The Expanded Program on Immunization (EPI) has played a key role in global health, contributing to the eradication of smallpox and the elimination of polio and neonatal tetanus. As a national initiative, EPI provides free vaccines, while the immunization schedule specifies the timing and sequence needed for full protection. These achievements align with Sustainable Development Goals (SDG) 3 (Health) and SDG 10 (Reduced Inequalities).1 Globally, EPI prevents 2–3 million deaths annually;[1] in Vietnam, it averted an estimated 5.7 million disease cases and 26,000 deaths between 1980 and 2010.2

However, ethnic minorities and other vulnerable groups still face barriers to vaccination, including poverty, limited education, and remote geography.3 In Vietnam, third-dose DPT-Hep B-Hib and first-dose MCV coverage in 2023 were 65 % and 82 %, respectively.4 Children in rural and ethnic minority communities often receive delayed or incomplete vaccinations.5 According to the study conducted in 2008 in The Netherlands, 35 % of Orthodox Protestant children were unvaccinated;[3] in 2017, along the Thailand–Myanmar border, coverage for DTP, HBV, OPV, and measles vaccines ranged from 54.6 % to 56.3 %.6 These gaps increase the risk of outbreaks, threatening both minority groups and the general population.7

This study adopts the Health Belief Model (HBM) to examine factors influencing vaccination adherence among minority communities. HBM suggests that health behaviors are shaped by perceived susceptibility, severity, benefits, barriers, and cues to action.8 Since parental beliefs are central to vaccination decisions, applying HBM provides valuable insights into how these perceptions affect adherence to the EPI.9

Despite EPI’s global success in improving child health, gaps remain in addressing the needs of minority populations facing complex barriers—such as poverty, language, and geographic isolation—which are often underrepresented in research.3610 In Vietnam, national reports rarely disaggregate vaccine coverage data by minority status, and studies often overlook timeliness, focusing only on overall coverage.11 A study found significantly lower rates of timely vaccination among rural and minority children, emphasizing the need for Vietnam’s EPI to prioritize timeliness in reaching these underserved groups.5 Yet, the critical factor in building immunity remains underexplored in current research.1012

Beyond timeliness issues, disparities in access to immunization services worsen vaccination inequities. Disadvantaged groups in Vietnam, particularly ethnic minorities, face significant barriers - including language limitations that hinder understanding of immunization messages.13 In Dong Thap Province, Khmer and Cham communities encounter additional challenges such as cross-border migration, sociocultural beliefs about disease, and limited healthcare infrastructure, all of which impede timely and equitable vaccination.

Although parental decisions impact vaccination adherence, they are shaped by broader systemic factors like healthcare access and vaccine availability. Addressing these structural barriers is key to improving vaccination rates and equity among minority populations. This study hypothesized that socio-demographic factors, HBM constructs, vaccine provision, and healthcare accessibility predict adherence among ethnic minority parents in Dong Thap Province, Vietnam. Therefore, this study aimed to determine levels of parental adherence to EPI and identify key predictors of vaccination adherence among ethnic minorities in Dong Thap province, Vietnam. The findings aim to inform practical strategies for improving vaccination timeliness and equity in this vulnerable group.

A key component of public health initiatives to stop vaccine-preventable diseases is making sure that everyone is vaccinated on time and completely. However, achieving high vaccination compliance rates remains a challenge in underprivileged communities. This study underscored the importance of assessing not just vaccination coverage but also adherence to schedules to achieve comprehensive population immunity. Despite Vietnam's high vaccination coverage rates, adherence among ethnic minority communities remained low, with only 18.9 % of parents adhering to recommended schedules. This discrepancy exposed a gap in the effectiveness of the EPI in addressing the needs of underserved populations.

The existence of global and regional disparities in adherence might reveal as follows: the adherence rates among ethnic minorities in Vietnam were lower compared to similar studies conducted among Arab communities in Israel, where DPT/Hib and measles adherence rates were 92 % and 82 %, respectively.22 Higher adherence rates in Israel have been linked to supportive societal norms, universal health coverage, and robust follow-up and pediatric services. In comparison, the 56.7 % adherence rate among migrant children on the Thailand–Myanmar border was notably higher than the 18.9 % among ethnic minority children in Vietnam. This may reflect Thailand’s more proactive policies and mobile immunization programs targeting hard-to-reach and undocumented populations.6 In contrast, Vietnam’s EPI in border areas still relies heavily on fixed-site delivery, with limited outreach strategies.

In addition to parental decisions, healthcare infrastructure plays a critical role in vaccination adherence. Children in areas with better access to hospital deliveries and community prenatal care had significantly higher rates of timely immunization.5 Seasonal agricultural work may also cause delays, as parents deprioritize vaccination visits. In rural minority areas, children often receive delayed rather than refused immunizations—most are eventually vaccinated, but adherence to the schedule remains challenging.5

Adherence further declines as children age, consistent with findings from the Korean CDC.23 Parents may initially follow schedules closely but become less vigilant as children grow, especially if they appear healthy or have previously fallen ill despite vaccination.23 Older children are also more likely to have comorbidities, which can complicate vaccination.24 Other factors—such as fear of needles.25 past side effects.26 and competing demands—can also reduce adherence.

Age-related declines may reflect gaps in health system outreach. While newborns benefit from routine postpartum contacts, booster doses (e.g., JE2, JE3) often rely on parental initiative. This reduced engagement by the health system over time may lead to missed or delayed vaccinations. Given Vietnam’s cumulative EPI schedule, older children face more chances of falling behind. Our strict adherence definition—timely for all vaccines—may lower adherence rates in older age groups, not due to reduced parental effort but due to longer exposure to potential delays.

Firstborn children showed higher adherence rates than later-born siblings, consistent with findings from multiple countries.27 Parents often prioritize the health needs of their first child and follow medical advice more strictly, while in larger families, divided attention and increased childcare experience may reduce the urgency for subsequent children.28 Health centers should implement targeted reminder systems for these higher-risk families to support timely vaccination.

HBM analysis identified perceived barriers—such as time constraints, vaccine unavailability, and limited information—as key obstacles to adherence, echoing studies from South Korea and Saudi Arabia.2930 In Saudi Arabia, high self-efficacy was sometimes linked to lower adherence due to exposure to anti-vaccine narratives and distrust in vaccine safety.30 In contrast, our study found that higher self-efficacy was associated with better adherence, as confident parents trusted local health workers and the EPI system.

This study's predictive model (binary logistic regression) reliably identified families at risk of non-adherence to the EPI using demographic (child’s age, birth order) and psychosocial factors (perceived barriers, self-efficacy). Health workers can apply priority screening to guide follow-up visits, targeted counseling, reminder systems, or educational outreach. This approach allows for efficient resource allocation by focusing on families most likely to miss or delay vaccinations.

Several limitations should be noted. First, without survey weighting, adherence estimates may not represent all communes equally, affecting generalizability. Second, while vaccination cards were verified, self-reported data on socio-demographics, parental perceptions, vaccine provision, service access, and child characteristics may introduce recall bias. Third, clustering effects were not adjusted for; though households were randomly selected, unmeasured commune-level factors (e.g., infrastructure, outreach programs) may have influenced outcomes. Future studies should consider cluster-adjusted analyses to account for these contextual variables. Lastly, future models may be improved by including factors such as parental trust in healthcare, knowledge of the immunization schedule, and exposure to vaccine misinformation.

Parental adherence to the EPI for children under five in Dong Thap’s minority population was low (18.9 %). Key predictors included perceived barriers, self-efficacy, child age, and birth order. Interventions should prioritize older and higher birth order children and address logistical and psychological barriers such as access, time constraints, and concerns about adverse events.

Strengthening collaboration with local leaders and health volunteers and using multilingual education materials can improve outreach and confidence. Integrating vaccination with maternal-child health visits, along with SMS reminders and home visits, may support timely immunization in mobile populations. In cross-border areas, robust tracking systems are vital due to migration challenges. Training healthcare workers in culturally sensitive communication and involving experienced parents in peer-support programs can further strengthen adherence efforts.