One of the questions which this research set out to answer was at what rate was Korean children having age-appropriate immunizations. Results from this research showed that indicate 50.3% of the children in Nosan city of Korea received all doses in their 4 : 3 : 1 series on time. Age-appropriate immunization was higher for antigen-specific series that required fewer doses than series requiring more doses. Single dose MMR compliance rates were high, while 4 dose DTaP compliance was low. DTaP vaccine compliance was low in achieving age-appropriate immunization levels because non or delayed immunization was high in the 4th dose.3,22-24
Since the period between the 3rd and the 4th dose of DTaP is long and most national mandatory vaccination is completed within 12 months after birth, non and delayed immunization for DTaP and Polio are high, compared with other immunizations. An intervention focusing on the 4th dose of DTaP vaccination enhances the age-appropriate immunization rates of 4 : 3 : 1 series, and has been shown to result in a 10% increase in completed series.25,26
As the importance of the age-appropriate immunization is well established, why are the rates not higher? Perhaps, it is difficult for caregivers to know whether their children have received/completed all mandatory immunizations as there are so many. This is further complicated by poor record keeping. A review of research reveals that very little is known about the risk factors for age-appropriate immunization in childhood immunization.
According to our present study, the related factors for age-appropriate immunizations were residential district, birth order, the number of siblings, primary care-giver, mother's educational level, marital status, and economic status. In regards to residential district, the age-appropriate immunization rate was higher in the urban area than in the rural area. This can in part be interpreted by easy accessibility to a medical institution, because there are more facilities in the urban areas, and shorter traveling times make them more accessible to the facilities. The results from this study are consistent with those which showed that age-appropriate immunizations are low when people residing in a region have difficulty to access to a medical institution because of long distance.13,27
In Korea, residential districts can be a barrier for accessibility to medical institutions as they can limit the date and time when immunizations are provided. If a working caregiver is denied time-off from her work place and the local health district is also inflexible with providing available times for immunization, there is clearly a conflict. Given the fact that more than 60% of national mandatory immunization were implemented by public health centers in the survey area, this may alone explain the difference in the age-appropriate immunization; the degree of the perception of barriers in terms of distance and time. Until now, the strategies to reduce barriers to immunization have carried out by emphasizing the economic aspects (free immunization). However, it is necessary in the future to consider the accessibility, in terms of distance and time, to the institution providing immunization.
In the present study, birth order and the number of siblings were related with the implementation of the age-appropriate immunization. These findings are consistent with what has often been reported; i.e., later birth order and more siblings result in less compliance to age-appropriate immunization.6,7,25,28-37
The present study showed that being born first lowered the perceived barrier scores, and raised the knowledge of the next immunization dates, resulting in higher compliance to age-appropriate immunization. There might be a number of factors contributing to the failure of late order siblings to complete immunization series. Lack of time and resources with larger families can result in attention dispersal. Also, growing confidence in the role of being a mother, as their children grow, may result in perceptions of children as less threatened or better protected by a mother's perceived competency.
The economic level and income conditions are important factors for compliance of immunization and age-appropriate immunization.34
Nevertheless, attempts to relieve economic impacts on immunization rates by providing free immunization12
have not resulted in increased immunization rates among those in lower economic categories.38,39
In Korea, free immunization is also provided when people use public health centers, and the present study also indicates that barriers-including economic factors affect age-appropriate immunization.
There was no difference in the scores of benefit of immunization and the susceptibility and severity of a disease between two groups. Surveys were conducted after immunization, not before it. Therefore, if the side effect of vaccine didn't happen and an infant didn't suffer from an infectious disease, we can conclude that the immunization of the past was good. Since it is perceived that there is little possibility to be sick because of the previous immunization, it is thought that the relationship between the susceptibility and severity of a disease and the fulfillment of behavior wasn't discovered.
There was no statistically significant difference in the scores of benefit of immunization. In addition, the susceptibility and severity of a disease, cue to action, and self-efficacy were higher in the age-appropriate immunization group than in the no age-appropriate immunization group.
However, perceptions of immunization dates were high in the age-appropriate immunization group. Knowing when immunization is scheduled enhances the possibility of carrying out the behavior.29
Especially, if the caregiver thinks that it is important to be immunized timely, the caregiver is Universilikely to act in a timely way.2,9,28,30,40
These results imply that it is necessary to make a policy to reduce the barrier to time and distance.
To our best knowledge, this study is the first report on the factors associated with age-appropriate immunization for 4 : 3 : 1 series (4 DPT, 3 Polio, 1 MMR) in Korea. However, our data were obtained from one limited area, and therefore, are not representatives of entire Korea. We conducted household survey and provider check using the questionnaire and checklist to obtain data. But, immunization history was identified according to a child's vaccination card with additional immunization data collected from medical records of private clinics and immunization registry data of public health center. Therefore, children without a vaccination card and/or medical data, and registry data were excluded. As a result, we might have overestimated the immunization rate. This study was cross-sectional design, therefore, it was not possible to clearly state that exposure or outcome was the cause and which effect. Our study was focused primarily on socio-demographic characteristics and mother's beliefs about childhood immunization, therefore, did not include many other factors, especially clinical condition of children and more objective measurement about the geographic distance between house and hospital or clinic. Children with clinical conditions (e.g., URI) that were contraindication of vaccination were 22.2-60.0%, which ranged widely, depending on the type and dose of vaccination. Prevalence of clinical condition was 60% in 1 dose of DTaP/DTP, 25% in 2 doses of DTaP/DTP, 35.3% in 3 doses of DTaP/DTP, 25.6% in 4 dose of DTaP/DTP, 40.0% in 1 dose of polio, 22.2% in 2 doses of polio, 35.3% in 3 doses of polio, and 40.0% in 1 dose of MMR. These were not categorized according to age-appropriate immunization; therefore, we might have underestimated the immunization rate.