Coronavirus disease, commonly known as COVID-19, is an infectious disease caused by the SARS-COV-2 virus which was first diagnosed in Wuhan, China, in 2019. The infected patients will experience mild to moderate respiratory illness and recover without requiring special treatment; However, some will require serious medical attention. Older people and those with underlying medical conditions like cardiovascular diseases, diabetes, chronic respiratory disease, or cancer are more likely to get sick more seriously (World Health Organization, 2020). The increasing death rate of COVID-19 has increased anxiety, fear, and stress among people especially those with underlying illnesses (Bagheri et al., 2021). Cautious measures such as using face masks, keeping social distance, preventing formation of crowds, and home quarantine for affected people, to name a few, have taken to prevent the virus spread. This has led to changes in people's lifestyles and habits. Research has shown that the recovered patients may still experience some effects including long term respiratory problems, muscle atrophy, weakness, lethargy, lung fibrosis, secondary bacterial and fungal infections, and cardiovascular involvement such as myocarditis, pericarditis and pulmonary artery emboli (Iranpour & Ghaderian Jahromi, 2021). Thereby, post-acute COVID-19 syndrome (PCS) was introduced and defined as an ongoing symptomatic illness in patients who have recovered from their initial COVID-19 infection (Mahase, 2020). Evidence show that Patients with PCS may experience helplessness and depressive symptoms (Durankuş & Aksu, 2020). During these hard times, resilience can help people cope with stressful events and recover from illnesses (Morse et al., 2021).

The Resilience can be defined as the ability of an individual to adapt and cope with stress and adversity, and to bounce back from difficult situations (American Psychological Association, 2020, Brinkhof et al. 2021). These are the most common method for measuring resilience, which are comprised of participants answering a series of questions about their ability to cope with stress and adversity. Some well-known resilience scales include the Connor-Davidson Resilience Scale (CD-RISC) and the Resilience Scale. Resilience is considered as a positive and sustaining outcome which allows individuals to flourish despite their present adversities (Molina et al., 2014). Wood and Bhatnagar (2015) stated that increasing people's resilience and their resistance threshold toward stressful events can reduce their helplessness and increase their spirit and mental health. These symptoms along with low resilience can lead to low health-related quality of life (HRQoL) in PCS patients (Weya-Kaso et al., 2021). The HRQoL is a health care indicator for any type of diseases which measures patients' overall wellbeing in terms of physical, mental and emotional aspects (Foster et al., 2021). The HRQoL refers to an individual's subjective perception of their physical, mental, and social well-being, as well as their ability to perform daily activities and engage in meaningful relationships (WHO, 1997). The World Health Organization Quality of Life Module (WHOQOL) is the most commonly used criteria for quantitative assessment in healthcare research and clinical practice to assess the effectiveness of interventions and treatments. Guangbo et al. (2021) state that physical and emotional roles, vitality and social functioning were highly affected dimensions. In a retrospective study in China, Chen et al. (2020) stated that COVID-19 can substantially jeopardize the physical and psychological dimensions of patients' HRQoL.

Fortunately, there are treatments especially designed for such frustrating situations. Acceptance and commitment therapy (ACT), a third-wave behavioral therapy, aims to develop psychological flexibility (Hayes et al., 2012). The main goal of the ACT is to develop capability to make the most practical, fit decision between different choices instead of avoiding thoughts, feelings, or distressing desires (Hayes et al., 2005). In a study, Mirzaeidoostan et al. (2019) deployed ACT for female patients with HIV and found that the therapy could effectively decrease patients' anxiety about death and dying and improved their mental health. They, also, reported that the therapy was successful in increasing patients' psychological flexibility and help them to follow-up their treatment procedure with more seriousness. Since little is known about the PCS and its effects on patients and research literature on ways of psychologically helping them is scarce, the current research aimed to examine effectiveness of the acceptance and commitment therapy on resilience and health-related quality of life of patients with PCS.

Resilience and mental health are two critical factors that contribute to the positive mental health of a person, including doctors and patients who have recovered from COVID-19 and may suffer from various problems after recovery. Studies related to coronaviruses, particularly COVID-19, have shown that patients may experience severe complications after recovery from the disease, and their quality of life may be affected for years. Given the social, personal, economic, and health effects resulting from this disease, better disease management and mental health can be achieved through social support programs, organizational support, education, and preventive programs. This study examines the efficacy of the Acceptance and Commitment Therapy (ACT) treatment method on patients who have recovered from COVID-19 and shows that this method can enhance the resilience and mental health quality of life. Moreover, the findings of this study indicate that psychologists and doctors can use this treatment method to help patients with other conditions such as cancer and HIV with an expected high efficacy. The beneficiaries of this study are people affected by diseases such as COVID-19, cancer, HIV, and others, as well as those who care for these patients (such as doctors, nurses, and families). The results of this study can also be used to apply cognitive therapy methods like the ACT method to improve resilience and mental health among healthy individuals.