The Use of Mobile Medical Units for Populations Experiencing Homelessness in the United States: A Scoping Review

Abstract

With annual point-in-time counts indicating a rise in unsheltered homelessness in the United States, much attention has been paid to how to best provide care to this population. Mobile medical units (MMUs) have been utilized by many programs. However, little is known regarding the evidence behind their effectiveness. A scoping review is conducted of research on MMU provision of medical services for populations experiencing homelessness in the USA to examine the extent and nature of research activity, summarize available evidence, and identify research gaps in the existing literature. Following guidelines for scoping reviews, PubMed and Google Scholar were used to identify an initial 294 papers published from January 1, 1980, to May 1, 2023, using selected keywords, which were distilled to a final set of 50 studies that met eligibility criteria. Eligible articles were defined as those that pertain to the provision of healthcare (inclusive of dental, vision, and specialty services) to populations experiencing homelessness through a MMU in the United States and have been published after peer review. Of the 50 studies in the review, the majority utilized descriptive (40%) or observational methods (36%), with 4 review and 8 controlled studies and no completed randomized controlled trials. Outcome measures utilized by studies include MMU services provided (58%), patient demographics (34%), health outcomes (16%), patient-centered measures (14%), healthcare utilization (10%) and cost analysis (6%). The studies that exist suggest MMUs can facilitate effective treatment of substance use disorders, provision of primary care, and services for severe mental illness among people experiencing homelessness. MMUs have potential to provide community-based healthcare services in settings where homeless populations reside, but the paucity of randomized controlled trials indicates further research is needed to understand if MMUs are more effective than other care delivery models tailored to populations experiencing homelessness.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11606-024-08731-9.

INTRODUCTION

An estimated 582,484 individuals experienced homelessness on a given night in 2022; this number has risen consistently since 2016.¹ People experiencing homelessness (PEH) have increased burden of chronic disease, increased rates of infectious diseases and fatal and non-fatal accidents, and 12 times the risk of overdose compared to the general population.^2,3 These factors, combined with high levels of exposure to chronic stress and trauma, contribute to PEH having accelerated rates of aging and four times the rate of mortality of the general population.^4–6 Reduced rates of preventative health screenings such as colorectal cancer screening and vaccinations among PEH also likely contribute.^2,7 PEH face many barriers to accessing primary care or other preventative healthcare, including behavioral and cognitive challenges with daily activities as well as difficulties with transportation, insurance, legal status, or stigma barriers.⁸ These barriers result in PEH having higher hospital utilization for mental, behavioral, and neurodevelopmental disorders and longer hospitalizations compared to the general population, which is estimated to cost the healthcare system over $9 billion annually.⁹

There are various approaches to providing healthcare to PEH, such as ambulatory intensive care, low-barrier clinics that allow walk-in appointments, shelter-based clinics, and other types of integrated care with legal aid and social services.^10–13 However, these approaches may be too intensive, may not reach, or may not be appropriate for all PEH. Many of these approaches also represent more passive outreach as opposed to direct community outreach. Mobile medical outreach has been trialed as a method to overcome the barriers that PEH face in accessing care. These services range from traditional street medicine to more formalized mobile medical units (MMUs) in which providers see patients in a vehicle that serves as a mobile health clinic.¹⁴ Mobile medical outreach strategies engage hard-to-reach populations such as certain minority groups and groups with multiple competing health and social needs.¹⁵ These outreach strategies may improve health outcomes through increased health screenings, preventative care, and chronic disease management as well as reduced emergency department (ED) utilization.⁸ Most MMU programs are funded through philanthropy, with one mobile health clinic reporting a return on investment of $36 for every $1 invested in the program.^16,17 Between 2007 and 2017, over 811 mobile health clinic programs nationwide provided a median number of 3491 patient visits annually.¹⁶ Mobile care recipients generally view these programs favorably, reporting they create a culture of respect and inclusivity, are expeditious, free, and convenient.¹⁸

There has been little research focused specifically on the impact of MMUs on PEH, despite the number of programs that have quickly established. Given the limited amount of peer-reviewed studies and the paucity of controlled trials, we selected a scoping review approach to systematically map the research conducted in this space.¹⁹ We formulated the following research question: What is known from the existing literature about the state of research on MMU provision of medical services to people experiencing homelessness, and are there studies exploring the effectiveness of these services? In this study, we define effectiveness as the degree of beneficial effects under “real world” clinical settings, inclusive of cost-effectiveness analyses.²⁰

METHODS

Review Protocol

A review protocol was developed based on the Preferred Reporting Items for Systematic Reviews and Meta-analyses Protocols (PRISMA-P).²¹ The two major search databases used were PubMed and Google Scholar. Eligible articles were defined as those that pertain to the provision of healthcare (inclusive of dental, vision, and specialty services) to PEH through some type of MMU in the United States. Included studies were in English and were published from January 1, 1980, to May 1, 2023, and peer-reviewed.

Search Method

Searches were conducted in PubMed using MESH terms like “mobile health unit”: “mobile health unit” OR “mobile clinic” OR “mobile health” AND “homeless”, and in Google Scholar using keywords like: “homeless” AND “mobile health unit” AND “United States”. The full syntax of keywords and strategies used in these searches are available upon request from the corresponding author. The initial search returned a total of 294 unduplicated studies (Google Scholar 192, PubMed 102).

Review Process

Between May and September 2023, two reviewers (NC, JH) reviewed all 294 studies independently. In total, 237 studies were removed after preliminary review as they did not meet the inclusion criteria (Google Scholar 173, PubMed 64) leaving 57 remaining studies. After a secondary review (NC, JH, JT), six more studies were removed as they were not peer-reviewed publications, and one study was removed as it did not pertain to medical service provision. This resulted in a final selection of 50 studies. Figure 1 includes a flow chart summarizing the study identification and selection process.

Figure 1.

Flow chart of study identification and selection process.

A data chart form was jointly developed by the reviewers, which included key study characteristics, the type of study, study outcome measures, and a summary of the findings. The two reviewers independently completed the form for each study, with a third reviewer available to adjudicate and resolve discrepancies. A secondary review of the type of study was conducted by all study authors, and a simplified categorization was developed: review studies (including integrative, symposia, and systematic reviews), controlled studies (including prospective and retrospective controlled cohort studies, nonrandomized experimental and randomized controlled trials, and time-lag design cohort studies), observational studies (including cost analysis, cross-sectional, spatial mapping, mixed methods, and retrospective analysis studies), and descriptive studies (including case studies and qualitative studies). A broad categorization of study outcomes was also developed. We use the term “study outcomes” here to refer to the type of findings generated by the study and not specifically in terms of clinical outcomes of patients (i.e., outcomes were of the study and not necessarily of patients). For example, some study outcomes fell into the category of services/logistics, which included descriptions of quantity of services or program structure. The study outcome category of patient populations included demographic descriptions or beliefs/attitudes of patients served by MMUs; the study outcome category of health outcomes included studies that reported a measurable clinical outcome inclusive of mental health and substance use; the category of patient-centered outcomes included studies that use patients’ self-reported perspectives of MMU care, changes in housing, or well-being; the category of healthcare utilization outcomes included studies that report patient utilization of healthcare resources during or after implementation of MMU care; and the category of cost-effectiveness outcomes included studies of associated savings from MMUs considering associated costs. We followed PRISMA-ScR in reporting our findings.²²

RESULTS

A total of 50 articles were included in this review. Table 1 contains the number and proportion of studies according to research design and study outcome measure (an additional table of study type by study outcome category displayed as a matrix is available in the Appendix). Tables 2, 3, 4, and 5 include descriptions and primary clinical outcome measures for all review (n = 4), controlled (n = 8), observational (n = 18), and descriptive (n = 20) studies, respectively. The following section summarizes the included studies by study outcomes in order of frequency to highlight potential gaps in the literature.

Table 1.

Number and Proportion of Studies According to Type of Research and Outcome Measure (n = 50)

Research methodology	N	%
Review methods (n = 4)
Integrative review	2	4
Symposia review	1	2
Systematic review	1	2
Controlled methods (n = 8)
Experimental (nonrandomized)	1	2
Prospective controlled cohort	3	6
RCT	1	2
Retrospective controlled cohort	2	4
Time-lag design cohort	1	2
Observational methods (n = 18)
Cost analysis	1	2
Cross-sectional	2	4
Mixed methods	1	2
Retrospective analysis	13	26
Spatial mapping	1	2
Descriptive (n = 20)
Case study	17	34
Qualitative	3	6
Study outcome	N	%
MMU services/logistics	29	58
MMU patient population	17	34
Health outcomes	8	16
Patient-centered measures	7	14
Healthcare utilization	5	10
Cost-effectiveness	3	6

Table 2.

Description of Included Review Studies

First author, year	Study description and sample size	Outcome measure(s)	Key findings	Other findings/limitations
Post, 2007	Integrative review of 33 National Health Care for the Homeless (HCH) Council programs	MMU patient population: -Demographics MMU services/logistics: -Description of services -Program design	Largely adult, high-risk, urban populations 76% provide primary care services, 48% screenings Most programs staffed by medical provider; many community partnerships; most funded by HRSA	Most common reason for MMU service delivery is to create welcoming services for PEH that otherwise are unable or will not seek care at fixed-site clinics; report common barriers to healthcare for PEH
Coaston, 2022	Integrative review of 12 studies investigating MMU provision of primary care	Health outcomes: -Chronic disease management (asthma medication ratio, blood pressure, HgbA1c) Healthcare utilization	↑ AMR compared to usual care ↓ Systolic BP by 10.7 mm Hg ↓ HgbA1c ↓ ED visits ↓ Hospital length of stay	25% of studies included PEH; MMUs improve the management of asthma, hypertension, and diabetes; MMUs appear to be efficacious and cost-effective for vulnerable populations through emergency department avoidance and decreasing hospital length of stay
Howe, 2009	Symposia review of 21 presentations at the 2007 and 2008 International Street Medicine Symposia investigating quality management of programs	MMU patient population: -Demographics MMU services/logistics: -Description of services Patient-centered measures	Largely adult, unhoused, and socially vulnerable, transient population Many services provided in mobile units utilizing electronic medical records The “in reach” and “Goal-Negotiated-Care framework” were two exemplar patient-centered models	Best practices to serve hard-to-reach populations include the use of MMUs; MMUs provide enhanced privacy to perform a more thorough history and physical examination; patient engagement and patients’ assessment of their well-being are two short-term quality measures for MMU programs
Yu, 2017	Literature review of 51 studies investigating mobile health clinics in the United States	MMU patient population: -Demographics MMU services/logistics: Cost-effectiveness	Increased engagement among minority, socially vulnerable populations Increased screening, preventative care, and chronic disease management ↓ ED visits ↓ Hospitalizations/readmissions	Review not exclusive to PEH; reports MMUs increase cost savings, improve equity in accessing healthcare, and are effective at addressing medical and social determinants of health; maps and describes programs using MMUs

Table 3.

Description of Included Controlled Studies

First author, year	Design, N	Intervention/comparison arm	Outcome measure(s)	Key findings
Rosenblum, 2002	Experimental (nonrandomized), N = 250	Mobile medical van with 2 part-time physicians, a physician assistant, and a driver/medical aid; one arm received social work (SW) intervention; reported 4-mo outcomes	Health outcome: -Drug use Healthcare utilization: -ED visit Patient-centered measure: -Housing status	↓ drug use in both interventions ↓ ED visits w/ SW intervention ↑ housing and public assistance in both interventions
Guo, 2001	Prospective controlled cohort, ex post matched control, N = 2200	Mobile mental health crisis service delivered by a team of crisis intervention specialists, registered nurses, and psychiatrists providing crisis intervention and case management services; control group received hospital-based intervention	Healthcare utilization: -Hospitalization rate and timing	↓ 30-day hospitalization rate by 8% in mobile crisis intervention cohort compared to hospital-based intervention
Rife, 1991	Prospective controlled cohort, N = 176	Mobile case management assigned to people with severe mental illness experiencing homelessness	Patient-centered measures: -Quality of life	↑ global well-being (perceived living situation, use of leisure time, finances, and physical health)
Swanson, 2003	Prospective controlled cohort, N = 974	Homeless-focused healthcare sites (shelter/outreach clinics and mobile vans) treating women experiencing homelessness compared to same population seeking care in county/government clinics or private offices	Patient-centered measures: -Patient satisfaction	↑ patient satisfaction at homeless-focused healthcare sites
Springer, 2022	Type 1 effectiveness-implementation RCT protocol (ongoing), Target N = 864	Patient navigation vs MMU services linking justice-involved individuals to community-based HIV and substance use disorder prevention and treatment	Health outcomes: -Chronic disease management (Pre-exposure prophylaxis prescription rates for HIV; HIV outcomes)	Describes the protocol for a 5-year hybrid type 1 effectiveness-implementation randomized controlled trial that compares PrEP uptake and HIV outcomes among justice-involved individuals with high rates of homelessness
Fine, 2023	Retrospective controlled cohort, N = 138	Mobile addiction medicine clinic in Boston targeting PEH compared to a propensity score matched control cohort	Healthcare utilization: -Outpatient visits -Hospitalizations -ED visits	No statistically significant changes in outpatient visits, hospitalizations, or ED visits in mobile addiction medicine clinic cohort
Hall, 2014	Retrospective controlled cohort, N = 2259	The New Jersey Medication Assisted Treatment Initiative delivering mobile addiction care in MMU (opioid agonist treatment, syringe exchange program and counseling) compared to patients entering treatment at fixed-site methadone clinics or non-medication assisted treatment	MMU patient population: -Demographics	MMU cohort more likely to be black, unhoused, uninsured, have intravenous drug use and were less likely to have had previous addiction treatment
Morris, 2001	Time-lag design cohort, N = 25	Mobile Outreach and Crisis Services (MOCS) unit utilizing a Program for Assertive Community Treatment model	Health outcome: -Psychiatric symptoms Patient-centered measure: -Housing status -Functional status	↓ psychiatric symptoms ↑ housing ↑ global functioning

Table 4.

Description of Included Observational Studies

First author, year	Design, N	Intervention	Outcome measure(s)	Key findings
Song, 201323	Cost analysis, N = 5900	An urban MMU in Massachusetts addressing blood pressure among other primary care concerns for PEH	Health outcomes: -BP reduction Cost-effectiveness: -Return on investment	↓ in systolic and diastolic blood pressure of 10.7 mmHg and 6.2 mmHg, respectively, resulting in reduced risk in stroke and heart attack and ED visits, thereby producing a positive lower bound for the clinic’s return on investment of 1.3
Matteoli, 201524	Cross-sectional study, N = 21	Volunteer outreach podiatry unit serving PEH	Health outcomes: -Diabetic foot wound healing	86% of patients showed resolution of their foot wounds
Bartek, 199625	Cross-sectional study, N = 3144	MMU visiting homeless shelters addressing chronic and acute medical conditions	Healthcare utilization: -Prescriptions by MMU	Respiratory products, antibiotics, and NSAIDSs were the most commonly prescribed medications for MMU patients
Regis, 202026	Mixed methods, N = 328	MMU staffed with clinicians and other staff to increase access to harm reduction services, addiction treatment, and primary care by deploying caregivers to overdose “hotspots” in Boston	MMU services/logistics: -Harm reduction/addiction services provided	MMU established 9098 contacts with people with OUD, distributing a total of 96,601 syringes, 2956 naloxone kits, and provided 854 buprenorphine prescriptions to 164 unique patients
Barry, 199427	Retrospective analysis, N = 764	MMU staffed by resident physicians offering on-site urgent care in New Haven	MMU patient population: -Demographics	26% of patients were homeless with a mean length of homelessness of 15 months and 41% had been victimized within 1 year
Slagg, 199428	Retrospective analysis, N = 1430	Mobile psychiatric unit in Chicago	MMU patient population: -Demographics	65% of patients were members of minority groups, 60% were male, 28% had alcohol or other substance abuse problems, and more than 37% had a history of hospitalization in a state psychiatric hospital
Nuttbrock, 200329	Retrospective analysis, N = 1042	Project Renewal’s mobile medical services for PEH in New York City	MMU patient population: -Reasons for visiting medical van -Most prevalent infectious diseases -Most prevalent substances used	Acute medical concerns, prevention, and chronic medical condition were the most common reasons for seeking care at the MMU. Hepatitis B and tuberculosis were the most common infectious disease and cocaine the most common substance used
Busen, 200830	Retrospective analysis, N = 95	MMU located in an urban community that attracts a large number of homeless, street-involved youth	MMU patient population: -Demographics -Reason for leaving home MMU services/logistics: -Description of services	Mean age of patients was 20.5 years with most having transient living situations. Abuse accounted for the majority leaving home Success of the program was associated with providing treatment and counseling for psychiatric conditions, risky sexual behavior, and substance use
Moore, 201031	Retrospective analysis, N = 235	MMU with HEENT provider conducting cancer screening and providing education to PEH about head and neck cancer risk factors	MMU patient population: -Reasons for visiting MMU -Understanding of cancer risks MMU services/logistics: -Description of services	41.4% of the participants reported at least 1 otolaryngologic sign or symptom. Most individuals did not recognize that smoking can lead to head or neck cancer Approximately 10% of participants were referred for HEENT evaluation and underwent biopsy revealing a malignancy that was later treated. Mobile HEENT screening appears to be effective at educating PEH of cancer risks and providing early detection of cancer
McNamee, 199432	Retrospective analysis, N = 175	MMU serving children experiencing homelessness in a midwestern metropolitan area; results over 1-year period	MMU patient population: -Demographics MMU services/logistics: -Description of services	Majority of patients were under the age of 6 Most common diagnoses treated were upper respiratory infection, bronchitis, otitis media, asthma, scabies, and rash
Spanowicz, 199833	Retrospective analysis, N = 1171	MMU providing primary healthcare to men living in a shelter; results over 4-year period	MMU patient population: -Demographics -Reasons for visiting MMU MMU services/logistics: -Description of services	Majority of men were under the age of 40; most common reason for seeking care were respiratory problems, depression, ENT problems, skin disorders, and preventive health maintenance Most common MMU interventions were acute care treatments on site, education on management of health, prescribing medications, and referrals Documented 2086 encounters with male patients
Paris, 199634	Retrospective analysis, N = 7500	Mercy Mobile Health Care staffed by an interdisciplinary team with a fleet of vans and mobile clinics to provide primary care, including HIV counseling and education, to PEH in the inner city of Atlanta	Health outcomes: -HIV seropositivity rates and risk factors	↓ HIV seropositivity from 12 to 6% during the 2-year study period. HIV seropositivity was associated with age, intravenous drug use, and history of sexually transmitted disease. Patients who have not completed high school and those who have been homeless for more than one year exhibited the highest risk for contracting HIV
Gibson, 201735	Retrospective analysis, N = 8415	Community Health Care Van (CHCV) providing healthcare services such as primary care, infectious disease screening and treatment, and opioid agonist treatment for PEH in New Haven	MMU patient population: -Demographics MMU services/logistics: -Description of services	Being foreign-born, having injection drug use, and having hypertension were associated with increased MMU use Clients receiving buprenorphine had the highest visitation rates
Malone, 202015	Retrospective analysis, N = 811	Mobile Health Map, a comprehensive database of mobile clinics in the United States that compiles data to measure, improve, and communicate the impact of MMUs in the United States; results over 10-year period	MMU patient population: -Demographics MMU services/logistics: -Description of services -Program design -Funding sources	Over half of clients represent racial minorities MMUs provided a median of 3491 annual visits; primary care is most common service model; most are independent or university-affiliated and receive at least some financial support from philanthropy
Zucker, 201236	Retrospective analysis, N = 202	MMU providing screening, counseling and vaccinations for hepatitis A and B for subjects at high risk for hepatitis C virus and HIV in western Massachusetts	MMU patient population: -Demographics Health outcomes: -HCV screening outcomes and risk factors	Majority male with history of nasal inhalation of substances, tattoos, and incarceration ↑ rates of HCV were found in people using injection drugs, who had a history of sharing needles, or a history of receiving treatment for substance use disorder
Hudmon, 202337	Retrospective analysis, N = 639	Pharmacist-led tobacco use screening and brief cessation interventions during MMU encounters in urban Indiana	MMU patient population: -Demographics -Readiness to quit smoking MMU services/logistics: -Description of services	23.7% of food pantry clients and 66.7% of homeless shelter clients reported smoking 49.2% indicated readiness to quit within 2 months and of these 90% accepted a tobacco quit line card
McGee, 199538	Retrospective analysis, N = 408	MMU providing primary care services to sheltered, homeless, women in a Midwestern urban setting over 3-year study period	MMU patient population: -Demographics -Reasons for visiting MMU	Majority of women were between ages of 26 and 40; respiratory disorders were the most commonly treated condition; preventive health maintenance also a major concern
Gibson, 201439	Spatial mapping, N = 8404	Community Health Care Van (CHCV) providing healthcare services such as primary care, infectious disease screening and treatment, and opioid agonist treatment for PEH in New Haven	MMU patient population: -Demographics -Distance traveled for services	A minority of CHCV clients travel sizable distance with frequent utilization; high-frequency MMU users show overlap of substance use disorder, violence, and AIDS

Table 5.

Description of Included Descriptive Studies

First author, year	Design, N	Intervention	Outcome measure(s)	Key findings
Leo, 202140	Descriptive study, N unspecified	Van-based buprenorphine induction unit, granting low-barrier buprenorphine access to dozens of individuals experiencing street homelessness in urban Chicago	MMU services/logistics	A partnership fostered collaboration between MMU and telehealth addiction provider to provide medication delivery by the mobile health van, resulting in access to buprenorphine with little to no cost to patients
Knight, 199041	Descriptive study, N unspecified	A mobile clinic employing nurses to provide health assessment and group education in NJ	MMU services/logistics	A nurse-led mobile clinic was started in 1990 in NJ
Stickler, 202142	Descriptive study, N unspecified	A mobile COVID-19 unit to provide timely person-centered care to COVID-19–positive individuals experiencing homelessness	MMU services/logistics	13 people experiencing homelessness were served in first 2 weeks of this mobile van program in Rochester, MN
King, 199443	Descriptive study, N unspecified	A mobile medical outreach van in Wilmington, DE, designed to service the needs of under-resourced and homeless individuals	MMU services/logistics	What started as an outreach van in Wilmington, DE, then started offering a variety of family-related services to the indigent and now offers a more comprehensive, holistic healthcare service to those in need with the goal of lowering the infant mortality rate by identifying high-risk pregnancies early on and managing complications during pregnancy and after birth
Paris, 199444	Descriptive study, N unspecified	A mobile clinic program that deploys four vans in greater Atlanta to treat medically at-risk, hard-to-reach clients experiencing homelessness	MMU services/logistics	Mercy Mobile Health Program in Atlanta is a model of care for PEH reaching people living with HIV and other chronic illnesses
Caires, 201745	Descriptive study, N unspecified	A mobile clinic program that provides acute care services to individuals who are homeless while fostering interprofessional teams through nurse practitioner–physician collaboration	MMU services/logistics	An NP-led mobile clinic successfully provided acute care services to people experiencing homelessness
Sharon Attipoe-Dorcoo, 202046	Descriptive study, N unspecified	A mobile health van webinar sharing experiences, challenges, and best practices of responding to COVID-19	MMU services/logistics	Mobile clinics are a longstanding community-based service delivery model that fills the gaps in healthcare delivery safety-nets, and can also reach social-economically underserved populations in both urban and rural areas, of particular importance during the COVID-19 pandemic
McManus, 1992 47	Descriptive study, N unspecified	A mobile health van serving homeless and under-resourced children in urban New York	MMU services/logistics	The “Big Blue” van of The Children’s Aid Society brings much needed health services (mainly dental) to homeless and underserved children of New York City
McCarley, 1998 48	Descriptive study, N unspecified	A mobile outreach crisis service in urban Oklahoma helping those suffering from severe and persistent mental illness	MMU services/logistics	A mobile outreach crisis service in Tulsa, OK, is an effective way to assist those suffering from severe mental illness and homelessness in acute settings
Zeien, 202349	Descriptive study, N unspecified	A student-led interprofessional volunteer organization that provides medical care and other essential services to individuals experiencing homelessness in urban Arizona	MMU services/logistics	A student-led mobile clinic is an effective way to provide direct outreach to underserved populations and to address public health concerns such as emerging disease outbreaks like Mpox
Patti, 199050	Descriptive study, N unspecified	A clinic system aiming to break the cycle of homelessness through provision of continuing healthcare services in Atlanta, GA	MMU services/logistics	Describes the foundation and history of what would become the Atlanta Community Health Program for the Homeless program
Duford, 201951	Descriptive study, N unspecified	A service-learning model that uses a mobile health clinic experience as part of the community medicine rotation for physician assistant students	MMU services/logistics	The mobile health clinic service-learning model provides diverse, meaningful experiences for PA students
Redlener, 199452	Descriptive study, N unspecified	A mobile primary care pediatrics clinic for children experiencing homelessness in urban New York	MMU services/logistics	A mobile primary care clinic successfully served children experiencing homelessness in NYC
Schotland, 2021 53	Descriptive study, N unspecified	A mobile van providing primary care and basic laboratory testing to children experiencing homelessness in urban Massachusetts	MMU services/logistics	Historical description of 1970’s Bridge Over Troubled Waters (Bridge) Medical Van, (Steve Disenhof)
Fraino, 201554	Descriptive study, N unspecified	A nurse practitioner-led mobile health clinic providing primary care and wraparound support services to individuals experiencing homelessness in Marin County, CA	MMU services/logistics	Nurse practitioners can play a critical role in mobile health units, which in turn can aid individuals experiencing homelessness
Hastings, 200755	Descriptive study, N unspecified	A mobile clinic with volunteer undergraduate students, medical students, and physicians providing healthcare and education to people experiencing homelessness in urban California	MMU services/logistics	The UCLA mobile clinic is an effective mode of caring for people who are homeless; this population continues to struggle with substantial challenges in between episodes of care
Gao, 201956	Descriptive study, N unspecified	A mobile dental clinic providing oral healthcare to individuals in rural regions	MMU services/logistics	Mobile dental vehicles may expand access to dentistry for rural people, some experiencing homelessness
Swigart, 200457	Qualitative, N = 55	A qualitative study of 55 individuals experiencing homelessness in urban PA and their decisions on whether or not to accept free tuberculosis screening	Patient-centered measures	The reasons for accepting/rejecting care for homeless persons vary substantially on an individual basis
Paradis-Gagné, 2023 58	Qualitative, N = 12	A critical ethnography of 12 individuals experiencing homelessness exploring needs and preferences of outreach nursing interventions	MMU services/logistics	Nurse-led clinics provide timely access to healthcare that can satisfy needs that aren’t typically considered “health needs”; these needs are important for people experiencing homelessness
Ramirez, 202259	Qualitative, N = 31	A qualitative study of 31 people experiencing homelessness at a shelter clinic (N = 16) and a mobile clinic located in a church (N = 15) in Austin, TX	Patient-centered measures	Patients of mobile clinics desire shared decision-making, trust with providers, continuity of care, and integrated social services

MMU Services/Logistics

Twenty-nine studies reported MMU services/logistics as a study outcome. Although this is the most reported outcome, the quality of evidence is marginal given most of the studies are observational (8) or descriptive (18). Most studies described programs treating adults, although some focused on women,³⁸ children,^32,47,52,53 or adolescent care delivery.³⁰ In terms of types of care delivered, many MMUs addressed primary care, urgent care, or preventative care concerns.^{15,23,25,27,29,33,35,45,52,59–62} One review of 51 studies found that MMUs largely provide increased screening, preventative care, and chronic disease management.⁶¹ MMU delivery of addiction care is also common,^{26,37,40,63–66} along with psychiatric crisis care.^28,41,48 Other specialty services provided via MMU include podiatry,²⁴ otolaryngology,³¹ dental care,⁵⁶ and infectious disease care such as COVID-19 response,^42,46 monkey pox response, ⁴⁹ HIV care,^34,67 and hepatitis C care.^34,36 The most common diagnoses treated among adults in one observational study were respiratory problems, depression, ENT problems, skin disorders, and preventive health maintenance.³³ The most common diagnoses treated among children in one study were upper respiratory infection, bronchitis, otitis media, asthma, scabies, and rash.³²

The majority of programs delivered healthcare through use of a physician.⁶⁸ However, there were several nurse-led models,^45,58,60 including incorporation of a psychiatric mental health nurse practitioner,⁴¹ one pharmacy-led smoking cessation program,³⁷ one social work intervention,⁶⁴ one case management intervention,⁶⁹ and several student-led MMU clinics.^51,55 In terms of affiliation and funding, an analysis of all MMUs registered on Mobile Health Maps noted that the majority of programs are independent or university-affiliated and receive at least some financial support from philanthropy;¹⁵ one review noted that most include community partnerships and are funded by HRSA.⁶⁸ Many descriptive studies provided a historical description of the planning, development, and implementation of MMU programs in various states.^{43–45,50,53}

MMU Patient Population

Seventeen studies reported demographic data on populations served by MMUs, with the majority being observational studies (13). There were three reviews^8,68,70 and one controlled study reporting demographic data.⁶⁵ The average age of adult MMU patients varied, with most being 40 years of age or younger.^33,38 One program serving street-involved youth noted an average age of patients of 20.5 years,³⁰ while another program targeting children experiencing homelessness noted that the majority of patients were under the age of 6.³² A literature review of 51 studies noted that MMUs found increased engagement by minority population;⁸ multiple studies identified MMUs as serving a high proportion of black, foreign-born, or other minority populations.^15,28,35,65 MMU patients exhibited high rates of being uninsured,⁶⁵ having a history of incarceration,³⁶ and being chronically homeless or victims of domestic violence.²⁷ Abuse was the most common reason for leaving home in one study of homeless youth.³⁰ MMUs also reached other high-risk populations: those with high rates of mental illness, substance use, intravenous drug use, or HIV.^28,35,39,65 Patients receiving buprenorphine had the highest visitation rates for one MMU.³⁵ Two out of every three homeless shelter clients smoked tobacco, with nearly 50% indicating readiness to quit within 2 months in one study.³⁷ Infectious diseases such as hepatitis B and tuberculosis were common in those using MMUs,²⁹ and one study noted older age, intravenous drug use, history of sexually transmitted disease, low education level, and being homeless for more than 1 year were associated with increased risk for contracting HIV.³⁴ Patients’ reasons for visiting a MMU were reported in several studies, citing acute medical concerns, prevention, and treatment for chronic medical conditions.^29,38 One study utilized spatial mapping to identify the distance traveled by participants to receive care, noting that only a minority of MMU patients traveled long distances to receive care.³⁹ One review of 33 National Healthcare for the Homeless Council programs found that MMUs primarily served largely adult, high-risk, urban populations,⁶⁸ a finding supported by a symposia review which added that MMU programs primarily served socially vulnerable and transient populations.⁷⁰

Health Outcomes

One review study, three controlled studies, and four observational studies reported health outcomes. One literature review found that MMUs increased asthma medication rates and reduced hemoglobin A1c, although only three of the 12 studies included PEH.⁶² One study noted mean reductions in systolic blood pressure of 10.7 mmHg among unhoused MMU patients.²³ One podiatry care MMU found that 86% of patients showed resolution of diabetic foot wounds while under their care.²⁴ One MMU crisis intervention targeting PEH with severe mental illness showed a reduction in psychiatric symptoms among patients.⁷¹ One program proving HIV care and prevention education saw a reduction of HIV seropositivity from 12 to 6% during the 2-year study period.³⁴ One randomized controlled trial is currently underway to investigate HIV outcomes among criminal justice involved individuals at high risk for homelessness.⁶⁷ One program measured hepatitis C virus (HCV) screening rates and seropositivity.³⁶ Two studies demonstrated reductions in drug use by MMU patients from socially vulnerable populations.^64,65

Patient-Centered Measures

One review, four controlled, and two qualitative studies measured patient-centered outcomes such as housing status, patient satisfaction, or quality of life. One prospective controlled cohort study noted that women experiencing homelessness had increased patient satisfaction receiving care from homeless-focused care sites (inclusive of MMUs) as compared to county/government clinics or private offices.⁷² A study of a mobile outreach and crisis service MMU showed improvements in housing status and global functioning.⁷¹ One program noted improved housing and public assistance after a social work intervention,⁶⁴ and one program that included a case management intervention improved a global well-being measure.⁶⁹ One review described the “in reach” and “Goal-Negotiated-Care (GNC) framework” as two exemplar patient-centered models of care for MMUs that serve PEH, and proposed that patient engagement and patients’ assessment of their well-being are two short-term quality measures for MMU programs.⁷⁰ One qualitative study identified that fear of a bad test result, stigma, and confidentiality concerns were reasons PEH rejected services of a MMU.⁵⁷ Another qualitative study identified shared decision-making, developing trust, continuity of care, and integrating social services into healthcare as important components of care to appropriately serve PEH.⁵⁹

Healthcare Utilization

One review, three controlled, and one observational study assessed healthcare utilization. The review noted reductions in ED utilization and decreased hospital length of stay among MMU programs providing primary care.⁶² One MMU social work intervention was associated with reductions in ED visits.⁶⁴ Thirty-day hospitalization rate decreased by 8% in an MMU crisis intervention cohort compared to a hospital-based intervention.⁷³ One study found that a mobile addiction clinic for homeless-experienced adults did not lead to reductions in healthcare utilization in the first year.⁶⁶ One cross-sectional study investigated prescriptions filled by MMU patients, noting that respiratory products, antibiotics, and NSAIDSs were the most commonly prescribed.²⁵

Cost-effectiveness

Three studies addressed the cost-effectiveness of MMU programs. One review of mobile health clinics in the United States included a large number of studies including PEH, and noted that cost savings are driven through preventing ED visits, reducing hospitalization and readmission rates, providing symptom-free days, and improving quality-adjusted life years.⁸ Another review found that MMUs appear to be both efficacious and cost-effective for vulnerable populations through ED avoidance and decreasing hospital length of stay.⁶² Finally, one cost analysis study demonstrated improvements in blood pressure for MMU care recipients would result in reduced risk in stroke and heart attack and ED visits, thereby producing a positive lower bound for the clinic’s return on investment.²³

DISCUSSION

This scoping review highlights the current evidence around MMU provision of healthcare for PEH, with most of the evidence being observational or descriptive. Below we describe the implications of the current available evidence supporting MMU utilization for certain outcomes based on the strength of the available evidence, followed by current gaps existing in the literature to guide future research.

Strength of Evidence and Implications

The most common result reported in all study types was the diversity of patients served by MMUs. This suggests that the strongest evidence, based on descriptive data, for MMU provision of care to PEH is that they provide easier access to care for diverse and traditionally hard-to-reach populations, a finding in alignment with previous work.⁷⁴ Given this finding, MMUs should be considered an important tool to improving health equity for PEH that have historically been challenging to reach. However, we could find no cost-effectiveness studies or analyses of return on investments to demonstrate exactly the dollar values of MMUs which should be considered in future studies, particularly within value-based care frameworks as payors shift to embrace this model.

In terms of proof-of-concept, most MMU interventions described in the literature are focused on the provision of primary and preventive care.^{8,16,23,39,52,59,62} Despite a plethora of observational studies describing these programs, there is very weak evidence that MMUs improve primary care outcomes. Only one review included a handful of trials to suggest improvements in hypertension, asthma, and diabetes;⁶² thus, more research is needed comparing MMU to clinic-based primary care for PEH. Outside of delivering primary care, many studies described MMU programs that engage PEH in addiction treatment.^{26,40,63,65,66} This appears to be a promising tool to address the current overdose crisis, although the current available evidence is mainly descriptive and therefore weak. Studies reporting treatment follow-up will be important to verify this model of addiction care. Based on the existing controlled studies, there are varied outcome domains that MMUs seem to confer benefits, although more than one study has reported reductions in drug use, improved housing stability, and reduced hospitalizations. However, since none has been a completed randomized controlled trial, it is premature to conclude MMUs are effective for specific health or social outcomes.

Three of the controlled studies reported healthcare utilization outcomes after MMU intervention, although the interventions in each of the studies were heterogeneous.^64,66,73 There is some, albeit weak, evidence to support that psychiatric and social work MMUs reduce ED visits. Although this should be verified in future studies, MMU programs targeting a reduction in ED visits should consider incorporating these disciplines into the care team. This is a particularly interesting line of research that should be pursued since MMUs may facilitate other forms of health and social care services. MMUs may not be suitable for providing long-term care but may serve as early engagement vehicles for more established healthcare services later.

Although the evidence is descriptive, it should be noted that many different healthcare professions serve in existing MMU programs. Primary care providers, nurses, psychiatrists, therapists, dermatologists, addiction specialists, case managers, podiatrists, otolaryngologists, social workers, dentists, dental hygienists, and pharmacists were all represented in the included studies. In serving PEH, MMUs appear to be an important strategy in reaching hard-to-reach patients in any field or discipline of medicine. The ability of MMUs to adapt to the needs of the community was a common theme and appears to be a strength of this type of healthcare delivery.^42,46,49,70

Gaps in Evidence

There is a strong need for controlled trials, especially in determining whether MMUs serve different groups of PEH than other service delivery models, and how outcomes might differ between different types of MMUs and more conventional approaches to primary care. Therein lies challenges regarding designing controlled trials for MMUs to include appropriate comparison groups and logistics of randomly assigning patients to MMUs in the community. Cluster randomized trials should be considered, although they would require participation from many sites. Another major gap in the literature on MMUs is specificity in their utility. This review demonstrated MMUs currently offer a variety of different services for different conditions and diverse populations. But it is not clear what services are best delivered through MMUs. Moreover, screening, preventive procedures, and full treatment courses may be offered by MMUs, but it may be prudent to identify practical limitations of service provision by MMUs. Importantly, PEH may be an especially salient target group for MMUs as some are reluctant to seek traditional office-based care. It may be relevant to consider which health conditions and outcomes are best suited for MMUs to address; alternatively, another approach is to not focus on health conditions but consider whether and how MMUs may facilitate a pathway to more traditional office-based care.

Our review also revealed the long-term outcomes of MMUs are unknown and relatively little evidence of cost-effectiveness which is critical to study to convince healthcare systems that MMUs are a worthwhile investment and inform policy-level decision-making on widespread implementation. The average annual cost to operate an MMU is $632,369 as reported by 173 MMU programs in a 2020 study.¹⁶ Despite the uncertainty regarding cost and funding, it is important to note that many MMU programs have been in existence for 10 or more years.^{35,43,44,53,66} This suggests that MMUs are sustainable on the local level despite less than half of MMU programs receiving federal funding.¹⁶

Limitations

This scoping review protocol limited our search to PubMed and Google Scholar, excluding grey literature. Given the limited funding and academic support for MMU programs and healthcare for the homeless, there is reason to believe that publishing articles in peer-reviewed literature is not a priority. Additionally, the majority of studies included in this review were observational or descriptive. Therefore, the strength of evidence in this field is weak and limits the ability to draw conclusions regarding best practices for MMU care delivery for PEH.

CONCLUSION

There are many MMU programs that provide primary care, preventive care, specialty care, addiction care, psychiatric crisis care, and care coordination for PEH. The best available evidence supports that this model of care engages a diverse and challenging population to engage in traditional care settings, and there is some evidence MMUs can help improve behavioral health, reduce hospitalizations, and increase housing stability. However, the state of the literature is underdeveloped and in need of more rigorously designed studies. More research is needed to understand if MMUs are more effective than other care delivery models tailored to populations experiencing homelessness and whether they might serve an important function in the continuum of healthcare services for PEH.

Supplementary Information

Below is the link to the electronic supplementary material.

Declarations

Conflict of Interest

The authors declare that they do not have a conflict of interest.