The research goals of this randomized controlled trial are to determine the feasibility and the mechanism of change of iMBT that has been developed using the Acceptance Checklist for Clinical Effectiveness Pilot Trials.

The primary research question is as follows:

What is the effectiveness of the iMBT in relation to improvements on depressive symptoms among people with clinical depression, relative to a usual care control after the intervention and in 3-month follow-up?

Secondary questions include the following:

Which facet(s) of mindfulness (i.e., observe, describe, act with awareness, non-react and non-judgement) improved during the intervention? How does the growth trajectory of different facets of mindfulness relate to the improvement of well-being and reduction of ill-being?

The investigators hypothesize that:

H1 Participants in iMBT group will have greater reduction in depressive symptoms and increase in all facets of mindfulness and mental well-being, than the usual care group at post-intervention, and 3-month follow-up.

H2 Using latent growth analysis, the intraindividual growth trajectory of the monitor and acceptance facets of mindfulness would mediate the effect of iMBT on the intraindividual changes in depressive symptoms.

H3 Using multi-group analysis, in accord with Acceptance and Monitor theory, the relationship between the growth trajectory of monitor facets of mindfulness and the growth trajectory of depressive symptoms will be moderated by the level of acceptance. People with greater acceptance of inner experience will benefit more from the change of monitor facets of mindfulness in iMBT.

1.1 Internet delivered Intervention for Depression Major depressive disorder (MDD) is a significant and common public health concern due to its high prevalence, high disease burden, and common comorbidity 1-3. While evidence-based psychological treatments are available, most of those affected by depression do not have access to these treatments or seek help5. Several reasons such as long waiting time of mental health service, barriers in access to care, and reluctance to seek help due to stigma are contributing to this situation6. One promising approach to enhance the accessibility and serviceability of psychotherapy is to complement the existing system using evidence-based self-management programs delivered via the Internet. Different forms of psychotherapy, could potentially be transferable to Internet-based interventions, especially when guided by coaches9 who provide online guidance, encouragement, and therapeutic activities 10,11. The initiatives of translating and scaling up mental health service via the Internet echo with the National Institute for Health and Care Excellence guidelines (NICE) for managing depression 12. The guideline recommended low-intensity Internet-based interventions as first line treatment prior to more complex higher-tier services. Internet-based interventions for major depression has not only been shown to be efficacious 13, but also cost-effective and able to generate societal savings14,15. Notably, a recent meta-analytic review revealed that Internet-based cognitive behavioral therapy (iCBT) was as effective as its face-to-face counterpart for clinical practice in treating depression16. Given the high accessibility and low recurring costs of Internet-based interventions for depression, these interventions are suggested to have a huge potential for public mental health impact.

1.2 Mindfulness-based Training as an Internet delivered Intervention for People with Depression In addition to iCBT, Internet-based mindfulness-based training (iMBT) has also gained evidence in improving mental well-being and reducing psychological distress. Mindfulness is defined "paying attention in a particular way: on purpose, in the present moment, and nonjudgmentally" 17,18. In the context of understanding the beneficial effect of mindfulness on depression, it is theorized that mindfulness training reduced depression through encouraging individuals to notice experiences regardless of its valence labelled, and to approach those experiences with gentleness, curiosity and interest without suppressing, judging, or pushing these experiences away 19. In turn, repetitive negative thinking, which involves cognitive over-engagement in attempt to control unpleasant inner experiences, would be attenuated by the facilitation of individuals' processing of their affective experiences17,20. Moreover, through observing that different experiences come and go over time, mindfulness practitioners come to know the impermanent and transitory nature of the inner experience and realize that it is not always necessary to react.

A recent meta-analysis that included 209 studies with 12,145 participants concluded that mindfulness-based intervention is an effective treatment for various psychological problems, and is especially effective for reducing depression, anxiety and stress21. Evidence of online mindfulness-based intervention has demonstrated its effectiveness among community samples and subclinical populations with elevated depressive symtoms23,24. In addition, iMBT may be more acceptable than intervention using the traditional cognitive behavioral approach. As reported in a recent study, intervention with mindfulness element was chosen as the first option of intervention by over 80% of people with depression/anxiety. Moreover, nearly half of the participants in a study reported preference of online formats for mindfulness interventions over group/individual formats28.

Given its acceptability and preference by individuals with mental health needs and its promising effects in reducing depressive and anxiety symptoms, another critical question lies in examining how MBT works so that further refinement of such approach can be made based on its theoretical roots and mechanism of change. The precise mechanisms underlying the effect of mindfulness have received recent theoretical attention19,29,30. Despite not having abundant studies, recent meta-analysis of mediation studies with 12 RCTs identified consistent evidence for the change in mindfulness as a mechanism underlying MBIs31. However, simply identifying mindfulness as the mechanism of change in MBT is too crude and intuitive. Further unpacking the effects of mindfulness is necessary to understand the process through which individuals experience changes. One possibility is to examine specific effect of each facet in mindfulness (i.e., observe, describe, act with awareness, non-react and non-judgement). Correlational study suggested that different facets of mindfulness have differential relationships with various psychological variables 32. Although most facets of mindfulness are frequently found to be associated with reduced psychological distress, the "observe" facets is often uncorrelated or even positively correlated with mood symptoms 33. In accord with the Acceptance and Monitor theory19, a recent study showed high observing skills was correlated with higher depressive symptoms with low acceptance. Yet, high observing skills in combination with high acceptance correlated with increased adaptive cognitive processing tendencies 34. Consequently, it is important to examine relationships between change of mindfulness and that of psychological symptoms at the facet level to provide a more fine-grained perspective on the contribution of mindfulness. This could also facilitate refinement of iMBT.

1.3 Aims and hypotheses The research goals of this randomized controlled trial are to determine the feasibility and the mechanism of change of iMBT that has been developed using the Acceptance Checklist for Clinical Effectiveness Pilot Trials (ACCEPT) framework35.

The primary research question is as follows:

What is the effectiveness of the iMBT in relation to improvements on depressive symptoms among people with clinical depression, relative to a usual care control after the intervention and in 3-month follow-up?

Secondary questions include the following:

Which facet(s) of mindfulness (i.e., observe, describe, act with awareness, non-react and non-judgement) improved during the intervention? How does the growth trajectory of different facets of mindfulness relate to the improvement of well-being and reduction of ill-being?

The investigators hypothesize that:

H1 Participants in iMBT group will have greater reduction in depressive symptoms and increase in all facets of mindfulness and mental well-being, than the usual care group at post-intervention, and 3-month follow-up.

H2 Using latent growth analysis, the intraindividual growth trajectory of the monitor and acceptance facets of mindfulness would mediate the effect of iMBT on the intraindividual changes in depressive symptoms.

H3 Using multi-group analysis, in accord with Acceptance and Monitor theory, the relationship between the growth trajectory of monitor facets of mindfulness and the growth trajectory of depressive symptoms will be moderated by the level of acceptance. People with greater acceptance of inner experience will benefit more from the change of monitor facets of mindfulness in iMBT.

A two-armed parallel RCT following CONSORT statement1 will be conducted to examine the efficacy of an Internet-based mindfulness-based training (iMBT) to a treatment-as-usual control group (TAU). Eligible participants will be randomized to either iMBT or TAU by block randomization with block number of 6 with allocation ratio of 1:1. Intervention (iMBT) will be delivered over a 6-week period via an internet e-learning mental health platform. Both groups will be assessed at the following time points: (1) before intervention (T0), (2) 2,4 weeks since the commencement of group (T1,2), (3) 6 weeks after (i.e., when the intervention ends) (T3), (4) at 3-month follow-up(T4).

Inclusion Criteria:

• Participants aged 18 years old or above
• Have access to computer and mobile phone (since this is an internet-based therapy)
• Score >9 on PHQ9
• Have the ability to read and type Chinese

Exclusion Criteria:

• Self-reported presence of psychosis or bipolar disorder, post-traumatic stress disorder, drug or alcohol dependence, current use of antipsychotic medications
• Self-reported frequent suicidal ideation (more than half of the days in the past two weeks)
• Completion of an online mental health program/research for depression in the past 3 months