GRAPHICAL ABSTRACT

INTRODUCTION

Undocumented migrants are individuals residing in a country without valid residence or work permits. Major international organizations recommend the use of the term “undocumented migrants” rather than “illegal migrants” [1]. Accordingly, we use the term “undocumented migrants” throughout this study.

As of June 2025, approximately 380 000 undocumented migrants are estimated to reside in South Korea (hereafter, Korea), accounting for 14% of all foreign residents [2]. Because they are excluded from the Korean National Health Insurance (NHI) system, they must bear the full cost of medical care, often incurring expenses that are 5–20 times higher than those for individuals covered by NHI [3]. Previous studies have reported that financial barriers, complex administrative procedures, language and cultural differences, and unstable legal status limit their access to healthcare services [4].

The principle of universal health coverage emphasizes the obligation to ensure health rights for all individuals, regardless of race, ethnicity, or socioeconomic status [5]. However, in Korea, the current health coverage system excludes undocumented migrants from such protection. Although the Korean government has implemented limited policies to improve healthcare access for non-citizens without legal residency status, migrant advocacy organizations and the National Human Rights Commission of Korea have repeatedly called for expanded healthcare coverage for undocumented migrants [6,7].

International studies indicate that undocumented migrants experience a wide range of health conditions [8–10]. Musculoskeletal disorders are particularly common (approximately 15%), reflecting physically demanding working conditions [11,12]. In addition, unstable living conditions and psychological stress contribute to high rates of non-specific pain and chronic diseases [10,11]. However, domestic research in Korea has primarily focused on healthcare barriers and behavioral factors [11,12], resulting in limited quantitative evidence on the clinical characteristics of this population.

According to the monthly statistical report published by the Korea Immigration Service (Ministry of Justice), approximately 2.87 million migrants resided in Korea as of May 2026 of whom 347 006 (12.1%) were undocumented [13]. These individuals often face precarious working conditions and unstable legal status, both of which may exacerbate health risks [14]. Given that the living and working conditions of undocumented migrants in Korea resemble those reported in international studies, we hypothesized that similar clinical patterns would be observed. Accordingly, we analyzed medical records from a single free clinic to characterize their clinical profiles. Although data from a single clinic may not be representative of the broader population, this study represents an initial effort to empirically examine the clinical characteristics and healthcare-seeking behaviors of undocumented migrants in Busan.

METHODS

Study Design and Data Collection

This retrospective study analyzed medical records from the Solidarity with Migrants free clinic between January 2020 and December 2024. Eligible participants included all undocumented migrants who received care during this period. Patients with Korean citizenship and those with incomplete records for age or gender were excluded. The final analytic dataset included 2111 clinical encounters from 750 patients. Detailed information on the study setting is provided in Supplemental Material 1.

Clinical data were extracted from physician-written summaries and anonymized by replacing personal identification codes with randomly generated numbers. To ensure data integrity, 2 researchers (LY and MH) independently reviewed the digitized records for errors and missing values.

Variables and Measurement

Study variables included patient characteristics, such as birth year, medical insurance status, gender, nationality, height, weight, smoking status, and alcohol consumption, as well as healthcare utilization data, including visit date and diagnosis code. Diagnoses were coded using the International Classification of Primary Care, Second Edition (ICPC-2), as applied in previous studies of undocumented migrants [8–10]. Up to 3 primary codes were extracted for each encounter. Patients diagnosed with hypertension (K86), non-insulin-dependent diabetes (T90), or lipid disorder (T93) were classified as having chronic disease. These 3 conditions were selected because they were the most frequently diagnosed chronic conditions in our dataset, are identifiable and manageable even in the resource-limited setting of a free clinic for migrants, and represent leading contributors to the global burden of non-communicable diseases [15]. For quality assurance, 2 researchers independently coded a random 5% sample, achieving 82.8% agreement (Cohen’s κ=0.81).

Statistical Analysis

Patient demographic and clinical characteristics were summarized using descriptive statistics. Continuous variables were presented as medians and interquartile ranges. To identify factors associated with visit frequency, multivariable mixed-effects negative binomial regression was performed, with nationality specified as a level 2 random intercept. The negative binomial distribution was selected because the outcome variable showed substantial overdispersion (variance-to-mean ratio=8.63; overdispersion parameter α=0.554), and the negative binomial model accounts for excess variability through an additional dispersion parameter.

Model 1 included age, gender, health behaviors, including smoking and alcohol consumption, body mass index, and chronic disease status as fixed effects. In model 2, interaction terms between gender and all other independent variables were introduced to assess whether the associations between these factors and visit frequency differed by gender. Linear combination testing was then conducted to evaluate the statistical significance of these interaction effects. Statistical analyses were conducted using Stata/SE 18 (StataCorp., College Station, TX, USA) and Python 3.12.3. Code verification was assisted by Claude Sonnet 4.6 (Anthropic, 2025).

Ethics Statement

The study was approved by the Inje University Paik Hospital Institutional Review Board (IRB No. 2025-08-012).

RESULTS

Of the 750 patients, 530 (70.7%) were men and 220 (29.3%) were women. The prevalence of chronic disease was 27.2% among men and 22.7% among women. Current alcohol consumption (25.3 vs. 16.8%) and smoking (25.1 vs. 12.7%) were more common among man patients than among woman patients (Table 1). No substantial gender differences were observed in the mean number of visits per year (Supplemental Material 2). Patients originated from 25 countries, with gender distributions varying by country (Supplemental Material 3). China accounted for the largest proportion of patients overall (23.5%) and of man patients (25.1%), whereas the Philippines accounted for the largest proportion of woman patients (22.3%). Pakistan showed the greatest gender disparity, with marked man predominance (22.3 vs. 3.6%).

Table 2 presents the distribution of health problems among undocumented migrants according to ICPC-2 classification. Cardiovascular (24.5%) and endocrine/metabolic conditions (22.7%) were the most common, together accounting for nearly half of all visits. Musculoskeletal (17.6%), skin (7.9%), respiratory (7.0%), and digestive conditions (6.2%) were also frequently recorded. Hypertension (K86, 23.5%) was the most common diagnosis, followed by non-insulin-dependent diabetes (T90, 16.1%) and lipid disorder (T93, 5.9%). Sexually transmitted infections (n=3) and pregnancy-related or childbirth-related conditions (n=1) were rare.

Mixed-effects negative binomial regression models were used to identify factors associated with the number of clinic visits (Table 3). In model 1, which included only main effects, age, smoking status, and chronic disease status were associated with visit frequency. Each 1-year increase in age was associated with a 4.2% higher visit frequency (β=0.041; 95% confidence interval [CI], 0.032 to 0.051; p<0.001), and chronic disease status was associated with a 75.8% increase in visit frequency (β=0.564; 95% CI, 0.383 to 0.745; p<0.001). Smoking was negatively associated with visit frequency (β=−0.211; 95% CI, −0.412 to −0.010; p=0.040), suggesting that non-smoking migrants tended to visit the clinic more frequently.

After gender interaction terms were introduced in model 2, the association with age remained significant (β=0.030; 95% CI, 0.014 to 0.045; p<0.001), whereas the main effect of chronic disease status was no longer statistically significant (β=0.151; 95% CI, −0.234 to 0.535; p=0.443). However, the interaction between gender and chronic disease status was positive and statistically significant (β=0.488; 95% CI, 0.053 to 0.924; p=0.028), indicating that the association between chronic disease status and visit frequency differed by gender.

Among women, chronic disease status showed little association with visit frequency (β=0.151, p=0.443). In contrast, among men, chronic disease status was associated with substantially higher visit frequency. Based on linear combination estimates, men with chronic disease had an 89.5% higher visit frequency than men without chronic disease (β=0.639; 95% CI, 0.058 to 1.220; p=0.031).

DISCUSSION

This study had 3 main findings. First, the predominance of man patients (70.7%) mirrored the national undocumented migrant population (62.9%) [16], a pattern that likely reflects labor market demographics and may also be influenced by the availability of predominantly man physicians at the clinic. In contrast, the median age of patients in this study (43 years) was higher than the national average, suggesting greater healthcare demand among older migrants. The nationality composition—predominantly Chinese, Pakistani, and Filipino patients—also differed from national trends, likely reflecting community networks and the availability of interpretation services.

Second, chronic diseases—specifically hypertension, non-insulin-dependent diabetes, and lipid disorder—accounted for 45.5% of the primary reasons for clinic visits, a proportion substantially higher than that reported among Korean patients attending primary care clinics (13.9%) [17]. This pattern may partly reflect the operational characteristics of the free clinic, which provides regular health check-ups once a year. Patients newly diagnosed with chronic conditions often continued follow-up visits, contributing to repeated utilization. Moreover, because the clinic operates only once per week, patients may be less likely to use it for acute or minor health problems. A similar pattern has been documented in a clinic serving undocumented migrants in Switzerland, where hypertension was the most prevalent condition and most patients had at least 1 chronic disease [9].

Third, gender differences were observed in free clinic utilization among undocumented migrants with chronic diseases. This finding contrasts with studies of the general Korean population, which have reported more frequent hospital visits and higher treatment adherence among women [18]. The discrepancy suggests that healthcare utilization among undocumented migrants with chronic conditions may vary according to citizenship status and healthcare entitlements and may intersect with gendered social structures that shape differential access to resources and care [19]. This pattern is consistent with prior research indicating that undocumented migrant women occupy the lowest tier in the healthcare hierarchy because of compounding structural and social constraints, including language barriers, administrative exclusion, financial precarity, and fear of deportation [20].

To better elucidate these gender-differentiated patterns, future studies should incorporate multisite datasets from migrant-serving institutions. Such data would enable comparative analyses across institutions and support more robust examination of interaction effects between gender and nationality in healthcare utilization. Complementary qualitative evidence from in-depth interviews with undocumented migrants living in Korea may also help identify contextual factors that limit follow-up care and health-seeking behaviors.

Based on this exploratory analysis of undocumented migrants in Busan, we highlight 2 policy implications for the Korean healthcare system. First, current healthcare policies may emphasize reactive intervention rather than continuous chronic disease management. Clinic visits due to chronic diseases among undocumented migrants at this center (45.5%) suggest substantial unmet demand for chronic disease management. The observed demand for primary care underscores the need to strengthen institutional access to primary healthcare for undocumented migrants, as early management of chronic diseases may prevent avoidable complications and reduce long-term health system burden. Given the aging and increasing settlement of migrant populations, restricting access to primary care based on citizenship is detrimental to migrant health and may perpetuate the risk of medical impoverishment.

Second, the limitations of volunteer-based services underscore the need for government-led health coverage for undocumented migrants. Physician volunteers working with Solidarity with Migrants have noted that services provided by free clinics are largely limited to basic diagnostic tests and that available treatments are highly constrained, making it difficult to provide care comparable to that available to Korean patients (personal communication, August 2025). As integral members of Korean society, undocumented migrants should be recognized within the Korean healthcare system and provided equitable access to essential medical services. Beyond continued reliance on volunteer-based community services that seek to secure healthcare access for the most vulnerable in the absence of public provision, more formal and publicly accountable systems of migrant health coverage are urgently needed.

This study is significant because it examines healthcare utilization patterns among undocumented migrants who remain part of Korean society but are excluded from the healthcare system. Notably, the analysis was based on data from a real-world primary healthcare setting in which undocumented migrants receive medical services.

Several limitations should be acknowledged. First, the characteristics of the free clinic may have influenced healthcare utilization patterns among undocumented migrants, limiting the generalizability of the findings. Second, the use of data from a single clinic further limits generalizability. Third, some ambiguity arose during the conversion of clinical diagnoses into ICPC-2 codes. For instance, non-specific diagnostic descriptions such as “dermatitis” could only be classified as “skin disease, other” (S99), rather than more specific categories such as “dermatitis seborrheic” (S86) or “dermatitis/atopic eczema” (S87), resulting in limited diagnostic specificity.

In conclusion, this study demonstrates that healthcare utilization among undocumented migrants attending a free clinic in Busan is largely focused on chronic disease management. Clear gender differences were also observed in healthcare utilization patterns related to chronic disease. Taken together, these findings underscore the need for more inclusive health policies for marginalized populations in Korean society.

Supplemental Materials

Notes

Conflict of Interest

The authors have no conflicts of interest associated with the material presented in this paper.

Funding

None.

Acknowledgements

This research was conducted with the official approval and cooperation of Solidarity with Migrants. The authors wish to express sincere gratitude to the activists, volunteer physicians, and the migrant community members whose participation and support were essential to this research.

Author Contributions

Conceptualization: Kim S. Data curation: Lee Y, Moon H. Formal analysis: Kim S. Funding acquisition: None. Writing – original draft: Lee Y, Moon H, Kim S. Writing – review & editing: Lee Y, Moon H, Kim S.

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