The determinants of immunization coverage among children aged between 12 and 35 months: a nationwide cross-sectional study in Lao People’s Democratic Republic

It is important to prevent vaccine-preventable diseases through immunization and maintain and improve the population’s health. Immunization protects individuals and reduces the number of such infections in society by improving collective immunity when many people are immunized. The Expanded Program on Immunization (EPI) was initiated in 1974 by the World Health Organization (WHO) and the United Nations Children’s Fund (UNICEF) to promote vaccination to protect children worldwide against preventable infectious diseases [1]. Initially, tuberculosis, polio, diphtheria, tetanus, whooping cough, and measles were selected for vaccination. Subsequently, with new vaccines added, EPI works with other public health programs to improve the health of all people everywhere [1].

In the Lao People’s Democratic Republic (PDR), the national immunization program launched the measles vaccine in 1982/1984 [2‐5]. However, measles is still endemic due to immunization difficulties and surveillance in remote mountainous areas where most people live. Therefore, a pilot measles campaign targeting children aged 9–59 months was conducted in two provinces in 2000, with the remainder of the country covered in 2001 to improve measles control [2, 6]. In Lao PDR, supplementary immunization activities (SIAs) for measles were performed in 2007 and 2011 [6]. The rubella vaccine was introduced in the 2011 SIA and subsequently included in the National Immunization Program (NIP) in 2012 [6]. Lao PDR introduced a routine second dose of the measles-containing vaccine in the NIP in 2017. In 2019, a subnational immunization campaign was conducted for children aged between 6 months and 10 years [6]. Between 2000 and 2020, hepatitis B, Haemophilus influenza type B, polio (inactivated polio vaccine), pneumococcal bacteria, rotavirus, Japanese encephalitis virus, and human papillomavirus vaccines were introduced into the NIP [6]. The first national multistage random cluster sampling survey to identify the sociodemographic factors affecting immunization was conducted in 2014. Following the survey, the national EPI conducted two SIAs specifically on measles. The Eighth National Health Sector Development Plan 2016–2020 aims for at least 90% immunization coverage by 2020. Interventions to increase routine immunization coverage and to improve access to vaccines have been implemented in Lao PDR, for example, by developing cold chains to build the capacity of healthcare workers to deliver vaccines [7]. Conversely, the measles and hepatitis B vaccines (Hep B) administered at birth in 2019 were still low, at approximately 70% [8]. Therefore, there is a need to conduct a national survey to evaluate the interventions implemented recently and broaden the understanding of the factors influencing child immunization.

To the best of our knowledge, this is the first study to examine the immunization coverage nationwide among children aged 12–35 months using multistage cluster sampling and investigate the determinants of full immunization with a birth dose of Hep B after introducing recently implemented efforts to improve vaccination.

Our study indicated that childbirth at a hospital or health facility was significantly associated with complete immunization. This is related to access to health services, which is similar to previous studies in Lao PDR [10‐14]. Lao PDR is ethnically diverse, including 49 ethnic groups, most of whom live in rural and remote mountainous areas, with limited communication, transport, and social service provision [15]. Therefore, access to health services remains a significant barrier to immunization coverage. In addition, comprehensive and appropriate information dissemination is important for immunization against EPI-covered diseases [16]. The utilization of micro-planning for immunization sessions and the activities of mobile teams have also been suggested to be successful in providing routine immunization [5]. Therefore, adapting these activities to local conditions is necessary to improve immunization coverage.

However, the proportion of children (173 children, 67.6%) who were fully immunized was lower than the national target of 90%. The definition and rates of full immunization varied among studies in Southeast Asian (SEA) countries. Previous studies have reported complete immunization rates of 59.0–80.8%, 79.3–86.4%, and 55.4% in Lao PDR [14, 16, 17], Malaysia [18, 19], and Myanmar [20], respectively. Several factors are associated with immunization coverage in SEA countries. Sociodemographic characteristics affecting complete immunization are the number of children in the family, child’s age, child’s ethnicity, mother’s age, mother’s ethnicity, mother’s religion, mother’s education, mother’s occupation, father’s education, father’s occupation, zone of residence, travel time to health facilities, and willingness to pay for immunization [14, 16‐22]. Health system and service utilization factors, including mother’s antenatal care attendance, tetanus vaccination during pregnancy, and delayed immunization schedule, are also associated with immunization coverage [18, 20]. Here, the univariate analysis showed that non-residency in a fixed house and a greater number of children were associated with partial immunization. However, several countermeasures have been explored to improve vaccine coverage. In low- and middle-income countries, education may be more effective than incentives to increase vaccination [23]. Similarly, it has been suggested that soft skills, including communication by community outreach teams regarding immunization activities in Lao PDR, also help residents’ vaccine acceptance [13]. Therefore, the effective utilization of the health system and services should also be considered to achieve the national target of full immunization.

Here, full immunization was defined as having received eight doses of vaccines included in the WHO definition of full immunization (one dose of BCG vaccine, three doses of the polio vaccine, three doses of diphtheria-tetanus-pertussis vaccine, and one dose of measles-containing vaccine) [24], plus a birth dose of Hep B and three doses of PCVs considering the vaccine introduction situation in Lao PDR [8]. In addition, the following vaccines were identified as factors contributing to partial immunization: a birth dose of Hep B, three doses of PCVs, and one dose of measles-containing vaccine, and non-coverage rates for these vaccines were similar to those previously reported [8, 25]. Furthermore, the immunization coverage we identified was similar to that previously reported (59.0–80.1%) [14, 16]; however, differences in the definition of full immunization may have had an impact.

The strength of our study is that we surveyed individuals with documented immunization records on a nationwide scale and selected participants using random sampling, which has the advantage of accurately assessing the situation throughout the country. However, this study had some limitations. First, the survey has the advantage of being able to assess immunization status on a national scale; however, groups with varying immunization statuses, including ethnic minorities, may be elusive. For example, in Lao PDR, a measles outbreak was reported in 2019 in an ethnic minority group with low immunization coverage. Therefore, national policies should recognize this heterogeneity. Second, this study analyzed subpopulations of the nationwide measles and rubella seroepidemiological survey. Therefore, the study design and sample size followed this survey, which assessed measles and rubella immunity nationwide. Thirdly, against the targeted 416 pairs, 256 pairs were finally included in the analysis and 160 pairs were excluded from the analysis. Among them, 98 pairs without vaccination records were excluded from the analysis. Therefore, although the current situation requires immunization records, including those used in this study, the results may have selection bias [13]. However, the issues raised in our study warrant further investigation into subgroups of ethnic minorities using mobile device apps for immunization records.

Our study elucidated that the immunization status among children aged between 12 and 35 months in Lao PDR is satisfactory in improving access to healthcare by strengthening communication with residents regarding health service utilization, and expanding mobile outreach services may play a pivotal role in this endeavor. Further research is warranted to evaluate efforts to increase immunization coverage and target populations with limited access to healthcare.