynecologists

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