The negative impacts on physical and psychological health brought by COVID-19 seem to perpetuate in the post-pandemic era. It is estimated that there will be an impending mental health crisis in the general population in the aftermath of the pandemic due to the delayed negative impacts of COVID-19 and the associated non-pharmaceutical public health interventions such as social distancing, quarantining, and lockdown. Smart Health, defined as the provision of medical and public healthcare services by using mobile technologies, is a cost-effective and easily operated intervention that can provide various functions and can bring significant changes in people's health behaviour, such as prompting them to adopt a physically active lifestyle. Despite the rapid growth of digital health technology, most of them were developed during pandemic with a focus on delivering non-systematic, general virtual healthcare to people. The aim of this study is 1) to develop a Health Apps for Post-Pandemic Years (HAPPY) driven by the Transactional Model of Stress and Coping Theory, and 2) to evaluate its efficacy of alleviating people's physiological and psychosocial distress during post-pandemic era. A total of 814 eligible participants, both COVID-19 victims and non-COVID-19 victims exhibiting physiological and/or psychosocial distress during post-pandemic era, will be recruited and randomized to either the experimental or the waitlist control group. The experimental group will receive a 24-week intervention combined with an 8-week regular supervision phase plus a 16-week self-help phase. Participants will receive different coping strategies, namely physical training, energy conservation techniques and mindfulness-based coping in the intervention hub at Level 2 based on participants' assessments at Level 1, and enhance self-management at Level 3. Participants' physical activity levels will be measured using commercial wearable sensors. The waitlist control group will receive materials on the promotion of physical and psychological health during waiting period and receive the same intervention as the experimental group in week 25. It is hypothesized that the experimental group will exhibit milder symptoms of physiological and psychosocial distress, and have a more positive appraisal mindset, greater self-efficacy, and more sustainable self-management ability than participants in the waitlist control group.

Studies indicate that during the pandemic, people, particularly those COVID-19 survivors, are more likely to experience distress symptoms, including physical fatigue, decreased sleep quality, and body pain than during normal times. The abovementioned impacts on health can affect everyone during the COVID-19 pandemic. These negative impacts on physical and psychological health also seem to perpetuate in the post-pandemic era. It is estimated that there will be an impending mental health crisis in the general population in the aftermath of the pandemic due to the delayed negative impacts of COVID-19 and the associated non-pharmaceutical public health interventions such as social distancing, quarantining, and lockdown. Studies showed that around 10-30% of people who have contracted COVID-19 present with post-COVID-19 conditions, or "Long COVID", and among all the symptoms, physical and psychological complications such as depression, anxiety and reduced quality of life are commonly reported. This indicates an urgent need to identify an evidence-based intervention to address the health issues of people whose health has been affected by COVID-19.

Smart Health is defined as the provision of medical and public healthcare services by using mobile technologies, such as mobile phones, tablet devices, personal digital assistants (PDAs), and other wireless devices. A smart health intervention is a cost-effective and easily operated intervention that can provide various functions, including self-monitoring, data collection, real-time feedback, and notifications. Appropriate use of smart health-based interventions can bring significant changes in people's health behaviour, such as prompting them to adopt a physically active lifestyle. In a systematic review of 12 studies with a total of 3,469 participants, in which the effects of mobile app-based interventions were compared with those of the usual care, significant effects on health outcomes were found from using mobile apps to manage chronic diseases, such as improved physical functioning and increased medication adherence.

Despite the rapid growth of digital health technology, most of them were developed during pandemic with a focus on delivering non-systematic, general virtual healthcare to people. This research gap gives us the impetus to develop a 24-week intervention guided by an innovative three-level (Prevention, Protection and Progression) Health Apps for Post-Pandemic Years (here-and-after as HAPPY) driven by the Transactional Model of Stress and Coping Theory (TMSCT) to address COVID-related physical and psychosocial distress symptoms of people during the post-pandemic era. The TMSCT emphasizes the use of appraisals to evaluate the harm, threats, and challenges that result in the process of coping with stressful events. The level of stress experienced (in the form of thoughts, emotions, and behaviours) as a result of external stressors depends on appraisals of the situation, which involves a judgement about whether the internal or external demands exceed an individual's resources and ability to cope when the demands exceed the resources. Positive emotions will be elicited when individuals are capable of resolving stressors with the utilisation of both internal (e.g. cognitive appraisals, emotion regulation) and external resources (e.g. social support, access to new knowledge and skills), while negative emotions will be elicited as a result of unresolved distress.

This protocol aims to evaluate the immediate effects (eight weeks after weekly supervised sessions, i.e. on the 8th week) and the mid-term effects (six months when the intervention has been completed; i.e., on the 24th week) of the HAPPY for alleviating people's physiological and psychosocial distress during the post-pandemic era. It is hypothesised that the experimental group will 1) exhibit greater improvement in physiological and psychosocial distress and 2) have a more positive appraisal mindset, greater self-efficacy, and more sustainable self-management ability than participants in the waitlist control group.

Inclusion Criteria:

• Aged 18 and older;
• Exhibiting physiological and/or psychosocial distress:
• A cutoff value of ≥ 20 in the Kessler Psychological Distress Scale (K10) will be used to indicate participants with psychological distress.
• A cutoff value of ≥ 4 on the Brief Fatigue Inventory (BFI) indicate participants with physical fatigue;
• A cutoff value of > 5 on the Pittsburgh Sleep Quality Index (PSQI) indicate participants with decreased sleep quality;
• A cutoff value of > 3 on the Numerical Pain Scale (NPS) indicates participants with pain, including headaches, upset stomach, and other forms of pain
• Has access to the Internet and a smart phone

Exclusion Criteria:

• Individuals with any health conditions that could hamper participation in the Health Apps for Post-Pandemic Years (HAPPY), such as severe cognitive, visual, or hearing impairments.

For confidentiality, the data will be kept anonymous and the information of all participants will be replaced by reference codes. The data collected will be kept in a locked place and electronic versions will be encrypted, and only be accessible by the researchers. All data will be destroyed within 7 years after the completion of this research.

The researchers who perform the outcome assessment and analysis will be blinded to the group allocations of participants.