Introduction

Immunization has been regarded as one of the most cost-effective preventive health measures worldwide. Achieving and maintaining high levels of immunization coverage in whole population, especially in its most vulnerable members of a population is important for control and elimination of vaccine preventable diseases. In China, the routine immunization coverage have maintained higher levels. However, immunization coverage by immunization services and different groups varies widely. In recent years, internal migrants have increased dramatically with the remarkable economic development, and children taken along with their migrant parents have been found to be a particularly vulnerable group that is underimmunized. Given the increasing number and mobility of children taken along with migrants, the immunization status of them matters not only for themselves, but also for the entire society, and therefore, it is an increasing concern in China.

In contrast to many other countries, migration process and migrant definition in China are tightly related to a household registration system or hukou system. Generally, children's hukou status is inherited from their parents, despite their places of birth or current places of residence. Like other studies, the term "migrant children" is defined in this study as those children without hukou status of current place of residence. They are more vulnerable and the reported immunization problems were mainly related to them.

Despite their increasing magnitude and particular vulnerability with underimmunization, information on their immunization status is lacking. Especially, no estimates of immunization coverage among migrant children at the national level were available. Furthermore, the contributing factors to immunization status among migrant children were not well documented even by several previous studies, because of a small size of study subjects. Thus, using the data of the National Immunization Survey in 2004, this study examined the immunization status among migrant children at the national level.

The purpose of this study was to describe immunization status among migrant children, determine whether disparities in complete immunization coverage exist between migrant and non-migrant children, explore factors affecting immunization status among migrant children in China.

Methods

Data were drawn from the 2004 National Immunization Survey, a latest national survey that provides the nationally representative estimates of immunization status for children born in China between 2001 and 2003 as well as basic demographic information of both children and their parents or caregivers. Totally 155,899 children older than 12 months residing in the surveyed area for at least 3 months were included in the study.

The demographic information of each eligible child was collected by face-to-face interviews with one of the parents or caregivers using a standard questionnaire, and immunization history was abstracted from the child's immunization certificate or immunization record in immunization clinic. Immunization status was defined as complete immunization status if all the required immunizations with the 1:3:3:1 vaccines series (one dose of BCG, three doses of OPV, three doses of DPT and one dose of MV) had been given by age of 12 months. Migrant status of children was dichotomized by whether they have hukou status of current place of residence.

The analysis followed two stages with the first stage focused upon a descriptive analysis of immunization status with respect to migrant status and other variables. Differences in proportions and coverage levels were calculated using the chi-square test with at 5% of significance level. Timeliness of individual vaccine immunization was illustrated with the Kaplan-Meier method. The second stage utilized multivariate methods to determine the significant factors associated with complete immunization status. The predictor variables included factors related to child's gender, birth year, birthplace, place of residence, ethnic group, parents or caregivers' awareness, region, and immunization certificate availability. Two sets of models were utilized, with the first including both migrant children and non-migrant children in the model, investigating whether there is difference between two groups with respect to complete immunization coverage. The second set of models focused on migrant children only, enabling the exploration of covariates relative to migrant children.

Utilization and analysis of the 2004 NIS dataset was approved by the National Immunization Program of China CDC. The names of the study children and their parents or caregivers were not included in the available dataset or in any of the findings.

Results

Of 155,899 study children, 150,421(96.5%) were non-migrant, 5,478 (3.5%) were migrant. Compared with non-migrant children, migrant children were more likely to be Han ethnic group, female, live in urban area and eastern region, have immunization certificate, have lower immunization awareness parents or caregivers and receive immunization in immunization clinic.

For each individual vaccine and complete series vaccines, significant differences in coverage were identified between two groups. Among non-migrant children 87.6% were completely vaccinated, while only 78.2% migrant children were. Immunization with each single vaccine was delayed in both migrant and non-migrant children according to the recommended immunization schedule. Vaccination coverage by age for each single vaccine among migrant children was significantly lower than that of non-migrant children during the study period.

The result of logistic regression shows that place of delivery, place of region, place of residence, immunization certificate availability, parents' or caregivers' awareness and birthplace are significant factors associated with complete immunization status among migrant children. Those migrant children who resided in western region or rural area, delivered at home, without immunization certificate, non-locally born and had a caregiver with lower immunization awareness were less likely to be complete immunization status.

Discussion

Observed Disparities in Immunization Coverage

The finding that the complete immunization coverage was lower among migrant children compared with that of non-migrant children is consistent with the results of previous studies. But, the findings of this study would be most reliable because they were estimated based on a nationally representative sample, whereas the previous studies in China have addressed immunization coverage mainly focused on urban areas and limited in sample size.

Timeliness of immunization for each single vaccine

The significant difference in vaccination coverage by age partially supports the disparity in complete immunization coverage between migrant and non-migrant children. Moreover, the delayed vaccinations may leave a group of children vulnerable for a certain period between the vaccine recommended age and completed age, increasing the potential for epidemics or outbreaks of vaccine preventable diseases.

Factors associated with complete immunization among migrant children

An important finding of this study is that non-locally born migrant children were much less likely to be completely immunized than locally born migrant children. The main reasons for such disparity were examined in this study. Most non-locally born migrant children might have moved from an area with lower immunization coverage into an area with a higher coverage.

In the present study, having an immunization certificate was a strong significant factor for being complete immunization among migrant children. Having immunization certificate may imply that such parents or caregivers of migrant children have a sense of responsibility and positive attitude toward childhood immunization. In addition, immunization certificate availability adds an information source of immunization to caregivers, given there are general knowledge on immunization attached on the certificate.

Awareness level on immunization of migrant children's parents or caregivers was also related to immunization coverage in this study. Those migrant children who have a high-level immunization awareness parents or caregivers are nearly 1.6 times more likely to be in complete immunization than those who have parents or caregivers with low level immunization awareness. This emphasizes the fact that if the information, education, and communication activities on immunization are properly implemented, they can increase the immunization status of children irrespective of the place of region and place of residence.

Some other factors were found associated with complete immunization status among migrant children. Migrant children born at home were found to be less likely to receive complete immunization. The reason might be related to their parents' socioeconomic status. Both place of region and place of residence (urban and rural) influenced complete immunization status among migrant children in this study. Health inequality all over the country might account for these disparities. The access to health services including availability of routine immunization is inadequate in some regions as well as rural areas.

This is the first study that was based on data from a nationally representative survey to assess immunization status among migrant children. The large sample size makes it possible to control for some confounding characteristics to explore difference among variables and also permits the ability to perform stratified analyses in this study. In addition, unlike previous studies, which focus on migrant children in urban area, this study concentrates on all migrant children in the country. Therefore, this study provides more comprehensive view on immunization status as well as information on factors of complete immunization status among migrant children in China.

Based on this study, for improving immunization coverage among migrant children, more attention and efforts are needed for migrant children, especially if they live in western region or rural area. In addition, improving access to immunization and awareness of immunization for parents and caregivers may help to improve immunization coverage among migrant children in China.

Conclusion

This study demonstrates that migrant children are less likely to be complete immunization status in comparison to non-migrant children in China. Migrant children with complete immunization status are more likely to be locally born, have an immunization certificate, be born in health facilities, live in urban area or eastern and central region, and have parents or caregivers with higher awareness on immunization.

本研究は中国の流動人口の子どもにおける予防接種(BCG、ポリオ、百日咳-ジフテリア-破傷風三種混合(DPT)、麻疹)の現状を明らかにするため、2004 年に中国保健省が実施した全国予防接種調査のデータに基づいて、流動人口の子どもに対する生後12か月までの各ワクチンの接種率及び完全予防接種率(全ワクチン接種完了率)を推定し、非流動人口の子どもとの差を分析し、そして、完全予防接種の有無に影響する可能性のある因子について検討したものであり、下記の結果を得ている。

1.対象となった子どもは155,899名で、流動人口の子どもは3.5% (5,478名)を占めた。分析の結果、流動人口の子どもにおける接種率はBCG90.9%、ポリオ3回87.7%、DPT3回85.3%、麻疹85.5%、完全予防接種率は78.2%であった。一方、非流動人口の子ども(150,421名)における接種率はBCG97.8%、ポリオ3回94.0%、DPT3回93.3%、麻疹92.9%、完全予防接種率は87.6%であった。これらは、中国の流動人口の子どもにおけるの予防接種率を初めて推定したものである。

2.流動人口の子どもは非流動人口の子どもに比べて、各々のワクチンの接種率及び完全予防接種率が有意に低かった(カイ2乗検定によりp<0.001)。完全予防接種率について、多重ロジステイックモデルを用いて分析した結果、他の因子の影響を除いても、両群の差は有意であった。これらの結果は、種々の対策により、流動人口の子どもの予防接種率を向上させる必要性を示唆している。

3.流動人口の子どもにおいて、完全予防接種率は、出生地で生まれ、かつ出生地で予防接種をうけた群は64.2%、出生地で生まれ、かつ出生地と現居住地両方で予防接種をうけた群77.3%、現居住地で生まれ、かつ現居住地で予防接種をうけた群85.8%であり、各群内で有意な差を認めた。

4.流動人口の子どもにおける予防接種をうけていない理由については、「予防接種を知らなかった」、「予防接種の場所や時間を知らなかった」、「親が忙しかった」と答えた者が合わせて78.7%を占め、今後親に対して健康教育を実施して予防接種率を向上させる可能性のあることが示唆された。

5.多重ロジステイックモデルを用いて、流動人口の子どもにおける完全予防接種の有無に関連する要因分析をしたところ、「現居住地で生まれた」、「子どもの予防接種証明書を持っている」、「医療機関で生まれた」、「東部の省あるいは中部の省に住んでいる」、「予防接種に関する親の知識レベルが高い」という6つの変数が完全予防接種と関連していた。

以上、本論文は中国の全国予防接種調査のデータ分析から、流動人口の子どもの予防接種の実態と完全予防接種の有無に関連する要因を初めて解明したものである。本研究は流動人口の子どもにおける予防接種の現状の理解と今後の予防接種率の向上に重要な貢献をなすと考えられ、学位の授与に値するものと考えられる。