Adaptability, Scalability and Sustainability (ASaS) of complex health interventions: a systematic review of theories, models and frameworks

Background Complex health interventions (CHIs) are increasingly used in public health, clinical research and education to reduce the burden of disease worldwide. Numerous theories, models and frameworks (TMFs) have been developed to support implementation of CHIs. This systematic review aims to identify and critique theoretical frameworks concerned with three features of implementation; adaptability, scalability and sustainability (ASaS). By dismantling the constituent theories, analysing their component concepts and then exploring factors that influence each theory the review team hopes to offer an enhanced understanding of considerations when implementing CHIs. Methods This review searched PubMed MEDLINE, CINAHL, Web of Science, and Google Scholar for research investigating the TMFs of complex health interventions. Narrative synthesis was employed to examine factors that may influence the adaptability, scalability and sustainability of complex health interventions. Results A total of 9763 studies were retrieved from the five databases (PubMed, MEDLINE, CINAHL, Web of Science, and Google Scholar). Following removal of duplicates and application of the eligibility criteria, 35 papers were eligible for inclusion. Influencing factors can be grouped within outer context (socio-political context; leadership funding, inter-organisational networks), inner context; (client advocacy; organisational characteristics), intervention characteristics (supervision, monitoring and evaluation), and bridging factors (individual adopter or provider characteristics). Conclusion This review confirms that identified TMFS do not typically include the three components of adaptability, scalability, and sustainability. Current approaches focus on high income countries or generic “whole world” approaches with few frameworks specific to low- and middle-income countries. The review offers a starting point for further exploration of adaptability, scalability and sustainability, within a low- and middle-income context. Trial registration Not registered. Supplementary Information The online version contains supplementary material available at 10.1186/s13012-024-01375-7.


Introduction
This systematic review examines the adaptability, scalability, and sustainability (ASaS) of complex health interventions (CHIs), which are increasingly used in public health, clinical research, and education to alleviate global disease burdens [1].The effectiveness of CHIs depends on various factors, including health resources, education levels, and economic status [2,3].
CHIs are interventions with multiple interacting components, posing unique evaluation challenges beyond the usual practical and methodological difficulties [4].
Adaptability, scalability, and sustainability are crucial concepts in implementing CHIs, addressed through stages of evidence efficacy, scaling-up, and long-term sustainability [5].Initial research phases focus on adapting interventions to local contexts and needs [6].Once effectiveness is proven, the goal shifts to broader implementation, aiming for sustainability in real-world settings [5].The definitions of the ASaS is shown in the Table 1.
Theories, models and frameworks are used extensively to advance implementation science [10][11][12][13], to guide the design and implementation of complex interventions, and to help in assessing their quality.The resultant models can also be used to elucidate causal mechanisms between influencing factors and to identify contextual factors associated with changes in outcomes [10,11,14].In turn, TMFs offer a lens for the exploration of the complex fields of public health, health policy and social care [10,13].
Generally, a theory is: "a set of inter-related concepts, definitions and propositions that present a systematic view of events or situations by specifying relations among variables, to explain and predict the events or situations [15,16].
Simply put, theories, are closely related to models.Specially, theories are characterized as combining the explanatory alongside the descriptive, and models are defined as theories with a narrowly defined scope of explanation [16].A framework is: "a structure, overview, outline, system or plan consisting of various descriptive categories including concepts, constructs or variables, and the relations between them that are presumed to account for a phenomenon" [16,17].
Compared with theories and models, frameworks do not seek to be explanatory; rather than describe the internal relationships of concepts, they simply present the concepts [16].However, despite some diverse characteristics, the terms "theory", "model" and "framework" (TMF) are often used interchangeably.
However, most existing frameworks and measures for determining implementation factors and outcomes have originated in high-income nations [18].A study indicated that although the number of frameworks related to program sustainability is increasing, these frameworks are rarely applied and evaluated in lowand middle-income countries or vulnerable communities within high-income countries.The primary reason for this phenomenon is the unique challenges faced by these countries and regions in terms of community and workplace capacities [19].Therefore, it cannot be assumed that current TMFs are suitable for resourcelimited settings [20].
Consequently, the aim of this review is to conduct a systematic review of theoretical frameworks concerned Table 1 The definitions of key concepts in this study

Concepts Definitions
Adaptability "the degree to which an intervention can be adapted, modified, or tailored to meet the needs of various contexts and populations while retaining its essential elements" [7].

Scalability
Scalability is defined by the World Health Organisation and ExpandNet [8] as "deliberate efforts to increase the impact of health service innovations successfully tested in pilot or experimental projects so as to benefit more people and to foster policy and programme development on a lasting basis".
Sustainability Scheirer and Dearing [9] (2011) defined sustainability as the "Continued use of intervention components and activities for the continued achievement of desirable health outcomes within the population of interest." with at least one of the three ASaS concepts, to deconstruct the constituent theories, and to analyze the influencing factors within these frameworks.Another aim of this study is to assess the applicability and feasibility of these TMFs in diverse settings.Specifically, the objectives of this review are: 1. To explore definitions of scalability, adaptability, and sustainability.2. To identify published theoretical studies concerned with at least one of the concepts of sustainability, scalability and adaptability of complex health intervention-related frameworks and to assemble and explore relevant models and frameworks; 3. To explore inter-relationships between factors influencing scalability, adaptability, and sustainability of the complex health interventions; 4. To analyse the applicability and feasibility of these TMFs; 5. To appraise the methodological quality and reporting quality of the included literature.

Search strategy
Systematic review methods were employed to identify and select TMFs.Specifically, the BeHEMoTh procedure was used as a systematic approach by which to collect theoretical frameworks [10].The BeHEMoTh procedure offers auditability and transparency when identifying published TMFs [21].Specific features of the BeHEMoTh search process are outlined in Additional file 1.The search begins with a structured BeHEMoTh question.First, the researcher reviewed TMFs identified from a scoping review in order to construct a systematic search procedure for retrieving ASaS related TMFs via Google Scholar [Step 1a].PubMed MEDLINE, CINAHL and Web of Science, were systematically searched using the same search strategy, in a process similar to a conventional systematic review search [step 1b].Titles and abstracts were screened for TMFs using a spreadsheet with each additional instance being added to the list of TMFs previously identified via step 1 [step 2].Named models retrieved from step 2, together with models found from scoping via Google Scholar, were then searched to retrieve additional related reports [step 3].Searching of source references for these TMFs was used to reveal cited studies [step 4a and step 4b].

Search terms
Search terms across all sources were organized within five search term groups including the three ASaS concepts (See Additional file 1).By searching for the three ASaS concepts individually rather than for their intersect, the search strategy recognises that few models involve all three factors of ASaS with many involving one or two factors.

Inclusion and exclusion criteria
Identified publications were imported to Endnote 9 software and duplicates were deleted.Specific inclusion criteria for factors that influence ASaS of CHIs are shown in Table 2.

Data extraction and appraisal
The titles and abstracts were screened, and the full papers of potentially relevant studies were obtained.Two authors independently assessed 10% of all titles and abstracts with a single reviewer then selecting full text papers for eligibility.An initial data extraction form was modified and adopted after revision.A single researcher independently extracted: (1) Study identification: year of publication, authors, name of study and name of the theories, models and frameworks; (2) Methods: study design, and study context; (3) any TMFs used; (4) Purpose of the theories, models, and framework; (5) Theories, models, and frameworks: definition, conceptual model, framework; (6) factors influencing ASaS of CHIs and inter-relationships between these concepts.These tables are shown in Additional file 3.

Quality assessment
Quality assessment criteria for assessing reports of TMFs are not common.Three papers were identified that either develop or utilize criteria for assessing theories [22][23][24] and these papers were used to compile the following quality assessment criteria: 1) Is the methodology identified and justified?2) Was a theoretical lens or perspective used to guide the study, with a reference provided?3) Is the theoretical framework described?4) Is the theoretical framework easily linked with the problem? 5) If a conceptual framework is used, are the concepts adequately defined? 6) Are the relationships among concepts clearly identified?7) Are the influencing factors of concepts clearly identified?8) Are the relationships among influencing factors clearly described?
Quality assessments were undertaken by a single reviewer, quality assessment judgements are reported in Additional file 4.

Analysis
Given that the literature relating to TMFs derives from multiple disciplines, the researcher decided to use a narrative synthesis approach, which allows for synthesis of diverse types, designs and contexts for studies [25][26][27].

CFIR framework
(3) Classic theories; Borrow classical theories from psychology, sociology and organisational science.Essentially, 'passive' -primarily explain, rather than guide, occurrence of change.
Include organizational theory [30], behavioural theories, and diffusion of innovation.
(4) Implementation theories; Adapted from classical theories, specifically for implementation COM-B (Capability, Opportunity, Motivation and Behaviour) and Normalization Process Theory).
(5) Evaluation frameworks Developed specifically to inform evaluation of outcomes from complex interventions.

RE-AIM and PRECEDE-PROCEED model
four key dimensions: Outer Context, Inner Context, Intervention Characteristics, and Bridging Factors.However, a more comprehensive understanding was needed, as the EPIS model alone did not fully capture the complexity of these factors.
To address this, features from the CFIR (Consolidated Framework for Implementation Research) and insights from the NASSS (Non-adoption, Abandonment, Scale-up, Spread, and Sustainability) framework and the Dynamic Sustainability Framework (DSF) were integrated.This meta-model enhancement involves expanding the descriptions within each EPIS dimension to cover additional critical elements found in these other frameworks.
Specifically, within the Inner Context, the organizational characteristics were elaborated to reflect deeper organizational dynamics affecting CHIs.In the Outer Context, the Sociopolitical Context was added, acknowledging its crucial influence on intervention outcomes.Further, the Intervention Characteristics were detailed more extensively to capture the nuanced nature of the interventions themselves.
This enriched model aims to provide a robust analytical framework that better reflects the complex interplay of factors influencing the ASaS of CHIs.By adopting this meta-model, the study offers a comprehensive theoretical foundation that underpins the examination of these complex interventions, paving the way for more targeted and effective implementation strategies in diverse settings.
Finally, the Theoretical Quality Tool, adapted from Hean et al. [31], was employed to rigorously assess the applicability of the collected (TMFs) in the context of Low-and Middle-Income Countries (LMICs).

Adaptability, scalability and sustainability definitions
All 37 included studies reported at least two out of the three concepts of ASaS, and the specific concepts involved in each study.(See Table 5).Definitions of ASaS found in the included studies are shown in the Additional file 2.

The process of implementation and relationships of adaptability, scalability and sustainability
Diverse TMFs support a strong correlation between the three ASaS concepts and the implementation process.Twenty-five of the TMFs explicitly mentioned implementation of CHIs; while the remaining ten did not [6,8,21,39,40,44,[54][55][56]62].
This review confirms the interconnectedness of adaptability, scalability, and sustainability in the implementation of complex health interventions (CHIs).The findings suggest that adaptability is crucial during the initial stages of CHI deployment, determining the potential for effective and cost-efficient implementation.As the intervention progresses, scalability becomes critical, ensuring that strategies effective on a smaller scale can be expanded to broader populations and regions.Ultimately, sustainability is achieved in the final stages, focusing on maintaining the benefits of the intervention over time and making necessary adjustments based on ongoing feedback and changing conditions.
In essence, the successful scaling and long-term sustainability of CHIs fundamentally depend on their initial adaptability.This streamlined approach highlights the critical progression from adaptability through scalability to sustainability, without delving into the specifics of various models and frameworks.

Influencing factors of adaptability, scalability and sustainability of complex interventions
This study collected and analyzed factors affecting the adaptability, scalability, and sustainability (ASaS) of complex health interventions (CHIs), systematically categorizing them into four distinct categories: outer context, inner context, intervention characteristics, and bridging factors.This classification helps clarify the various factors that influence the implementation of health interventions.
While all these factors impact the three concepts of ASaS, some have particularly close relationships with specific ASaS concepts.Subsequent sections will explore these factors in detail, emphasizing those closely linked to individual ASaS concepts.This approach highlights the multidimensionality of these factors and their varying impacts on the adaptability, scalability, and sustainability of CHIs.The overview of the factors influencing the ASaS is shown in the Table 6.

Outer context
Name of influencing factors, suggested definition, frequency of influencing factors of Outer context is shown in Table 7.
Client advocacy Three of the 37 studies identify client advocacy as an important influencing factor [9,45,50].
During implementation client advocacy assists healthcare workers, participants and their families in navigating the healthcare system [67].
Strong leadership can promote effective use of resources while encouraging personnel to work towards a common goal.

Structure
Organisations with strong organisational power may be likely to implement CHIs because they have stronger leadership and more frequent communication than those with weak or decentralised organisational structures [68].

Readiness for change
In addition, strong organisations are prepared and aware of possible encountered changes and can adjust their strategies and approaches of working in time to enable CHIs to be carried out well.Readiness for change is related to other factors including organisational culture, and individual attitudes [69][70][71][72].

Absorptive capacity
During the exploration and preparation phases of a CHI, an organisation's absorptive capacity (the ability to identify, assimilate, transform, and use external knowledge, research and practice [73]), readiness for change and receptive environment exert a significant impact on the adaptability of CHIs.

Individual adopter or provider characteristics
Individual adopter or provider characteristics include participants' personal characteristics, age, race/ethnicity, education, training, foundation subjects, professional experience, adaptability, personal values and goals, and personal character creative ability.

Suggested Definition
Frequency of influencing factors

Sociopolitical context
Relating to, or involving a combination of social and political factors [45].The political context focuses on the distribution of power, assets and interests within a population, as well as the range of organisations involved, their interests and the formal and informal rules that govern interactions between them.Also comprises health care system and its accessibility (e.g., delivery of services, leadership and governance, health information, human resources and financing) [ 1 Interorganizational Networks Includes the linkages and connections among organizations and other stakeholders that enable social support and flows of information within a community or healthcare system [38] 18 Funding Fiscal support can target multiple levels (e.g., staff training, fidelity monitoring, provision of the innovation) involved in implementation and delivery/use of the innovation [45] 19 Client Advocacy Support/marketing for system change based on consumer needs, priorities and/or demographics [45].
3 Leadership Characteristics and behaviors of key decision-makers pertinent at all levels who are necessary but not sufficient to facilitate or promote the implementation process and delivery/use of the innovation [45].
Supervision, monitoring and evaluation Supervision, monitoring and evaluation refers to the collection, storage, analysis and use of data to assess

INNER CONTEXT
Organizational characteristics Structures or processes that take place and/or exist in organizations that may influence the process of implementation [45] Absorptive capacity A set of organizational routines and processes by which [organizations] acquire, assimilate, transform, and exploit knowledge to create a dynamic organizational capacity [61].Absorptive capacity also describes an organizations pre-existing knowledge/skills, ability to use new knowledge, specialization and mechanisms to support knowledge sharing [50] Organization readiness Relationship between people, processes, systems and performance measurement.It requires synchronization and coordination without which no implementation will be successful Organizational structure Each department or unit able to make semiautonomous decisions) [61].Relates to structure and way an organization operates, including differences in mission, size, decision-making process, and services offered.Organizational attributes such as larger size and greater differentiation in personnel and structure are associated with adoption of new organizational forms [38] Values or visions Encompasses range of attitudes and knowledge about particular health conditions, expectations and priorities toward types of treatments or client populations, and collectively held beliefs and values that may affect the receptivity of individual and organizational stakeholders to adopt or adhere to a new care practice or intervention.Principles of social cognitive, motivation expectancy, and other social learning theories [38] Working environment organizational climate (shared perceptions of the psychological impact of the work environment on the provider) [50] Tension for change The degree to which stakeholders perceive the current situation as intolerable or needing change [6] Organization culture Combines the normative beliefs and shared expectations of the organization [50] Credibility and reputation the believability of the current intention; reputation is a historical notion based on the sum of the past behaviours [45] Leadership Characteristics and behaviors of key decision-makers pertinent at all levels who are necessary but not sufficient to facilitate or promote the implementation process and delivery/use of the innovation [45] individual adopter or provider characteristics Shared or unique characteristics of individuals (e.g., provider, supervisor, director) that influence the process of implementation [45] Ability or capacity

Staffing
Processes or procedures in place at an organization related to the hiring, review, and retention of staff involved in the active delivery of the innovation/EBP and/or its implementation.
whether complex interventions are achieving their intended objectives, and further influences improvement, policy development and advocacy of complex intervention [75].Nine of the 35 studies argue for the vital role of monitoring and evaluation in providing an effective approach by which to assess the effectiveness of complex interventions [21,30,40,43,44,50,51,58,64].

Intervention characteristics
The characteristics of the intervention itself is also an important factor.Specifically, the physical and community environment, the cost of the intervention and access to resources (8 studies) [6,21,30,34,38,51,52,56] and the source of funding all exert a direct impact.Project champions are committed to supporting and promoting the implementation of CHI, along with a strong belief in the value of carrying out CHIs [76].The factors within intervention characteristics is shown in the Table 9.
The included frameworks attest to how the characteristics of the CHI decide whether an intervention can be adapted, scaled-up and sustained [6,8,33,36,47,51,53].Although researchers hope that CHIs can be adapted and conducted as quickly as possible, it takes time for both healthcare providers and participants to adapt to new interventions [77].Also, when interventions change significantly within a short period of time, the lack of sufficient time to adapt to the intervention and adjust to relevant cultural factors prevent staff and participants from adopting a new CHI [78].

Bridging factors
Factors influencing the inter-relationship of outer and inner context are described as "bridging factors" in the EPIS framework.Bridging factors include community engagement and Purveyors/Intermediaries.
Twelve studies stress the importance of the community.Development of complex interventions within a community may be facilitated when they utilise existing community resources, available structures and staff, reducing dependence on external funding [21].For example, community members were proud to participate in a project to improve malaria prevention through insecticide-treated mosquito nets and thereby contribute to disease control within their community.Consequently, the project was speedily adapted, replicated and scaled-up locally [79].The community function is also affected by socio-political factors [80].If the visions and beliefs of the policy are inconsistent with community objectives, the policy hinders spread and sustainability even where the community possesses powerful leadership, project champions and sufficient resources [80,81].
Purveyors/Intermediaries take on a critical bridging role for key processes in the implementation of CHIs [45].Purveyors, who may be individuals, groups or communities, aim to facilitate the effective and sustainable implementation of CHIs [82].Intermediaries provide consultancy and training services to governments, organisations, etc., and also develop and implement different

Suggested Definition
Frequency of influencing factors

INTERVENTION CHARACTERISTICS
Factors relating to the characteristics of the innovation to be implemented.Innovation factors can also relate to the relationships of various stakeholders with intervention developers and the flexibility or rigidity in use of the innovation.
Communication effectively communication among healthcare workers and participants 6 Quality and fidelity monitoring/support continuous data collection plus collection across the sites to promote quality monitoring … costs and its consistency with the initial plan of the project.reflexive monitoring (formal and informal appraisal of the benefits and costs of the intervention).

5
Geographical circumstances associated with a physical location that affect humans living within a specific area.

5
Project champion Individuals who dedicate themselves to supporting, marketing, and 'driving through an [implementation]' [6] 11

Resources
Resources dedicated for implementation and on-going operations, including money, training, education, physical space, and time [6]; mobilising community resources.health-services and projects for them [82].They also provide monitoring, support, quality improvement and evaluation services at the end of the project [82].The factors within bridging factors is shown in the Table 10.

Applicability and feasibility of the collected TMFs
This study employs the Theoretical Quality Tool, adapted from Hean et al. [31], to rigorously assess the applicability of various Theoretical Models and Frameworks (TMFs) in the context of Low-and Middle-Income Countries (LMICs).The detailed outcomes of this assessment are presented in the Additional file 6.The summary table highlights the applicability and feasibility of TMFs in LMICs.
Of the 37 TMFs reviewed (two studies identify EPIS), seven demonstrate high applicability and feasibility, readily integrating into LMIC healthcare environments without necessitating significant overhauls.For example, models like the AIDED and NPT are readily implementable in LMICs due to their practicality and context-sensitive design.They integrate seamlessly into existing healthcare systems, offering solutions without the need for extensive system overhauls, crucial in resource-limited environments.Twenty-five TMFs require adaptations to align with the local conditions of LMICs, entailing modifications to fit cultural, economic, and healthcare infrastructure nuances.For example, the EPIS framework, CFIR framework, PRISM Model and Chronic Care Model, though broadly applicable, need customization to fit the unique cultural, economic, and healthcare infrastructures of LMICs.
For the remaining five TMFs, their inherent theoretical complexity and the fact that some were specifically designed for High-Income Countries (HICs) pose significant barriers to adoption in in Low-and Middle-Income Countries (LMICs).This finding highlights an important disconnect between their foundational assumptions and the practical realities of healthcare systems in LMICs.The evaluation outcomes of the applicability and feasibility of the collected 37 TMFs are shown in the Table 11.

Discussion
This theoretical systematic review identified common features and differences across 37 TMFs associated with ASaS.

Similarities and differences between the TMFs
All identified TMFs emphasize the importance of one or more of the three ASaS concepts.These frameworks aim to enable CHIs to adapt to new contexts and populations, scale up interventions, and ensure long-term effectiveness.The components of different TMFs share broadly similar descriptions, even if the terminology varies.For example, the EPIS framework divides the implementation process into four phases: exploration, preparation, implementation, and sustainment whereas Sarma's framework [5] describes three domains: i: evidence -efficacy to effectiveness; ii: Scaling-up; and iii: sustainability.A further study [36] describes four stages 1.Training (dissemination); 2. adoption (planning); 3. implementation; 4. practical improvement and two key points (preparation and maintenance).
In the EPIS framework, during exploration and preparation, adaptability is considered to determine whether the complex intervention can be conducted effectively with affordable cost.Domain I of Sarma's framework [5] includes the four vital components of intervention sources, evidence strength and quality, relative advantages, adaptability and complexity.The Framework -oral health [36] emphasizes adoption within the second stage.Hence, these three stages have the similar key components.The EPIS framework describes how a pilot study is further implemented across diverse participants and areas, which is similar to Domain ii: Scaling-up in Sarma's framework [5] and the implementation stage in Framework -oral health [36].Finally, the sustainment stage in EPIS framework, Domain iii: sustainability in Sarma's framework [5] and the maintenance point in Framework -oral health [36] all convey a shared understanding of sustainability.

Suggested Definition
Frequency of influencing factors

Community engagement
Mobilising community resources [34], community-academic partnerships [45], facilitated community support to meet the needs of patients [34] 13 Purveyors/intermediaries Individuals, groups or communities, who aim to facilitate the effective and sustainable implementation of CHIs [82].Intermediaries provide consultancy and training services to governments, organisations, etc., and also develop and implement different health-services and projects for them [82].
1        Similar stages may be presented in a different order within various models, reflecting the inherently multistage and non-linear nature of CHI implementation.Significant differences across different TMFs primarily relate to influences on ASaS.Furthermore, even when different TMFs use the same terminology to describe influencing factors, the meanings may differ due to the inherent complexity and dynamics of these factors.

The complexity of influencing factors of adaptability, scalability and sustainability
The TMFs reflect how CHIs and associated influencing factors do not operate in isolation, but are non-linear, interacting and interdependent.Some influencing factors appear across multiple studies.For example, researchers share a consensus about the importance of funding [6-9, 21, 34-36, 38, 39, 44, 45, 50, 54, 55, 58, 64].Some studies emphasise adequate and sustained financial support from governments and foundations as prerequisite to the sustainability and spread [5,44,50,61], while Sarma [5] recognizes the need to sustain interventions in the absence of adequate funding [21].In addition, the sociopolitical context, leadership and organizational characteristics are repeatedly mentioned as essential components for implementation.Furthermore, all the factors mentioned in the literature are bi-directional; the same influencing factor may act differently under diverse conditions, either as a facilitator or as a hindrance.
Leadership remains an important factor in relation to the inner context.Given that complex interventions are often run by the state, an organisation or a group, strong leadership can facilitate complex interventions.Also, the organisational culture, the vision/belief and the structure of the organisation interact with each factor and are influenced by funding, leadership and staffing.
Strong leadership needs to be accompanied by a structured organisation with a common vision in order to achieve the objectives of complex interventions.People as the carriers of culture, organisation, professional and personal attitudes, norms, interests and affiliations [84] also fulfil an important role.Individual adopter or provider characteristics are important influencing factors.When people within the organisation are aligned with the organisation's philosophy and culture, along with sufficient financial support, strong leadership and effective supervision, adaptation, scale up and long-term sustainment become possible for CHIs.Finally, intervention factors are influenced by both the outer context and the inner context, and bridging factors serve to unite the outer context, the inner context, and the intervention factors.

The dynamics of influencing factors of adaptability, scalability and sustainability
Factors that influence complex interventions are dynamic in both temporal and geographical terms.The role of these factors may change over time [85]; anticipated barriers may become facilitators [85].For example, in the early stages of an intervention, individual adopters may exhibit skepticism and distrust, presenting a barrier to CHI delivery.However, in later stages, if the intervention proves effective, participant attitudes may shift, motivating them to cooperate and thus becoming facilitators.Similarly, in the early stages, newly recruited or local staff may be unfamiliar with the intervention, posing a hindrance.Conversely, as staff become familiar with the intervention, they are better equipped to implement it, thereby becoming facilitators.
Identical influencing factors may have different effects in various geographical and national contexts.For example, women are generally considered a vulnerable group worldwide, particularly in LMICs, where they tend to have lower income and social status compared to men, making it difficult for them to access better health care resources [86].However, in the matrilineal community in Indonesia, women occupy similar or even higher social status than men, with a cultural tradition of controlling family finances [87].Therefore, in this context, gender and culture may facilitate interventions, especially maternal and child health related interventions.In relation to funding, reliable sources of funding help to sustain interventions [5], and one of the challenges to sustainability is the lack of long-term available funding [21].In summary, this systematic review offers a comprehensive understanding of factors influencing ASaS and provides a theoretical framework for effective CHIs in the future.

Have gaps in knowledge been addressed?
This is the first systematic review of ASaS related TMFs of CHIs.By focusing on the three factors of adaptability, sustainability and spread the review has been able to explore complex interactions of each with each other and with other important factors.

How have authors defined scalability, adaptability, and sustainability?
Additional file 2 consolidates definitions of scalability, adaptability and sustainability as identified across the included studies.It is noticeable that "sustainability has evolved from being considered as the endgame of a translational research process to a suggested 'adaptation phase' that integrates and institutionalizes interventions within local organizational and cultural contexts." [7] This literature argues that sustainability is, in fact, a manifestation of adaptability, and that the two concepts are closely related.

Which theoretical studies explore at least one of the concepts of scalability, adaptability and sustainability of complex health intervention within a relevant model/frameworks;?
This review reveals the scarcity of theoretical models for LMICs.The review identified four main categories of theoretical models, (i) the generic TMFs (e.g.RE-AIM and CFIR), with no obvious geographical target (26/37); (ii) tailored TMFs developed by some high-income countries (e,g.[52,53,56].for local needs (6/37); (iii) adapted TMFs (e.g.EPIS and Framework of Dissemination in Health Services Intervention Research), originally designed for high-income countries but now adapted to CHIs worldwide; (iv) TMFs specific to low and middle income countries (5/37) (e.g.[21,57]).85.7% of the included theories are either generic or specific to high-income countries, with a lack of TMFs specifically targeted at LMICs.As a result of this literature review the team have proceeded to develop a framework for Adaptability, Scalability and Sustainability that is suited for a low-and middle-income country context.
Thirty seven studies explore at least one of the concepts of sustainability, scalability and adaptability.However, no previous studies have explored all three ASaS concepts within a single TMF.Although some studies invoke the need to explore influencing factors and correlation among ASaS, no studies have actually conducted this research.

What inter-relationships have been demonstrated between factors influencing scalability, adaptability, and sustainability of the complex health interventions?
The meta-framework provides a comprehensive structure to explore the complexities of CHI implementation, emphasizing the interplay among four critical domains: outer context, inner context, intervention characteristics, and bridging factors.
In the outer context, the interplay between strong leadership, sociopolitical factors, and interorganizational networks is crucial.Strong leadership promotes resource optimization and strategic alignment toward CHI goals, essential for ASaS [35,39,44,54,58,64].Sociopolitical factors, including legislation, policies, and norms, provide a regulatory framework that guides the adaptation of CHIs to local settings, enhancing their feasibility and long-term integration [83].Additionally, robust interorganizational communication facilitates effective adaptation of CHIs to local contexts, potentially lowering costs and enhancing sustainability.
Within the inner context, organizational culture, structure, and leadership significantly interact, affecting CHI outcomes.Strong, visionary leadership is crucial for fostering an organizational culture that supports CHIs and aligns with broader intervention goals [84].The organization's structure further influences the implementation of these interventions, with well-structured organizations likely to achieve better scalability and sustainability.Additionally, the characteristics of individual providers and adopters play a critical role, impacting their ability to effectively implement and sustain CHIs.
Bridging factors like community engagement and the role of purveyors/intermediaries are vital for linking the outer and inner contexts of CHIs.Community engagement leverages local resources and capacities, which is essential for the localized adaptation and sustainability of interventions [8,9,21,30,34,38,42,44,45,54,56,58].Purveyors and intermediaries facilitate the transfer of knowledge and best practices, enhancing the overall effectiveness and reach of CHIs [45].These bridging roles ensure that interventions are not only well-integrated within communities but also maintain fidelity to their objectives and outcomes over time.

Lack of TMFs designed for LMICs
The lack of specifically designed TMFs for LMICs presents significant challenges in effectively implementing complex health interventions (CHIs) in these settings.Evaluating existing TMFs reveals a gap in their suitability and feasibility for application within the unique healthcare environments of LMICs.
Of the 37 TMFs assessed, many were found to require adaptations to align with the local conditions of LMICs, necessitating modifications to fit cultural, economic, and healthcare infrastructure nuances.For instance, frameworks such as EPIS, CFIR, PRISM Model, and Chronic Care Model, though broadly applicable, need customization to fit the unique contexts of LMICs.
Five of the TMFs reviewed were identified as inherently complex and primarily designed for high-income settings, posing substantial barriers to their adoption in LMICs.This highlights a critical disconnect between the foundational assumptions of these models and the practical realities of healthcare systems in LMICs, which face challenges such as limited resources, differing disease burdens, and varied healthcare delivery systems.
Despite these challenges, some models demonstrate higher applicability and feasibility.For example, the Dynamic Sustainability Framework (DSF) and the AIDED model are noted for their practicality and context-sensitive design, aligning with the continuous adaptation and learning required in LMICs.These models integrate seamlessly into existing healthcare systems, offering solutions without the need for extensive system overhauls, which is crucial in resource-limited environments.
The findings underscore the need to develop or adapt existing TMFs specifically tailored to the conditions of LMICs.This involves considering local healthcare practices, resource limitations, and cultural factors to ensure that the frameworks are both applicable and feasible in supporting the effective implementation and sustainability of CHIs in these settings.

Strengths and limitations
This systematic review retrieved relevant literature through a comprehensive search across four databases.Only studies published in English were included, potentially missing those from the grey literature.Identifying relevant implementation TMFs proved challenging due to the complex and diffuse terminologies used in this field.Exhaustive lists of synonyms would have been prohibitive, resulting in lack of specificity and numerous false positives.The authors sought an optimal balance between sensitivity and workload.Although the included studies were evaluated using a quality assessment tool, the risk of bias remains, particularly since only one author was responsible for data extraction.
Furthermore, although this review has identified how influencing factors interact, no clear theoretical model charts the specific TMFs, routes, and pathways from the influencing factors to the ASaS of CHIs.Finally, concepts such as acceptability, fidelity, and feasibility, are recognized as important features of CHIs [88] but fell outside the remit of this review.
Only one classic theory and one implementation theory are included.There are two possible reasons.Classical theories are borrowed from such disciplines as psychology, sociology and organisational development (e.g. the Diffusion of Innovation theory [89].Similarly.the Health Belief Model was published in 1950 [90] and the Theory of Planned Behavior in the late 1980s [91].Given that inclusion requires publication after 2000, many classic theories predate the study period.On the other hand, other theories, such as the implementation climate theory [92], may not be conceptually related to ASaS, resulting in their exclusion.The Detailed classification for collected TMFs is described in Table 4.

Conclusion
This review synthesizes 37 TMFs that document factors influencing the ASaS of CHIs.It confirms the wide variety of definitions used for adaptability, scalability, and sustainability within current TMFs, which typically do not include all three components.Current approaches focus on high-income countries or generic "whole world" approaches, with few frameworks specific to low-and middle-income countries.Numerous attempts have been made to describe and explore the interrelationships between implementation components.Of these, the EPIS and CFIR frameworks seem to possess the greatest inherent value, particularly within a model consisting of outer context, inner context, intervention characteristics, and bridging factors.This review offers a starting point for further exploration of adaptability, scalability, and sustainability, particularly within a low-and middle-income context.

Fig. 1
Fig. 1 PRIMA diagram of article selection Moving alcohol prevention research forward-Part I: introducing a complex systems paradigm Adaptation, complex adaptive system Table 4 (continued) Category of the framework Name of the framework (underlying theory if any) evaluating the scale-up and sustainability of digital solutions for front-line health workers A tale of 'politics and stars aligning': analysing the sustainability of scaled up digital tools for front-line health workers Theory of Change: a theory-driven approach to enhance the Medical Research Council's framework for complex interventions Sustainable Scalable NASSS Framework Beyond Adoption: A New Framework for Theorizing and Evaluating Nonadoption, Abandonment, and Challenges to the Scale-Up, Spread, and Sustainability of Health and Care Technologies Sustainability Scale-up/spread A comprehensive conceptual framework for implementation science Developing a conceptual framework for implementation science to evaluate a nutrition intervention scaled-up in a real-world setting Sustainability Scaling-up The systems transformation framework (STF) The Power of the Frame: Systems Transformation Framework for Health Care Leaders Sustainable Complex adaptive system the Context and Implementation of Complex Interventions (CICI) framework Making sense of complexity in context and implementation: the Context and Implementation of Complex Interventions (CICI) framework Sustainability Adaption Spoken languageImplementers, stakeholders and participants share a common spoken language Training or educationViewsTension for change The degree to which stakeholders perceive the current situation as intolerable or needing change(Damschroder, Aron et al. 2009)      Individual culture characterized by individualism, which is the prioritization or emphasis of the individual over the entire group.In individualistic cultures people are motivated by their own preference and viewpoints.Individualistic cultures focus on abstract thinking, privacy, self-dependence, uniqueness, and personal goals.Monitoring and evaluationProcesses or procedures undertaken to ensure adherence to active delivery of the innovation/EBP and/or an implementation strategy.(Moullin,Dickson et al. 2019); Fidelity Support System; Quality assurance evaluation; Continuous quality improvement[45] process, or occupation of supervising especially : a critical watching and directing (as of activities or a course of action) 6 support system or tool any hardware, software and other tools and/or utilities used to support complex health interventions.Example: information and communication systems facilitated rather than hindered the implementation and sustainability of a new CCM(Davy, Bleasel et al. 2015) 9 technology advance or environment 8 Time-cost 4

Table 2
Eligibility criteria

Table 3
Classification of models used in implementation science

Table 4
Overview of frameworks and models used in the

Framework of Dissemination in Health Services Inter- vention Research
A Person-Focused Model of CareAPerson-Focused Model of Care for the Twenty-First Century: A System-of-Systems Perspective Sustainable Complex adaptive systems Integrated sustainability framework The Sustainability of Evidence-Based Interventions and Practices in Public Health and Health Care Sustainable Adaptability FRAME The FRAME: an expanded framework for reporting adaptations and modifications to evidence-based interventions Adaptation/fidelity ExpandNet framework Practical guidance for scaling up health service innovations.Geneva: World Health Organization

Framework to Prevent Childhood Obesity Through Policy-Level Initiatives in Afterschool Pro- grams
Translating Policies Into Practice: A Framework to Prevent Childhood Obesity in Afterschool ProgramsAdapted from other settings

Table 5
The terminology used in the collected TMFs

Table 6
The factors influencing the ASaS of CHIs

Table 8
Inner context

Table 11
The applicability and feasibility of the collected TMFs