Influenza can cause a severe febrile disease and lead to death by itself or its serious complications. Influenza vaccination is the most important method of preventing influenza and recommended to all children more than 6 months of age and adults. It is recommended that people who are classified as high risk group or prone to spread influenza to people identified as high risk group are at the top of the priority list for vaccination [1]. Because pregnant women are likely to be hospitalized for influenza or at increased risk of its complication, such as premature birth or abortion of the fetus, they get priority for influenza vaccination. In addition, women who take care of neonates or infants can spread influenza to high-risk infants are also classified as priority group for influenza vaccination. However, previous studies of maternal influenza immunization rate in Korea is very low, the higher the rates of vaccination showed the higher the mother's perception about influenza vaccination [2]. In 2009, while undergoing an influenza pandemic, the recognition of influenza vaccination for women of childbearing age is expected to improve, but there is no concrete data on this yet. In addition, there is little data evaluating the perception of entire women of childbearing age on influenza vaccination. Therefore, we studied the influenza vaccine-related factors of childbearing age women in Korea.

Compared with healthy non-pregnant women, pregnant women infected with influenza virus are more likely to be hospitalized. And their risk of complication or death caused by influenza is high [3,4]. In addition, Influenza during pregnancy increases the risk of premature birth or stillbirth [5-7]. The factors suggested to increase the susceptibility of complicated influenza during pregnancy are linked to the physiologic changes that occur during pregnancy (most pronounced in the third trimester), including immunologic changes, increased cardiac output and oxygen consumption, decreased lung capacity and tidal volume [8]. In 2009, pregnant women infected with influenza A (H1N1) pdm09 had severe illness and high risk of mortality [9-11]. In contrast, the lack of harmful effects of inactivated influenza vaccination on both maternal and fetal health during pregnancy has been demonstrated in several studies. No differences in gestational age, proportion with cesarean delivery, birth weight, or length of stay for birth hospitalization were reported. Rather, it is suggested that maternal influenza vaccination can reduce small for gestational age birth [12-17]. Also, produced maternal antibodies are delivered through the placenta to the fetus to acquire passive immunity. The duration of passively acquired antibody of infants depends on the initial cord blood concentration, and it is probably less than 6 months. Thus maternal immunization during pregnancy can protect pregnant women from influenza as well as their neonates and young infants [12]. Therefore, inactivated influenza vaccination is strongly recommend for pregnant women or childbearing age women scheduled to be pregnant for an influenza pandemic [1,18]. But seasonal influenza vaccination rate of them is low in Korea. Kim et al. [2] reported that the seasonal influenza vaccination rate among the Korean pregnant women was only 4% during 2006-2007 influenza epidemic season.

The results of this study, the childbearing age women who have influenza vaccination at least once were 71% of entire respondents, did not differ with past study during 2006-2007 season. But this study showed the influenza vaccination rate during pregnancy of women pregnant who are now or has been at least once in the past was 16.4%, higher than 4% of those of past study. The improved vaccination rate during pregnancy in this study (16.4%) is explained by accumulation of past vaccinations for several influenza epidemic seasons. In the past study, a one season's influenza vaccination rate (4%) has been evaluated. However, the majority of respondents who have received influenza vaccine during pregnancy got vaccination since 2009. It means actual increased influenza vaccination rate of pregnant women due to improved level of public awareness and knowledge about influenza by nationwide promotion undergoing influenza A(H1N1) pdm09. In this study, 38.0% of all participants responded that they had a plan to receive influenza vaccination during pregnancy in the future. Influenza vaccination rate of pregnant women is expected to increase in the future.

Although influenza vaccination rate of pregnant women increased, it is still much lower than that of elderly 65 years old or more (77-79%) and healthy adults (30%), even compared with influenza vaccination rate of American pregnant women [19-21]. Most important factor associated with low influenza vaccination rate of pregnant women was lack of recommendation of healthcare providers. In this study, recommendation of healthcare providers is main determining factor for Korean childbearing age women. And 61.4% of all respondents had a concern about adverse reaction of general vaccines. Eighty-three point nine percent of respondents who are not planning influenza vaccination during pregnancy responded that they had a concern about harmful effect to their fetus. Additionally, 44.2% of entire respondents misunderstood that influenza vaccination of pregnant women was harmful to fetus. These results suggest that the concerns about safety of influenza vaccine are the major obstacles to increase vaccination rate of pregnant women. To improve influenza vaccination rate of pregnant women, it is important that healthcare providers explain and recommend influenza vaccination exactly and actively.

In multivariate analysis on factors associated with influenza vaccination, statistically significant factors were accurate understandings about influenza and positive roles of general vaccines. The results suggest that active education and promotion of influenza and vaccine can increase influenza vaccination rate. High income and high level of education have correlation with high vaccination rate in univariate analysis, but no significant correlation in multivariate analysis. However, 12.4% of non-vaccination group responded that the reason why did not receive influenza vaccination was expense of influenza vaccine. Thus, free vaccination for priority groups of influenza vaccination should be considered according to the economic level.

In multivariate analysis on factors associated with planning vaccination during pregnancy, significant variables are experience of childbirth, high level of education, previous influenza vaccination. Women who have experience of childbirth get more chance to exposure to medical information. And direct experience of efficacy and safety of influenza vaccination would lead to high vaccination rate during pregnancy.

In this study, the participants were not limited to pregnant women but childbearing age women including pregnant women. Influenza vaccination is highly recommended for not only pregnant women but also women planning to become pregnant and caring for neonates and infants.

Limitations of this study were as follows; first, the participants do not represent the whole childbearing age women as visitors to the university hospital were recruited. Second, vaccination history could be inaccurate because it was based on interview without confirmation by medical record.

In conclusion, influenza vaccination rate during pregnancy has been a slight improvement since 2009 but is still very low. To improve vaccination rate of childbearing age women including pregnant women, important factors are accurate education of efficacy and safety of influenza vaccine and strong recommendation of healthcare provider.