To examine the factors associated with the routine immunization status of children aged 2–3 years in China for gaining a better understanding of the Expanded Program on Immunization and to provide evidence for formulating specific strategies to guide the allocation of health resources.
We analyzed data from 45095 children aged 2–3 years in the 2013 National Immunization Coverage Survey to identify the sociodemographic and provider-associated factors affecting the full immunization status of children. Univariate and multiple logistic regression analyses were performed.
The immunization rate for children aged 2–3 years ranged from 95.9% (diphtheria and tetanus toxoid with pertussis vaccine, 4th dose) to 99.5% (Japanese encephalitis vaccine, 1st dose) and was 93.1% for full immunization. In terms of sociodemographic factors, male children [adjusted OR (AOR): 1.115; 95% confidence interval(CI):1.016–1.222], minority children (AOR: 1.632; 95% CI: 1.457–1.828), children of fathers with less than high school education (AOR: 1.577; 95% CI: 1.195–2.081), those born at home (AOR: 4.655; 95% CI: 3.771–5.746), those who immigrated from an adjacent county (AOR: 2.006; 95% CI: 1.581–2.546), and those living in urban-rural fringe areas (AOR: 1.807; 95% CI: 1.475–2.214) or mountainous areas (AOR: 1.615; 95% CI: 1.437–1.814) had significantly increased odds of not being fully immunized. In terms of provider-associated factors, administration of vaccines at home (AOR: 2.311; 95% CI: 1.316–4.059), household reminders (AOR: 2.292; 95% CI: 1.884–2.789), and travel time to vaccination providers of >40 minutes (AOR: 1.622; 95% CI: 1.309–2.010) were negatively associated with immunization rates. In addition, compared to 3-year-old years, 2-year-old children (AOR: 1.201; 95% CI: 1.094–1.318) were less likely to be fully immunized.
All included factors except maternal education level and distance from home to vaccination providers significantly affected immunization rates. Appropriate reminders and accessibility of immunization services played key roles in improving the immunization status. More attention to high-risk groups identified in this study may reduce the disparities in routine childhood immunization in China.
Citation: Cao L, Zheng J-S, Cao L-S, Cui J, Duan M-J, Xiao Q-Y (2018) Factors influencing the routine immunization status of children aged 2-3 years in China. PLoS ONE 13(10): e0206566. https://doi.org/10.1371/journal.pone.0206566
Editor: Italo Francesco Angelillo, University of Campania, ITALY
Received: July 16, 2017; Accepted: October 16, 2018; Published: October 31, 2018
Copyright: © 2018 Cao et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: Data cannot be made publicly available as survey respondents were assured raw data would remain confidential and did not consent to its being publicly shared. The data is from a national immunization survey conducted by the authors and led by the Ministry of Health of China and supported by the Chinese CDC (http://www.chinacdc.cn/en/contactus/). As the CDC cannot store or share the data, researchers interested in requesting access to the data should contact the authors directly: Jing-Shan Zheng (email: firstname.lastname@example.org) and Lei Cao (email: email@example.com).
Funding: This article was supported by the cooperation project between the World Health Organization and Chinese Center for Disease Control and Prevention. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
The Expanded Program on Immunization (EPI) is one of the most cost-effective public health interventions to protect children from vaccine-preventable diseases (VPDs) by delivering effective vaccines worldwide. The routine immunization program is a primary strategy of the EPI, which contributes to the reduction of both morbidity and mortality due to VPDs by vaccinating children aged 0–7 years and was initiated in 1978 with four vaccines in China . After China’s central government decided to introduce new vaccines into the EPI at the end of 2007, including hepatitis A vaccine (HepA); epidemic meningococcal polysaccharide vaccine (MPV); measles, mumps, and rubella vaccine (MMR); measles and rubella vaccine; and Japanese encephalitis vaccine (JE), 22 doses of 11 EPI vaccines have been introduced into the routine immunization program for all children aged 0–7 years in China . The Health Development Plan in China (2011–2015) proposed a goal of 90% for routine immunization coverage at the township level by 2015. Accordingly, a national immunization survey in China (CNIS) was conducted in 2013 to evaluate the completion of the goal and assess the immunization coverage level of children aged 2–3 years for 17 doses of eight vaccines administered before the age of 2 years .
Although China’s EPI has made remarkable progress in disease control in the past years, non-immunized children remain a public health concern [4,5]. A polio outbreak occurred in 2011 in the Xinjiang Uygur Autonomous Region of China after being polio-free for >10 years because of the introduction of wild poliovirus from neighboring Pakistan ; multiple measles outbreaks occurred in different geographical areas at different times [7,8], indicating that under-immunized populations remain an issue in China. To achieve the goal of disease control, understanding the factors that influence the full immunization of children holds practical significance. Therefore, we conducted this study using data from the 2013 CNIS to (1) identify factors that influence the immunization status of children in China aged 2–3 years using national representative samples, (2) improve our understanding of routine immunization in China, and (3) provide evidence for policy makers to improve immunization strategies going forward.
Materials and methods
Study setting and population
To date, the 2013 CNIS remains the most recent nationwide population-based household survey, which employed the methodology of the World Health Organization(WHO) and conducted strict quality control to ensure the validity of the survey. Children aged 2–3 years born between Sept. 1, 2009 and Aug. 31, 2011, randomly sampled from the 2013 CNIS were used in this study . The CNIS used a structured questionnaire to collect data, which can be found in Supporting information (S1 Table). Data on the immunization status of children were obtained from parents (61.9%), grandparents (36.0%), or relatives (2.1%). Over 99% of the children had their immunization records kept at home, which were used for assessing their immunization status. If children were without immunization records, surveyors were required to visit the local health center to retrieve their immunization cards. Of note, children with neither immunization records nor cards were classified as non-immunized .
At the end of 2012, China had >16.5 million children born each year in 31 provinces and 40446 townships . The 2013 CNIS was conducted in all 31 provinces, 963 counties and 1073 townships, accounting for 36.2% of the total counties and 2.6% of the total townships . The survey used the lot quality assessment sampling method [10,11] to determine the sample size of children to ascertain whether the immunization coverage of EPI vaccines reached 90% at the township level; overall, at least 42 eligible children were required to be sampled in each sampled township.
The survey employed a two-stage sampling method to identify the townships and households. Firstly, we randomly sampled 29–35 townships based on the number of townships in each province after merging those townships with a population of <10,000 to meet sample size requirements. Five villages were then randomly sampled in each township, and the village with a township government was selected. Finally, 10 children from each village with a township government and 8 children from other selected villages were randomly chosen. The sample frame of households with eligible children was established based on the household registration, health record, or rural cooperative medical record. If a household had more than one eligible child, then the surveyor randomly sampled an eligible child to interview.
The outcome variable in this study was the likelihood of children aged 2–3 years having missed at least 1 of 17 doses of eight required EPI vaccines (incomplete immunization), which included one dose of Bacillus Calmette–Guérin (BCG), three doses each of oral poliovirus vaccine (OPV) and hepatitis B vaccine (HepB), two doses each of measles-containing vaccine (MCV) and epidemic cerebrospinal meningitis group A polysaccharide vaccine (MPVA), four doses each of diphtheria and tetanus toxoid with pertussis vaccine (DTP), and one dose each of JE and HepA. If a child aged 2–3 years was immunized for all of the above 17 doses, then he/she was regarded as fully immunized. Immunization records at home and immunization cards at local health centers were used to identify the vaccination status of children. Children without records or cards were classified as non-immunized. Because routine immunization for JE has not been recommended in the province of Qinghai or in the Xizang and Xinjiang Autonomous Regions of China , children in these regions who had received 16 doses were considered to be fully vaccinated.
There were nine sociodemographic variables, including age (2 vs. 3 years old), gender (male vs. female), ethnicity (Han vs. non-Han nationality), area (rural, urban, or urban-rural fringe) and terrain of residence (plain, mountainous, or hilly), parental education level (college, high school, or less than high school), birthplace (hospital, township health center, or home), and residence status of children (permanent resident, migrant child from an adjacent county, or migrant child from another province).
The four provider-associated variables were as follows: (i) immunization providers, or whether immunization was administered by doctors in community health centers (in urban areas), township health centers (in rural areas), village clinics (in remote rural areas), or at home (in extreme remote areas); (ii) type of vaccination reminder, which could consist of advanced booking of the next vaccination appointment at the time of immunization service, notifications of an upcoming immunization appointment, or household reminders (i.e., immunization record kept at home or self-recall); (iii) distance from home to vaccination providers, grouped as <5 km, 5–10 km, or > 10 km; (iv) and travel time from home to vaccination providers, grouped as: < 20 minutes, 20–40 minutes, or > 40 minutes.
Data were analyzed using SPSS (SPSS Inc., Chicago, IL, USA) and statistical analyses were performed with the consideration of complex survey procedures. Univariate analyses were conducted to assess the relationship between immunization status and sociodemographic and provider-associated variables. Multiple logistic regression was utilized to determine the effect of the association. Variance inflation factors were calculated to exclude the possibility of multicollinearity between independent variables. None of these factors exceeded 10, which is the level suggested to be indicative of multicollinearity . P-values of <0.05 were considered significant.
The Ethical Review Committee of Chinese Center for Disease Control and Prevention (China CDC) approved the study protocol of the CNIS. During the survey, participants were explained the study purpose and their right to privacy. Informed consent was verbally obtained before interviewing participants. The data used in the study were considered by China CDC’s Ethical Review Committee to be exempt from IRB review and were approved to be used by the China CDC.
Recommended immunization schedule and vaccination coverage
Vaccination coverage in the 2013 CNIS and recommended immunization schedule for the EPI of China is shown in Table 1. In total, 45095 children aged 2–3 years were sampled. The immunization coverage of each reviewed vaccination in the 2013 CNIS was higher than 95%, ranging from 95.9% (DTP, 4th dose) to 99.5% (JE, 1st dose). The rate of fully immunized children aged 2–3 years was 93.1%.
Characteristics and univariate analysis
Table 2 summarizes the frequency of characteristics of children on sociodemographic and provider-associated factors. The results of univariate analysis on the association of exposure variables with incomplete immunization are shown with the crude odds ratio (OR) and confidence interval (CI) in Table 2. All variables except gender were significantly associated with an incomplete immunization status.
Multiple logistic regression analysis
All variables were entered into a multiple logistic regression analysis, the results of which are shown in Table 3. Eleven of the 13 included variables (all except maternal education level and distance from home to vaccination providers) were significantly associated with an incomplete immunization status.
The 2013 CNIS suggested that routine immunization coverage at the national level was high; however, it varied across provinces and type of vaccines because there were children with poor immunization status in some remote Midwestern provinces and Eastern urban areas . Low income , low maternal education level [14,15] and areas of residence  are risk factors for lower child vaccination rates. Although research in China has suggested that minority children, children from central and western rural areas of China with less education and younger parents , and those from migrant families in urban areas [18–20] were at a higher risk of not being vaccinated, the determination of factors influencing the immunization status of children awaits a national-level study. In the present study, we used the 2013 CNIS data to determine factors associated with the full immunization status of children to elucidate the progress of EPI to date and provide evidence for the formulation of specific strategies to guide the allocation of health resources.
We found that sociodemographic factors, except for the maternal education level, affected routine vaccinations to differing degrees. In general, it is broadly accepted that maternal education level is an exogenous socioeconomic determinant that significantly protects child health and varies across countries and regions; better vaccine coverage is associated with higher maternal education levels [21,22]. Unlike previous studies, the present study found that paternal rather than maternal education levels were strongly correlated with immunization status in multivariate analyses, and consistent findings have been found in some areas of China [13,23] and the Western Pacific . An explanation could be that higher paternal education levels are a proxy for higher family income and that family income positively affects the immunization status, although whether parental education level or income is more important in determining the health status of children remains debatable . Surveys in the cities of Chengdu and Wenzhou found that family income is positively correlated with the immunization status of immigrant children [14,25]. Moreover, the probability of being fully immunized increased with paternal education levels, suggesting that improving paternal understanding on immunization improves the immunization status of children. This may represent an alternative to solely focusing on maternal health education.
In some Eastern countries such as India and China, compared with male children, female children are less likely to have access to immunization services [15,26,27]. However, after controlling for various other factors, we found that female children were slightly more likely to be fully immunized than male children in China. Similar studies have found that gender discrimination in terms of health status and general knowledge of the concept of “son preference” has been attenuated or even eliminated in recent years in China . Overall, there are conflicting results on the effect of gender discrimination on immunization status, and results may depend on the research setting. Song  pointed out that in a family planning policy environment, the number of children that a family is permitted to have is subject to external restrictions. Son preference has changed to some extent, and gender discrimination may be reduced, particularly in families permitted to have only one child. However, we cannot dismiss gender differences in access to vaccinations because son preference continues to have a significant impact on children’s health in certain regions .
We also found that the age of children, area or terrains of residence, ethnicity, and residence status were significantly associated with vaccination status to varying degrees. The likelihood of being fully immunized appears to increase with age, most likely because older children have more opportunities to access immunization. Children living in urban-rural fringe areas were the least likely to be fully immunized than those living in other geographic locations, and children living in rural areas were more likely to be fully immunized than those living in urban areas. As a result of rapid urbanization, many immigrant populations have moved to urban-rural fringe areas, where a trend of lower immunization coverage and higher incidence of vaccine-preventable infectious diseases has been observed; these areas should be regarded as key areas for EPI management . Moreover, among migrant children, those who migrated from other provinces seem to have had a better immunization status than those who migrated from adjacent counties. To our knowledge, similar results have not been reported elsewhere. This could be because the local EPI paid considerably more attention to children from other provinces than to those from local provinces when managing routine immunization management. In addition, migrant parents may have been unable to access information on immunization services in a timely manner due to their limited socioeconomic status and knowledge of immunization, particularly for new vaccines introduced in China after 2008 .
Children residing in mountainous areas, those born at home, and minority children had the greatest risk of being incompletely immunized. These results were consistent with those of previous surveys conducted in a mountainous area  and those conducted in the provinces of Sichuan  and Fujian . Immunization services available to disadvantaged children in these areas were much weaker, lacked continuity, and were associated with a higher dropout rate because of transportation barriers, low socioeconomic status, limited access to health services, and inadequate immunization services .
Previous studies have shown that provider-associated characteristics are some of the most important risk factors for under-immunization [13,21,33], and information acquired from health-care workers was identified as one of the most influential factors for an adequate immunization among different population groups [34–36]. However, in China, several studies have primarily focused on parental satisfaction with immunization providers, rather than on other provider-associated factors, and on migrant children [19,26,37], rather than on all eligible children [14,30]. The present study found that vaccination providers, vaccination reminders, and travel time to vaccination providers were strongly associated with the immunization status among children. There were no significant differences in the immunization status among those vaccinated at community or township health centers compared to those vaccinated at village clinics. As mentioned by Linlin et al. , the administration of vaccines at home might be less effective than other vaccination settings in terms of achieving full immunization status. Consistent with findings from Nepal , travel time to vaccination providers was negatively associated with the immunization status, and travel time of <40 minutes may be preferable to avoid travel-related barriers to immunization in China.
Vaccination reminders are well-received and effective methods for improving immunization coverage . In China, vaccination reminders, such as advanced appointment bookings or notifications, have long been included as routine methods for improving immunization coverage; however, few studies have examined the effectiveness of such reminders. The present study found that advanced appointment bookings might be more effective than other types of reminders, and the greatest risk of missing immunizations occurred when parents depended on nothing more than household reminders, such as immunization records or self-recall. A study in Beijing  further demonstrated that advanced appointment bookings set up via a mobile phone application are more effective and met parental requirements better than those manually set.
It is important to note the limitations of this study. First, this study involved the secondary analysis of data collected in the CNIS. As with all secondary datasets, data were limited by questions asked in the initial survey and responses may have been provided by a guardian who was not necessarily most familiar with the child’s health history. Secondly, the cross-sectional survey design utilized was less powerful than an analytical design would have been in terms of evaluating risk factors for incomplete immunization status. Finally, to simplify the operation of the investigation, we did not require the surveyor to collect the response rate, even it was high.
This study was the first to assess a wide range of sociodemographic and provider-associated factors influencing the routine immunization status of children using nationally representative survey data in China. Further, this was a large population-based study with adequate power to detect small differences between groups, and 99% of immunization information based on the record minimizes the potential of information bias. Results revealed that all included sociodemographic and provider-associated factors, excluding only maternal education level and the distance from home to vaccination providers, significantly affected vaccination status among children. The probability of being fully immunized appears to increase with age, implying that children who are not able to be vaccinated on time were likely to be followed up and finish fully immunization. Furthermore, the study provided evidence that appropriate vaccination reminders and accessibility of immunization services play key roles in improving the routine vaccination coverage of children. Addressing these factors could contribute to identify at-risk children and geographic areas and could help develop corresponding strategies to reduce disparities in routine childhood immunization in China.
We would like to special thank Lance Rodewald for his support in polishing the manuscript and providing constructive comments.
- 1. Wang Long-De. Brief history of development of EPI in China. Immunization Practice and Management. Beijing: People’s Medical Publishing House; 2006. pp. 2–6.
- 2. Bai Jian-feng. China expaned the scope of the national immunization program. People’s Daily. Beijing; 19 Feb 2008: 2008.
- 3. Cao Lei, Zheng Jing-Shan, Yu Shi-Chen, Zhang Ye-Wu, Zhou Li-Wei, Lu Li. Sampling design of the China immunization coverage survey for national immunization program vaccines at the township level in 2013. Chinese J vaccines Immun. 2014; 20: 481–555.
- 4. Zheng Jing-Shan, Cao Lei, Guo Shi-Chen, Zhang Xiao-Shu, Li Lu, Wang Lei. Immunization coverage of the national immunization program vaccines at the township Level, based on a survey conducted by provincial CDCs in China, 2013. Chinese J Vaccines Immunization. 2014;6: 492–546.
- 5. Cao Lei, Wang Hua-Qing, Zheng Jing-Shan, Yuan Ping, Cao Lin-Shen, Zhang Guo-Min, et al. National immunization coverage survey in China after Integrated more vaccines into EPI since 2008. Chin J Vaccine Immun. 2012;18: 419–424, 478 [in Chinese].
- 6. Wen Ning, Fan Chun-Xiang, Fu Jian-Ping, Ning Jing, Ji Yi-Xin, Luo Hui-Ming, et al. Enhanced surveillance of acute flaccid paralysis following importation of wild poliovirus in Xinjiang Uygur Autonomous Region, China. BMC Infect Dis. 2014;14: 113. pmid:24576083
- 7. Nyangoma EN, Olson CK, Benoit SR, Bos J, Debolt C, Kay M, et al. Measles outbreak associated with adopted children from China—Missouri, Minnesota, and Washington, July 2013. MMWR Morb Mortal Wkly Rep. 2014;63: 301–4. pmid:24717816
- 8. Zheng Xiang, Zhang Ningjing, Zhang Xiaoshu, Ha L, Su Q, Wang H, et al. Investigation of a measles outbreak in China to identify gaps in vaccination coverage, routes of transmission, and interventions. PLoS One. 2015;10: 1–11. pmid:26208216
- 9. National Statistics Bureau. China Statistical Yearbook. 2013.
- 10. Lwanga S, Lemeshow S. Sample size determination in health studies: A practical manual, 1991. World Health Organization, Geneva. 1991. https://doi.org/10.2307/2290547
- 11. WHO. Monitoring immunization services using the LQAS. Geneva; 1996.
- 12. Robinson C, Schumacker R. Interaction effects: centering, variance inflation factor, and interpretation issues. Mult Linear Regres Viewpoints. 2009;35: 6–11.
- 13. Kim SS, Frimpong JA, Rivers PA, Kronenfeld JJ. Effects of maternal and provider characteristics on up-to-date immunization status of children aged 19 to 35 months. Am J Public Health. 2007;97: 259–266. pmid:17194865
- 14. Luman ET, Mccauley MM, Shefer A, Chu SY. Maternal characteristics associated with vaccination of young Children. 2008;
- 15. Sridhar S, Maleq N, Guillermet E, Colombini A, Gessner BD. A systematic literature review of missed opportunities for immunization in low- and middle-income countries. Vaccine. Elsevier Ltd; 2014;32: 6870–6879. pmid:25444813
- 16. Ozcirpici B, Sahinoz S, Ozgur S, Bozkurt AI, Sahinoz T, Ceylan A, et al. Vaccination coverage in the South-East Anatolian Project (SEAP) region and factors influencing low coverage. Public Health. 2006;120: 145–154. pmid:16260009
- 17. Jiang Zhen, Ju Li-Rong, Wang Zhi-Feng, Zhang Hui, Zhang You-Ran, Luo Da-Sheng. Current status on child essential immunization coverage in rural coverage in rural western and central China. Chinese J child care. 2014;22: 343–346.
- 18. Jiang-nan WU, Li-fang Huang, Zhi-qiang Lin. Study on coverage rates of the EPI vaccines and its influencing factors in Fujian, China. Strait J Prev Med. 2015;21: 1–3.
- 19. Zeng Li, Yang Rui-Pei, Zhang Liang-Zhi. Study on the immunization coverage rate and its influential factors among floating children in Chengdu. Mod Prev Med. 2016;43: 2014–2017.
- 20. Zhu Lin, Liu Ming, Chang Yi, Ma Jian. Primary immunization among preschool children in a community in the urban-rural fringe area. Chinese Gen Pract. 2012;15: 1257–1260.
- 21. Theeten H, Hens N, Vandermeulen C, Depoorter AM, Roelants M, Aerts M, et al. Infant vaccination coverage in 2005 and predictive factors for complete or valid vaccination in Flanders, Belgium: an EPI-survey. Vaccine. 2007;25: 4940–4948. pmid:17524528
- 22. Samad L, Tate AR, Dezateux C, Peckham C, Butler N, Bedford H. Differences in risk factors for partial and no immunisation in the first year of life: prospective cohort study. Bmj. 2006;332: 1312–1313. pmid:16740559
- 23. Shan Fu-Xiang, Gang Liu, Lu Zi-Yan. Analysis on the knowledge of immunization and influential factors among children’s parents in Shenzhen. Chinese J vaccines Immun. 2007;13: 590–592.
- 24. De Figueiredo A, Johnston IG, Smith DMD, Agarwal S, Larson HJ, Jones NS. Forecasted trends in vaccination coverage and correlations with socioeconomic factors: a global time-series analysis over 30 years. Lancet Glob Heal. 2016;4: e726–e735.
- 25. Case A, Lee D, Paxson C. The income gradient in children’s health: A comment on Currie, Shields and Wheatley Price. J Health Econ. 2008;27: 801–807. pmid:18191487
- 26. Lin Xian-Dan, Chen Ling-Ping, Zheng Xiao-Chun. Analyses of factors influencing the coverage of national immunization program vaccines in migrant children in wenzhou. Chinese J Child Heal Care. 2011;19: 51–53.
- 27. Pande R. Selective gender differences in childhood nutrition and immunization in rural India: The role of siblings. Demography. 2003;40: 395–418. pmid:12962055
- 28. Fang W, Xing Z. Family – related Factors Affecting Child Health in China. Popul Res. 2012;36: 50–59.
- 29. Song Yue-Ping, Tan Lin. The preference of son and gender discrimination in childhood health. Popul Res. 2008;32: 2008.
- 30. Deng Yu, Tan Xiao-Dong, Ni Zi-Ling, Yang Rui, Liu Yu-Zhong. Survey of expanded vaccination coverage of 0–6 year-old children and its impacting factors in Wuhan. China Trop Med. 2013;13: 1220–1223.
- 31. Hou Sai, Zhang Xiu-Jun, Huang Wei, Ge Xiao-Jiao. Survey on expanded program on immunization status of 1–7 years old children in Dabie Mountain areas back-land. Chinese J child care. 2011;19: 552–554.
- 32. Cao Yan. Investigation on status of preventive inoculation in health care services for children and its influencing factors. J Clin Med Pract. 2016;20: 84–87.
- 33. Napolitano F, Navaro M, Vezzosi L, Santagati G, Angelillo IF. Primary care pediatricians’ attitudes and practice towards hpv vaccination: A nationwide survey in Italy. PLoS One. 2018;13: 1–12. pmid:29596515
- 34. Smith LE, Amlôt R, Weinman J, Yiend J, Rubin GJ. A systematic review of factors affecting vaccine uptake in young children. Vaccine. The Authors; 2017;35: 6059–6069. pmid:28974409
- 35. Napolitano F, Napolitano P, Liguori G, Angelillo IF. Human papillomavirus infection and vaccination: Knowledge and attitudes among young males in Italy. Hum Vaccin Immunother. Taylor & Francis; 2016;12: 1504–1510. pmid:27070042
- 36. D’Alessandro A, Napolitano F, D’Ambrosio A, Angelillo IF. Vaccination knowledge and acceptability among pregnant women in Italy. Hum Vaccines Immunother. Taylor & Francis; 2018;14: 1573–1579. pmid:29863958
- 37. Gong Guo-Qin. Analysis of impact factors of the vaccination in national immunization program in community migrant children. ChinJ PHM. 2016;32: 212–214.
- 38. Lin Lin, Chen Chao, Tian X. Control research on routine children immunization service format. Chinese J vaccines Immun. 2007;13: 574–576.
- 39. Devkota S PB. Childhood immunization and access to health care: evidence from Nepal. Asia Pac J Public Heal. 2016;28: 167–177.
- 40. Morris J, Wang W, Wang L, Peddecord KM, Sawyer MH. Comparison of reminder methods in selected adolescents with records in an immunization registry. J Adolesc Heal. Elsevier Inc.; 2015;56: S27–S32. pmid:25863551
- 41. Jiang Long-Xun, Liu Si-Wei, Wang Xue-Hao, Gao Wei. Research of periodical inoculation distribution changes and intervention effect in eight vaccination clinics in Beijing. Med Recapitul. 2016;22: 1758–1761.