Different physical and mental morbidities such as pain, fatigue, depressed mood and cognitive impairment can be triggered by coronavirus infection. Transcranial direct current stimulation (tDCS), an easy-to-apply, non-pharmacological and safe technique, has been used to attenuate these symptoms caused by other diseases, and, therefore, it is expected that it can also attenuate them when generated by COVID-19. It is known that the persistent inflammatory state observed after COVID-19 would be related to the progression of these negative symptoms. As non-invasive brain stimulation can also attenuate acute and persistent inflammation, it can be estimated that tDCS can be a useful tool to recover immune function and reduce post-COVID-19 morbidity.

The aim of this study is to investigate if Direct Current Electrical Stimulation (tDCS) associated with motor and/or cognitive training is capable of reducing neuropsychiatric symptoms, as well as immune changes induced in COVID-19.

Volunteers will be randomized and allocated into four groups (Fatigue; Pain; Cognitive deficit and Depressed mood) and each group will be divided into two subgroups: 1) real tDCS + motor or cognitive training; and, 2) sham tDCS + motor or cognitive training according to each outcome. The volunteers will be evaluated in two aspect: immunometabolic evaluation (i) and clinical assessment (ii). The measures will pe performed in three moments: baseline (session 1); 72 hours after the last intervention (session 12) and follow up - 15 days after the last intervention (session 13).

(i) The immunometabolic evaluation will start with the participant's blood collection, after fasting for 8-12 hours, without having practiced intense physical activity and without drinking alcohol in the 24 hours prior to the collect. The blood will be allocated in 10 vacutainer tubes of 5ml containing EDTA for plasma separation, in 2 dry vacutainer tubes of 5ml for serum separation and 1 vacutainer tube of 5ml containing fluoride. In particular, 1 dry tube must rest for 1h, for subsequent BDNF analysis. 2 serum tubes, 2 plasma tubes and 1 fluoride tube should be centrifuged at 3,500 rpm for 15 minutes at 4°C, and the samples should be stored at -80°C for later analysis and the other fresh plasma tubes used (up to 2 hours after collection) for whole blood and PBMC stimuli.

Stimulation of whole blood with LPS, culture of peripheral blood mononuclear cells (PBMC), assessment of oxidative stress, assessment of adenine and LPS purine levels in plasma, analysis of immunophenotyping and apoptosis by flow cytometry, assessment of expression of purinoreceptors, mitochondrial assays in monocytes and T lymphocytes, evaluation of the generation of reactive oxygen species and quantification of several mediators by ELISA.

(ii) The clinical assessment will be performed using the following outcome measures:

COVID clinical situation - IInitially, modules 1 and 2 of the Case Report Form - CRF (Pcovid frameost COVID-19 CRF) of the Pan American Health Organization (PAHO) will be completed. Where module 1 comprises demographic data and clinical information related to the acute episode of COVID-19 and module 2 includes questions related to vaccination, occupational and functional status of the volunteer.

Fatigue - The Modified Impact of Fatigue Scale (MFIS) contains 21 items that analyze cognitive, physical and psychosocial issues in relation to fatigue, assessing the impact that is caused in the patient's life.

Pain - Brief Pain Inventory (BPI) assesses pain severity, impact of pain on daily function, pain location, pain medications, and amount of pain relief in the last 24 hours and last week; Visual Analog Scale (VAS) consists of a ruler numbered from 0 to 10 and divided into three parts, light, moderate and severe, with visual aid to facilitate the measurement of the patient's pain intensity.

Cognitive deficit - Montreal Cognitive Assessment (MoCA) instrument that quickly identifies cognitive declines in patients; FAS Test is a Verbal Fluency Test - Phonological Fluency Test that assesses verbal learning and the ability to produce words verbally; Random Number Generator (RNG test) assesses language and executive function; Digit span that will be used to assess the ability to focus, maintain attention, and mental manipulation. In the follow up, the CFL-test will be added to a verbal fluency test.

Depressed mood - Hospital Anxiety and Depression Scale (HADS) measures symptoms of anxiety and depression and consists of 14 items, seven for the anxiety subscale (HADS-Anxiety) and seven for the depression subscale (HADS-Depression); Brunel Mood Scale (BRUMS) used to quickly measure the patient's mood state.

Level of physical activity - The short version of IPAQ will be performed to identify if the volunteer has a life with active physical activities. It makes it possible to estimate the weekly time used for physical activities.

Vital signs - at the end of the scales, the signs of each patient will be measured: blood pressure, heart rate, respiratory rate and peripheral O2 saturation, parameters used to verify the hemodynamic status of the volunteer.

State of strength and effort - the handgrip strength test and perception of effort performed with the dynamometer will be used, quantitatively indicating the muscle strength of the hand and forearm.

Exercise capacity - the functional capacity and aerobic resistance of the participant will be evaluated through the 6-minute walk test. The test measures the distance that the volunteer covers in a period of 6 minutes walking at a stable speed.

At the end, a questionnaire of adverse effects of the evaluation will be carried out, containing information regarding any discomfort felt during the evaluation.

The intervention process will be carried out with Transcranial Direct Current Stimulation which will be applied for 30 minutes (2 mA; 0.057 mA/cm²) during motor or cognitive training. For symptoms of pain and/or fatigue, the anode will be positioned in the area referring to the left primary motor cortex (C3, according to the EEG positioning system). For symptoms of depressed mood and/or cognitive impairment, the anode will be positioned in the area of the left dorsolateral prefrontal cortex (F3). In both situations, the cathode will be positioned in the contralateral supraorbital region. In sham tDCS, the same parameters and setup as the real stimulation will be used, however, the application time will be 30 seconds, but the electrodes will be maintained for 30 minutes.

In motor training, the treadmill training protocol will last a total of 30 minutes and will be carried out during the application of tDCS. After positioning the tDCS electrodes, a cardiac monitor will be positioned on the individual to control HR. Before starting the protocol on the treadmill, the values of the perceived exertion scale (Borg) and HR should be noted.

Initially, the target HR for each part of the intervention will be determined. The formula by Tanaka et al., (2001) will be used to determine the maximum HR, with maximum HR = 208 - (0.7*age). For participants who use beta-blockers, the formula by Brawner et al., (2004) will be used, with maximum HR = 164 - (0.7*age).

The initial five minutes will be a warm-up. To achieve this, the target HR must be maintained at 50-60% of the maximum HR. The speed of the treadmill must be adjusted to maintain the HR within the pre-established target range. At five minutes, the Borg scale values, treadmill speed and HR must be noted. The central 20 minutes will be considered as the main part. To achieve this, the target HR must be maintained at 64-76% of the maximum HR. If the participant chooses to stop the treadmill session, the treadmill must be turned off and the participant must remain seated until the end of the tDCS application.

For cognitive training, carried out during tDCS, an n-Back task will be performed online in PsyToolkit (https://www.psytoolkit.org/). For a benefit on working memory learning curves. Performed for the outcomes of cognitive impairment and depressed mood. During the test, the volunteer will have to indicate which letter was projected by the application two letters ago. The system will show one letter followed by another, the volunteer must press the "space" key on the computer keyboard when the letter that was shown in the two positions above reappears.

The study will be carried out simultaneously in two research centers in Brazil: Laboratory of Applied Neuroscience (LANA), Federal University of Pernambuco (UFPE), Pernambuco, Brazil and at the Department of Physical Education, Faculty of Science and Technology - Campus Presidente Prudente (FCT/UNESP), São Paulo, Brazil.

Inclusion Criteria:

• Long COVID-19 (>3 months)
• The presence of at least one of the following symptoms, after diagnosed SARS-COV-2 infection:

Persistent pain Fatigue Depressed mood Cognitive impairment

Exclusion Criteria:

• Hemodynamic instability
• Historic of use or abuse of drugs or alcohol
• Use of medications that may interfere with the study results (anti-inflammatories and antibiotics)
• Metallic implant in skull and face
• Implant in the central nervous system
• Pacemaker