Table 1

Features of complex interventions (adapted from Lewin et al61) and complex systems (adapted from Petticrew et al28) and their impact on individual-level versus population-level and system-level interventions, as well as criteria in the WHO-INTEGRATE framework

Population-level and system-level interventions			Complexity-relevant differences between individual-level and population-level /system-level interventions	WHO-INTEGRATE framework criteria that are typically relevant*
Individual-level interventions
Features of complex interventions
Number of active components in the intervention61; interactions between components of complex interventions.28	+	++	Both types of interventions can comprise multiple components entailing synergistic or dissynergistic interactions among them. For population-level and system-level interventions, these interactions tend to occur among a greater number of more diverse components located at one or several organisational levels.	Balance of health benefits and harms. Human rights and sociocultural acceptability. Health inequity, equality and non-discrimination. Societal implications. Financial and economic considerations. Feasibility and health system considerations.
Number of behaviours of recipients to which the intervention is directed.	+	++	Both types of interventions can require behaviour change among recipients. For curative and preventative interventions at the individual level, these mostly relate to treatment adherence or tightly defined health-relevant behaviours, often among an ‘activated’ population seeking care or willing to engage in other ways. Population-level and system-level interventions tend to be concerned with a larger set of behaviours directly or indirectly linked to health, often in healthy general or at-risk populations.	Feasibility and health system considerations.
Range and number of organisational levels targeted by the intervention.	−	++	Individual-level interventions tend to target their recipients in a defined setting, for example, in a household or healthcare setting. Many population-level and system-level interventions target multiple levels, for example individuals living in households located in communities and influenced by community-level or national-level interventions; importantly, they often concern sectors beyond health.	Balance of health benefits and harms. Financial and economic considerations. Feasibility and health system considerations.
Level of skill required by those delivering the intervention.	++	++	The skills required for effective intervention delivery vary greatly depending on the nature of an intervention, and can be equally high for individual-level and population-level/system-level interventions. For population-level and system-level interventions, there may be a greater number of distinct implementation agents with a more diverse set of necessary skills.	Human rights and sociocultural acceptability. Feasibility and health system considerations.
Level of skill required by those receiving the intervention.	++	++	Both types of interventions can require a high level of skill among recipients, where skill can refer to specific (technical) abilities, as well as broader resources and characteristics, such as motivation and capacity (time, money, physical and mental energy). Interventions directed at individuals tend to require greater recipient skills and resources than many population-level and system-level interventions. Population-level and system-level interventions, on the other hand, often impact multiple behaviours related to diverse aspects of life and thus potentially rely on a more diverse set of skills and resources.	Human rights and sociocultural acceptability. Health inequity, equality and non-discrimination. Feasibility and health system considerations.
Features of complex systems
Interactions of interventions with context and adaptation28/degree of tailoring intended or flexibility permitted across sites or individuals in applying or implementing the intervention.61	+	++	Individual-level interventions tend to involve a small degree of tailoring, typically revolving around the health professional–patient relationship. In contrast, many population-level and system-level interventions are highly context-dependent and, in order to be effective, their design and delivery strategies must be tailored to the setting or context in which they are to be implemented.	Balance of health benefits and harms. Human rights and sociocultural acceptability. Health inequity, equality and non-discrimination. Societal implications. Feasibility and health system considerations.
System adaptivity (how does the system change).	−	++	Some population-level and system-level interventions may directly attempt to change or indirectly influence the context in which they are implemented. The system thus reacts and adapts in expected or unexpected ways to the intervention.	Balance of health benefits and harms. Societal implications. Feasibility and health system considerations.
Emergent properties.	−	++	Population-level and system-level interventions tend to impact diverse aspects of life and may produce emergent features in relation to one or several of these (eg, changes in social norms). Some individual-level interventions, when implemented and viewed at the population/system level, can yield emergent features (eg, herd immunity as a result of vaccination).	Balance of health benefits and harms. Human rights and sociocultural acceptability. Health inequity, equality and non-discrimination. Societal implications.
Non-linearity and phase changes.	−	++	Some population-level interventions may only begin to deliver meaningful outcomes once they have reached a certain scale (phase changes at a threshold); they may be highly effective at particular levels of coverage and less effective at others.	Balance of health benefits and harms. Human rights and sociocultural acceptability. Societal implications.
Negative and positive feedback loops.	−	++	Population-level and system-level interventions with their specific components or the set of interacting components can produce negative feedback loops and thus reduce the overall intervention effect (damping); similarly, positive feedback loops may result in an overall intervention effect that is greater than expected.	Balance of health benefits and harms. Human rights and sociocultural acceptability. Health inequity, equality and non-discrimination. Financial and economic considerations. Feasibility and health system considerations.
Multiple (health and non-health) outcomes and long complex causal pathways.	+	++	Both types of interventions can be characterised by multiple outcomes and long, complex causal pathways. Given their large number of components impacting health as well as non-health outcomes, this feature of complex systems is particularly prevalent among population-level and system-level interventions and complicated by often long lag periods. An individual-level intervention has to be sufficiently popular and impactful to diffuse through families, peers and among the broader community or nation to eventually have population-relevant impacts, whereas a population-level or system-level intervention tends to have more immediate impacts (intended and unintended).	Balance of health benefits and harms. Human rights and sociocultural acceptability. Societal implications. Financial and economic considerations.

−, indicates not relevant; +, indicates somewhat relevant; ++, indicates highly relevant.
↵*Each feature of a complex system tends to influence most or all criteria; here we highlight those criteria that may be of greatest relevance.
INTEGRATE, INTEGRATe Evidence.