This intervention will be a 12-week progressive and individualized exercise program with a frequency of 3 sessions per week and a session duration of 60 minutes (total: 36 exercise sessions). The intervention is designed to alleviate symptoms of dyspnea, improve functional capacity, and relieve mental health problems in the short term. A further aim is to preserve physical function while improving health-related QoL in the long term. As recommended by the Official American Thoracic Society/European Respiratory Society Statement [20], 12 education sessions (30 minutes per session) will be included in and delivered together with the first exercise session in each week to motivate and empower the patients to form regular exercise habits and improve health [21,22]. A steering group will be formed to design the exercise intervention, develop relevant documentation, and monitor the implementation. Group members will include exercise experts, public health specialists, occupational therapists, doctors, and other stakeholders. Textbox 1 and Table 1 provide a summary of the intervention.

This program will be delivered using a home-based tele-exercise approach to reduce the potential to infect other individuals [24], reduce health care burden and costs for both health care providers and patients, and improve participation and retention of rehabilitation [25]. Some evidence, although not for COVID-19, has suggested that home-based pulmonary rehabilitation with minimal exercise equipment is effective and may be a viable alternative to equipment-intensive clinic-based programs [18,26]. Other studies concluded that the web-based tele-exercise approaches were as effective as conventional face-to-face interventions [27] and could enhance the reach and engagement of the rehabilitation process [21,25]. We will build and develop a web-based platform in our International Rehabilitation Network Center for Patients With COVID-19 to deliver the intervention.

The intervention will include the following components: (1) a set of home-based tele-exercise sessions during which remote monitoring of vital signs will be required; (2) an individualized action plan to perform various daily physical activities; (3) educational sessions on self-management and habit formation; (4) access to a call center; (5) counselling sessions to enhance motivation to regularly engage in daily physical activities. The counseling intervention will be facilitated via a remote patient monitoring system. Patients will be equipped with an activity monitor (Mi Band 5, Xiaomi Corporation) that connects to a customized smartphone or tablet with a global system for mobile communication (GSM) connection that will receive, display, and send the data via the internet to a secure web-based server at the university. Patients will receive real-time feedback on the smartphone or tablet about their physical activity levels and their daily adjusted physical activity goals. A teleconference between the patient and the counselor will also be possible via the smartphone or tablet (using Zoom) in order to continually enhance adherence and goal achievement. Patients’ daily physical activities will be acquired by the Mi Band 5 via Bluetooth technology to the smartphone or tablet; from the smartphone or tablet, data will be sent to the central server. Following the completion of the intervention, all patients will receive detailed individual feedback in relation to the outcome measures obtained via a meeting with the research team. This will comprise a question and answer session with relevant experts in their field of expertise. Patients will also receive a detailed personalized report with recommendations and information relating to the outcomes of the intervention.

The content of the exercise intervention will be developed with reference to the Official American Thoracic Society/European Respiratory Society Statement published in 2013 [20]. Each exercise session (40-60 minutes) will include a warm-up, aerobic training, resistance training, a cool-down, and inspiratory muscle training (IMT) [20,28,29]. Aerobic exercise will be arranged before resistance training, as recent evidence has shown that this order produces greater benefits than performing resistance training first [30]. Additional IMT sessions will be performed without supervision according to a suggested frequency of twice daily [20]. Before the first exercise session of each week, an additional educational session (30 minutes) will be delivered to improve motivation, self-regulation, and habit formation relating to exercise [20]. The contents of the educational sessions will be developed mainly based on social psychological theories of intentional behavior and motivation and effective behavioral change techniques (Textbox 1) [20,22]. All patients will be briefed face-to-face initially to ensure that they can undertake the intervention properly and to enhance their compliance with the intervention. In addition, they will be videotaped during the exercise sessions, and real-time feedback will be given via the internet.

Aerobic exercise intensity will be determined using the Borg Category-Ratio 0-10 Scale (Borg CR10 Scale) modified for dyspnea, as recommended by the American College of Sports Medicine guidelines for patients with pulmonary diseases [23]. The exercise intensity will start from Rating 3 (equivalent to 53% peak work rate, ie, moderate intensity) in the first 2-4 weeks and progressively increase to Rating 6 (equivalent to 80% peak work rate, ie, vigorous intensity). Exercise intensity and session duration will be adjusted according to each patient’s response and tolerance. Lighter intensities than Rating 3 may be adopted when there is a need (eg, for those with severe conditions). Patients will be given specific and standardized instructions on the ratings in advance. A pulse oximeter (Beurer PO 40, Beurer GmbH) will be used to monitor oxygen saturation in the initial exercise sessions to avoid exercise-induced oxyhemoglobin desaturation. In terms of intensity of resistance training, a practical approach to determine the appropriate intensity will be adopted, in which workloads will be set at levels that evoke local muscular exhaustion in 6 to 12 repetitions, as the one repetition maximum approach may overestimate or underestimate the optimal resistance for individuals [20,31]. Progressive target goals and detailed training plans will be set up together with each patient based on their physiological measurement results [21].

The exercise intervention will be delivered and supervised by experienced instructors. We will recruit persons who have been educated in either exercise science or rehabilitation. Each exercise session will be facilitated by two instructors dedicated to a group of 5-7 patients through the web-based platform in our International Rehabilitation Network Center for Patients With COVID-19. They will also be responsible for monitoring developments and training during the intervention.

The Chinese herbal formula of Modified Bai He Gu Jin Tang (including Rehmanniae Radix 10 g, Rehmanniae Redix Praeparata 10 g, Ophiopogonis Radix 10 g, Lilii Bulbus 15 g, Paeoniae Radix Alba 10 g, Angelicae Sinensis Radix 10 g, Fritillariae Thunbergii Bulbus 5 g, Glycyrrhizae Radix et Rhizoma 10 g, Platycodonis Radix 15 g, and Salviae Miltiorrhizae Radix et Rhizoma 15 g) will be prescribed in granules. The rationale for the formula selection is as follows: (1) based on the syndrome differentiation, pulmonary fibrosis post–COVID-19 appears as qi-yin deficiency of the lung [32]; (2) a previous study has shown that many herbs using this formula and Salviae Miltiorrhizae Radix et Rhizoma have antifibrosis effects [33]; and (3) Bai He Gu Jin Tang has the effects of nourishing lung qi and ying deficiency [34]. Therefore, the formulation of Bai He Gu Jin Tang was modified to match the therapeutic purpose. A dose of 10 g per day (5 g, twice per day) will be ingested. Patients will dissolve a sachet of granules (5.0 g) in 200 ml of hot water twice per day after breakfast and dinner, 7 days per week, for 3 months. The entire manufacturing process will be in strict compliance with the standards of good manufacturing practice.

The waiting list control sign will be adopted to conceal the allocation results from the patients as well as to reduce selection and confounding bias and increase the participants’ adherence to the study. Patients in the waiting list control group will receive no treatment in the study period (including a 12-week intervention period and a 12-week follow-up period). However, they will receive Chinese herbal medicines after the completion of the study (ie, after the third wave of measurements in the 25th week).

Functional exercise capacity that reflects daily physical activities will be assessed using the six-minute walk test (6MWT) to assess the outcome of the intervention. This test is recommended by the American Thoracic Society to evaluate the global and integrated responses of all the systems involved during exercise, including the cardiopulmonary systems, systemic and peripheral circulation, blood, neuromuscular units, and muscle metabolism. Heart rate and blood pressure will be measured before and after exercise. The test has high test-retest reliability (intraclass correlation coefficient=0.88-0.91) [35].

Dyspnea is a common exercise-induced symptom of disease and usually occurs in patients with pathophysiology that results in inefficient gas exchange and ventilator impairments. The Borg CR10 Scale (Borg Dyspnea Scale) will be used to rate the dyspnea and overall fatigue levels of the patients at the beginning and end of the 6MWT. The scale will also be used during the cardiopulmonary exercise test (CPET).

Segmental muscle mass and BMI (kg/m²) will be used to assess the body composition of the patients. A bioimpedance analysis approach (InBody 770 analyzer, InBody Co, Ltd) will be used to assess the patients’ segmental muscle mass. A stadiometer (Seca 284, Seca Corporation) will be used to measure stature and body mass. Anatomical circumference will be measured using a distensible measuring tape.

The FVC test, forced expiratory volume (FEV₁), and FEV₁/FVC ratio will be assessed to indicate the functional severity and capacity of the patients’ lungs (Vmax Encore V229, VIASYS Respiratory Care Inc). Fractional exhaled nitric oxide (FeNO) will be used to assess inflammatory response to exercise and medicinal intervention (NObreath, Bedfont Scientific Ltd, England).

The CPET will be performed using the Vmax Encore V229. The CPET provides information concerning the level of exercise that the patient can perform without undue stress. The test results will guide the research team regarding the prescription of exercise for physical rehabilitation methodologies. It also provides quantitative evidence of the benefits of a rehabilitation program as well as information on the mechanisms involved. Improvement in exercise tolerance cannot be objectively assessed without CPET [36]. An incremental ergometry exercise test (Excalibur ergometer, Lode Ergometry), will be used to assess the responses of the cellular, cardiovascular, and ventilatory systems under precise conditions of metabolic stress. Breath by breath measurements of minute ventilation (VE), carbon dioxide output (VCO₂), O₂ uptake (VO₂), VE/VCO₂, VE/VO₂, deviation in the responses from the pulmonary to cellular respiration and arterial blood gas, partial pressure of carbon dioxide (PCO₂), and oxygen saturation as measured by pulse oximetry (SpO₂, Sentec AG), will be monitored continuously during the exercise test. Depending on the physical condition of the patients, modification of the protocols to smaller work rate increments may be warranted. Intensities and durations will be specific to each individual to facilitate measurement and data capture. Before each test, the O₂ and CO₂ analyzers of the CPET system will be calibrated using standard gases (Gas 1: O₂: 25.89%, CO₂: 0%; Gas 2: O₂: 16.44% CO₂: 3.89%), and the flow volume integration system will be was calibrated by applying a standard volume (3 L) of gas at various flow rates. Canopy studies within the Vmax software suite will be used to validate the accuracy of the gas-exchange measurement instruments that measure VO₂ and VCO₂ using the methanol burning technique. The ratio of VCO₂ to VO₂ in the combustion of methanol yields a respiratory quotient (RQ) of 0.667. We will use a 3% difference as the accuracy threshold for measured RQ (RQ range of 0.647-0.687).

All the devices used for measurement will be calibrated according to the manufacturer's specifications. The protocols of the measurements and sources of variability will be controlled by following the standards outlined in the American Thoracic Society calibration guidelines for spirometers and gasometers. Anatomical circumference measurements will be adopted from the anthropometry procedures manual provided by the US Centers for Disease Control and Prevention [37]. Repeated tests will be performed at the same time of day in a controlled laboratory environment of constant humidity, temperature, and pressure. Patients will arrive at the laboratory at designated times for all physiological and clinical assessments. All patients will be transported to the laboratory for testing and returned home by coach. A recovery period will be observed postassessment to ensure that the participants have no negative responses to the measurements. Once the recovery period is complete, all participants will be returned home.

Venous blood will be drawn at 3 time points, namely at pretreatment, at posttreatment, and after the 12-week follow-up. After coagulation at room temperature for 30 minutes, the samples will be centrifuged at 3000 rpm for 20 minutes. Serum as the supernatant will be extracted and stored at –80 ˚C until assay. Serum levels of insulin will be assessed by a commercially available enzyme-linked immunosorbent assay (ELISA) kit (Merck & Co). Serum will be isolated by centrifugation (1000 rpm for 30 minutes, 4 °C) and stored at –80 °C until analysis. The prothrombin time test will be used to measure blood coagulation. Venous blood will be collected by venepuncture in a tube with sodium citrate. Plasma will be isolated after centrifugation at 1000 rpm for 30 minutes at 4 °C. Thromboplastin will be added to the plasma and maintained at 37 °C for 2 minutes. Calcium chloride will be added to the mixture, and the plasma will be allowed to coagulate. The time needed for the coagulation will be recorded as the prothrombin time.

Flow cytometry for counts of main lymphocyte subpopulations (CD3, CD4, CD8, CD19, and CD16+CD56 T cells) will be performed to test immune function. Venous blood will be stored in tubes with EDTA (anticoagulant). 100 µL of blood will be stained within 24 hours of sampling by the BD Multitest 6-color TBNK reagent (BD Biosciences). The lymphocytes with the antigens CD3, CD4, CD8, CD19, and CD16+CD56, which are the main populations of lymphocytes, will be labelled by a fluorochrome-conjugated antibody in the reagent. The whole blood will be incubated in the reagent at room temperature for 30 minutes, and red blood cells will be lysed with ammonium chloride solution. Cell sorting and analysis will be performed with a BD Accuri C6 flow cytometer (BD Biosciences). Each subpopulation of lymphocytes will be expressed in absolute value and percentage of total lymphocytes.

As suggested by Hacimusalar and Eşel [38], the following markers related to emotional disturbances will be studied.

All patients will be requested to self-sample their first morning feces by following detailed printed instructions. Collected stool samples will be immediately frozen in a home freezer (–20 ℃), then transported to our facilities in a provided freezer pack and stored at –80 ℃ for long-term use. Total DNA of 200 mg fecal samples will be extracted and purified as described [39]. The DNA concentrations and size distributions will be estimated using a NanoDrop instrument (Thermo Scientific) and agarose gel electrophoresis, respectively. The DNA one-paired-end library will be prepared using a TruSeq DNA HT Sample Prep Kit (Illumina Inc), and whole-genome shotgun sequencing of the samples will be carried out using the HiSeq 2000 platform (Illumina Inc). Low-quality reads with N bases, adapter contamination, or human DNA contamination will be filtered from the raw data, and the remaining high-quality sequences will be mapped with the published gene catalog of reference genes in the human gut microbiome [40]. Taxonomic assignment of the predicted genes and

Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis will be performed as described previously [41]. The relative abundances of phyla, genera, species, and KEGG orthologs will be calculated from the relative abundances of the respective genes. For data analysis, the quantitatively measured metabolites for each participant will be merged into the participants’ clinical information through their individual sample IDs.

Human serum or plasma samples will be thawed on an ice bath to minimize sample degradation. Metabolites will be extracted by adding 0.5 mL of 50% methanol (–20 °C) followed by homogenization for 3 minutes using a Bullet Blender Tissue Homogenizer (Next Advance, Inc) and centrifugation at 13,500 g for 10 minutes at 4 °C. The resulting 50 μL of supernatant will be transferred to a 1.5 mL tube and mixed with 10 μL of 200 mM 3-nitrophenylhydrazine (3-NPH) solution and 10 μL of mixed 96 mM ethylene dichloride/pyridine methanolic solution. Derivatization will be conducted by incubation at 30 °C for 1 hour before evaporation to dryness under nitrogen. 400 μL of 50% aqueous methanol will be used to resuspend the samples. The supernatants will be used for ultra-performance liquid chromatography coupled with quadrupole/time-of-flight mass spectrometry (UPLC-QTOF-MS) analysis according to previous reports with minor modifications [42,43].

QoL will be measured using the Personal Well-being Index-Chinese Version. It is a subjective QoL measure that has been translated and validated. Anxiety will be measured using the Chinese version of the Depression Anxiety Stress Scale-21, which has been proven to have internal consistency [44,45]. The scale will discriminate between the negative emotional syndromes of depression, anxiety, and stress in Chinese populations. Only the subscales of anxiety and stress will be used. Loneliness will be measured using the Revised UCLA Loneliness Scale) [46,47]. This scale consists of 20 items and has been widely used to assess loneliness in research. General mental health will be measured using the General Health Questionnaire, which is commonly used to screen minor psychiatric symptoms [48].