Characteristics of included studies

The flowchart of the search results (Fig. 1) shows that the search identified 9763 studies. Following removal of duplicates and application of eligibility criteria, 37 studies remained for inclusion in the review. 25 studies provide macroscopic TMFs for CHIs [5,6,7,8,9, 30, 32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49] worldwide. A further seven included TMFs [50,51,52,53,54,55,56] that were developed in high-income countries and only five studies [21, 57,58,59,60] targeted LMICs.

Fig. 1

PRIMA diagram of article selection

Types of TMF

Overall, 28 [5,6,7,8,9, 21, 32,33,34,35,36,37,38,39,40,41,42,43,44,45,46, 48,49,50, 55, 57, 58, 61] of the 37 studies describe macroscopic TMFs and nine studies [30, 47, 51,52,53,54, 56, 59, 60] describe TMFs for specific relevant interventions. Categorising these according to the five categories of Per Nilsen's schema (Table 3) reveals that 14 of the 37 TMFs are process models, 14 are determinant frameworks, one is classic theory, one is implementation theory, and seven are evaluation frameworks (See Additional file 5). One classic theory and one implementation theory are included. The Detailed classification for collected TMFs is described in Table 4.

Table 4 Overview of frameworks and models used in the complex health interventionsAdaptability, 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.

Table 5 The terminology used in the collected TMFsThe 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.

Table 6 The factors influencing the ASaS of CHIsOuter context

Name of influencing factors, suggested definition, frequency of influencing factors of Outer context is shown in Table 7.

Table 7 Name of influencing factors, suggested definition, frequency of influencing factors of outer context Sociopolitical context

This study has identified multiple studies highlighting how sociopolitical factors deeply influence the adaptability, scalability, and sustainability (ASaS) of complex health interventions (CHIs) [8, 30, 35, 37, 39, 42, 44, 47, 49,50,51,52,53,54, 58,59,60, 64]. These factors, including religion [58], ethics [39, 56], legislation [30, 35, 39, 44, 54, 58, 59, 64], norms or regulations [7, 21, 35, 38, 49, 54, 56, 58], and policies [6, 7, 9, 21, 35, 37,38,39, 44, 50, 51, 54, 56, 57, 59], play a critical role in shaping health outcomes and addressing healthcare disparities. The influence of sociocultural factors such as common traditions, habits, patterns, and beliefs was also evident across different populations [35, 37,38,39,