Finding	Studies contributing	Assessment for each GRADE‐CERQualcomponent				Overall GRADE‐CERQualassessment and explanation
		Methodological limitations	Coherence	Relevance	Adequacy
1. Flexible interventions that were culturally appropriate, adaptable and/or able to be tailored to meet local needs were seen as key to successful implementation.	Blake 2020; Brown‐Johnson 2020; Cheung 2015; De Jong 2019; Ferranti 2016; Schreiber 2019; Waterman 2018	Moderate concerns as 2 studies had major limitations	No or very minor concerns	Minor concerns as studies had a good geographical spread and reported across several settings	No or very minor concerns as the finding was based on 7 studies, 4 of which included reasonably thick data	Moderate confidence Downgraded because we had moderate concerns regarding methodological limitations. We had no or very minor concerns about coherence, relevance and adequacy.
2. Interventions characterised as having a low level of complexity were seen as easier to implement.	Blake 2020; Brown‐Johnson 2020; Ferranti 2016; Son 2019	Minor concerns as 1 study had major limitations	Moderate concerns about the fit between the data from primary studies and the review finding	Moderate concerns as studies had limited geographical spread and reported across limited settings	Moderate concerns as the finding was based on 4 studies; 3 of these studies had moderately thin data	Low confidence Downgraded because we had moderate concerns regarding coherence, relevance and adequacy. We had minor concerns about methodological limitations.
3. Intervention costs and associated costs of implementing the intervention were seen as both hindering and facilitating implementation.	Blake 2020; De Jong 2019	No concerns	Moderate concerns about the fit between the data from primary studies and the review finding	Moderate concerns as studies had limited geographical spread and reported across limited settings	Moderate concerns as the finding was based on 2 studies with moderately thin data	Low confidence Downgraded because we had moderate concerns regarding, coherence, relevance and adequacy. We had no concerns about the methodological limitations.
4. Lack of awareness about the needs and resources of frontline workers was seen as a barrier to implementation. This included lack of awareness of frontline workers' of their own needs, and lack of awareness of organisations who employed and supported frontline workers.	Belfroid 2018; Cao 2020; Chang 2006; Chen 2020; Cheung 2015; Cunningham 2017; De Jong 2019; Ferranti 2016; Klomp 2020; Lee 2005; Schreiber 2019; Waterman 2018	Moderate concerns as 3 studies had major limitations	Minor concerns about the fit between the data from primary studies and the review finding	Minor concerns as studies had a good geographical spread and reported across several settings	Minor concerns as the finding was based on 12 studies, 5 of which included reasonably thick data	Moderate confidence Downgraded because we had moderate concerns regarding methodological limitations. We had minor concerns about coherence, relevance and adequacy.
5. Awareness of mental health needs by governments and political leaders was identified as a facilitator.	Cheung 2015; Klomp 2020	Serious concerns as both studies judged to have major limitations	Minor concerns about the fit between the data from primary studies and the review finding	Moderate concerns as studies had limited geographical spread and reported across limited settings	Moderate concerns as the finding was based on 2 studies with moderately thin data	Very low confidence Downgraded because we had serious concerns about the methodological limitations of these studies, and moderate concerns regarding relevance and adequacy.
6. Networking between organisations involved in providing frontline services, and co‐ordinating multiple external organisations in a crisis was seen as both a barrier and a facilitator to implementation.	Blake 2020; Cheung 2015; De Jong 2019	Minor concerns as 1 study had major limitations	Moderate concerns about the fit between the data from primary studies and the review finding	Moderate concerns as studies had limited geographical spread and reported across limited settings	Moderate concerns as the finding was based on 3 studies with moderately thin data	Low confidence Downgraded because we had moderate concerns regarding coherence, relevance and adequacy. We had minor concerns about the methodological limitations.
7. Effective communication, and cohesion through horizontal and vertical networks, was seen to strengthen social capital and improve team resilience and was considered to be a key factor in implementation.	Belfroid 2018; Blake 2020; Cao 2020; Chang 2006; Cheung 2015; Cunningham 2017; Klomp 2020; Lee 2005	Moderate concerns as 3 studies had major limitations	No or very minor concerns about the fit between the data from primary studies and the review finding	Minor concerns as studies had a good geographical spread and reported across several settings	No or very minor concerns as the finding was based on 8 studies, 4 of which included reasonably thick data	Moderate confidence Downgraded because we had moderate concerns regarding methodological limitations, and no or very minor concerns about coherence, relevance and adequacy.
8. Organisational incentives and rewards for frontline workers were seen as important in facilitating and engaging student healthcare workers and frontline staff with the intervention	Belfroid 2018; Chang 2006; Ferranti 2016; Waterman 2018	Minor concerns as 1 study had major limitations	Moderate concerns about the fit between the data from primary studies and the review finding	Moderate concerns as studies had limited geographical spread and reported across limited settings	Moderate concerns as the finding was based on 4 studies with moderately thin data	Low confidence Downgraded because we had moderate concerns regarding coherence, relevance and adequacy. We had minor concerns about the methodological limitations.
9. A positive learning climate for everyone involved in implementation of an intervention was seen to facilitate implementation.	Belfroid 2018; Brown‐Johnson 2020; Carvalho 2019; Chang 2006; Cheung 2015; Cunningham 2017; De Jong 2019; Lee 2005	Moderate concerns as 3 studies had major limitations	No or very minor concerns about the fit between the data from primary studies and the review finding	Minor concerns as studies had a good geographical spread and reported across several settings	Minor concerns as the finding was based on 8 studies, 5 of which included reasonably thick data	Moderate confidence Downgraded because we had moderate concerns regarding methodological relevance. We had no or very minor concerns about coherence, relevance and adequacy.
10. Resource constraints, including lack of equipment, staff time and skills, were described as hindering implementation.	Belfroid 2018; Brown‐Johnson 2020; Cao 2020; Chang 2006; Chen 2020; Cunningham 2017; De Jong 2019; Waterman 2018	Moderate concerns as 2 studies had major limitations	No or very minor concerns about the fit between the data from primary studies and the review finding	Minor concerns as studies had a good geographical spread and reported across several settings	Minor concerns as the finding was based on 8 studies, 5 of which included reasonably thick data	Moderate confidence Downgraded because we had moderate concerns regarding methodological limitations, and no or very minor concerns about coherence, relevance and adequacy.
11. Education, training, and access to information for frontline workers was considered an important step underpinning the readiness for implementation, and was seen to act as a barrier or facilitator depending on the quality provided.	Belfroid 2018; Chang 2006; Chen 2020; Cheung 2015; De Jong 2019; Ferranti 2016	Moderate concerns as 3 studies had major limitations	Minor concerns about the fit between the data from primary studies and the review finding	Moderate concerns as studies had limited geographical spread and was reported across limited settings	Moderate concerns as the finding was based on 6 studies with moderately thin data	Low confidence Downgraded because we had moderate concerns regarding methodological limitations, relevance and adequacy. We had minor concerns about coherence.
12. Frontline workers knowledge and beliefs about the intervention were seen to act as either a barrier or facilitator to implementation.	Belfroid 2018; Blake 2020; Carvalho 2019; Chen 2020; Cunningham 2017; De Jong 2019; Waterman 2018	Minor concerns as 1 study had major limitations	No or very minor concerns about the fit between the data from primary studies and the review finding	Minor concerns as studies had a good geographical spread and reported across several settings	Moderate concerns as the finding was based on 7 studies, 3 with moderately thin data	Moderate confidence Downgraded because we had moderate concerns regarding adequacy. We had no or very minor concerns about methodological limitations, coherence and relevance.
13. Frontline workers' confidence in their ability to deliver and implement an intervention was seen as an important factor in successful implementation.	Belfroid 2018; Brown‐Johnson 2020; Carvalho 2019; Cunningham 2017; Ferranti 2016	Minor concerns as 1 study had major limitations	Moderate concerns about the fit between the data from primary studies and the review finding	Moderate concerns as studies had limited geographical spread and was reported across limited settings	Moderate concerns as the finding was based on 5 studies all with moderately thin data	Low confidence Downgraded because we had moderate concerns regarding about coherence, relevance and adequacy. We had minor concerns about methodological limitations.
14. Individual personal characteristics and attributes of frontline professionals, such as their attitudes and motivation, were seen to act as either a barrier or facilitator to implementation.	Belfroid 2018; Chang 2006; Cheung 2015; Cunningham 2017; De Jong 2019; Lee 2005; Waterman 2018	Moderate concerns as 2 studies had major limitations	No or very minor concerns about the fit between the data from primary studies and the review finding	Moderate concerns as studies had limited geographical spread and was reported across limited settings	Moderate concerns as the finding was based on 7 studies, 3 of which had moderately thin data	Low confidence Downgraded because we had moderate concerns regarding methodological limitations, relevance and adequacy.
15. Planning to prepare individual frontline workers and organisations to implement changes was often reported to be overlooked, resulting in frontline workers feeling rushed and unprepared. Strategic plans at the level of the individual healthcare worker and organisation were considered to facilitate the success of the implementation.	Belfroid 2018; Brown‐Johnson 2020; Cao 2020; Chang 2006; Chen 2020; Ferranti 2016; Klomp 2020; Waterman 2018	Moderate concerns as 4 studies had major limitations	No or very minor concerns about the fit between the data from primary studies and the review finding	Minor concerns as studies had a good geographical spread and reported across several settings	Moderate concerns as the finding was based on 8 studies, 4 with moderately thin data	Low confidence Downgraded because we had moderate concerns regarding methodological limitations and adequacy. We had no or very minor concerns about coherence and relevance.
16. Meaningful engagement of people involved in the delivery of interventions to support mental health, and forming strong collaborations with champions and opinion leaders, were seen to positively impact on implementation.	Belfroid 2018; Blake 2020; Brown‐Johnson 2020; Cunningham 2017; Klomp 2020; Lee 2005; Son 2019; Waterman 2018	Moderate concerns as 2 studies had major limitations	Minor concerns about the fit between the data from primary studies and the review finding	Minor concerns as studies had limited geographical spread and was reported across limited settings	Moderate concerns as the finding was based on 8 studies, of which 3 studies had moderately thin data	Low confidence Downgraded because we had moderate concerns regarding methodological limitations and adequacy. We had minor concerns regarding coherence and relevance.
17. The opportunity for frontline workers to reflect on, evaluate or take part in a debriefing session was seen to promote a sense of safety, and support shared learning which facilitated the implementation process.	Belfroid 2018; Blake 2020; Carvalho 2019; Cunningham 2017; De Jong 2019; Klomp 2020	Minor concerns as 1 study had major limitations	Minor concerns about the fit between the data from primary studies and the review finding	Moderate concerns as studies had limited geographical spread and was reported across limited settings	Moderate concerns as the finding was based on 6 studies, of which 3 studies had moderately thin data	Low confidence Downgraded because we had moderate concerns regarding relevance and adequacy. We had minor concerns regarding methodological limitations and coherence.