An individual is eligible for inclusion if all of the following apply: 1. 18 to 70 years old, 2. Inciting event: Acute COVID documented by testing (PCR or antigen testing in a clinical setting). 3. Onset of COVID symptoms occurring on or prior to August 31st, 2023; and persistence of symptomatic expression of Long COVID (defined #6 below) for more than 3 months after COVID diagnosis. 4. Current symptomatic expression meets the case definition of ME/CFS. 5. PROMIS 29 score at screening of moderate to severe (≥60). 6. Meets National Academy of Sciences (NAS) criteria for Long COVID with the following provisions: 1. Allowance for normal illnesses of aging, such as hypertension and diabetes, if the conditions are treated and are in demonstrable stable and acceptable ranges at the time of screening and assessment. Specifically, blood pressures < 150 systolic and 90 diastolic mmHg are required. 2. Allowance of stable comorbid conditions common in post viral illness, such as fibromyalgia, irritable bowel, interstitial cystitis, dysautonomia that have not required hospitalization in the two years prior to recruitment. 7. Able to provide written consent to study. Agrees to participate in follow-up visits. Subject

An individual is ineligible to participate if any of the following apply: 1. Known active acute SARS-CoV-2 infection ≤ 4 weeks from consent. 2. Known severe anemia, defined as < 8 g/dL. 3. Known stroke that resulted in cognitive impairment within 3 months of enrollment. 4. Self-report of current treated or untreated major depression with psychotic or melancholic features, schizophrenia, bipolar disorder, delusional disorders, dementias of any type, or a history of CNS disorders that may affect cognitive function (i.e., epilepsy, stroke, brain tumor, multiple sclerosis, Parkinson's Disease, Alzheimer's disease), or substance abuse during the last two years, excluding cannabis products. 5. Allergy to any ingredient of the study drug (self-report) 1. Sipavibart is supplied as 150 mg/mL of active ingredient in 20 mM L-histidine/L-histidine hydrochloride, 220 mM L-arginine hydrochloride, and 0.04% (w/v) polysorbate 80, at pH 6.0. 2. Hypersensitivity to other humanized mAbs. 6. Current heavy alcohol or tobacco use (self-report). Alcohol consumption not to exceed approximately 15 drinks per week (with a drink defined as 12 oz beer, 5 oz wine, or 1.5 oz distilled spirits) and tobacco use not to exceed 20 cigarettes (or equivalent) per day during the last month. 7. Active chronic infections such as HIV, Hepatitis B Virus (HBV), and Hepatitis C Virus (HCV), indicated by self-report, and abnormal liver function tests (>3x upper limit of normal) or evidence in the health record of chronic active hepatitis or human immunodeficiency virus (HIV). 8. Renal disease (self-report; laboratory results: renal insufficiency with serum creatinine > 2.0 mg/dL or eGFR < 44; or currently on renal dialysis) 9. Liver disease (self-report or laboratory results: hepatic insufficiency (bilirubin >2.5mg/dL or transaminases > 3X the upper limits of normal) 10. Uncontrolled diabetes, evidenced by combination of morning blood glucose and previous diagnosis of diabetes, AIC>7 11. Diagnosed with congestive heart failure or significant arrythmia (ventricular tachycardia with a rapid rate at rest (> 100 bpm), persistent atrial fibrillation, or second- or third-degree heart block) 12. Pre-existing sustained severe hypertension (BP >180/110 mmHg in the sitting position) 13. Any of the following within 4 weeks of consent (Self-reported/medical record): 1. an acute myocardial infarction or unstable angina 2. uncontrolled arrhythmias causing symptoms or hemodynamic compromise 3. acute myocarditis or pericarditis, uncontrolled acutely decompensated heart failure (acute pulmonary edema) 4. acute pulmonary embolism 5. suspected dissecting aneurysm 6. severe hypoxemia at rest 7. any acute or chronic disorder that may affect exercise performance, or 8. if they are aggravated by exercise (e.g., infection, thyrotoxicosis, unable to cooperate) 14. Diagnosed bleeding disorders or use of blood-thinning medications. 15. Current or previous receipt of any COVID antiviral medication within 30 days prior to screening (self reported) 16. Currently have exclusionary diagnoses that could reasonably explain the symptoms of their fatiguing illness and their severity, using the exclusion criteria best described in the Ambiguities in case definition paper for CFS, as described in detail in [13] which clarifies exclusionary conditions. These exclusionary diagnoses that are not otherwise listed above comprise: 1. Organ failure 2. Chronic inflammatory diseases 3. Major neurologic diseases that could cause fatigue or neurologic deficits 4. Diseases requiring systemic treatment (i.e., transplantation, chemotherapy, radiation) 5. Major endocrine diseases 6. Untreated primary sleep disorders 7. BMI > 40 kg/m2 8. Temporary conditions discovered at screening, such as - Temporary effects of medications - Temporary sleep deprivation - Untreated hypothyroidism, hypothyroidism that has been inadequately controlled during the last 3 months, or free T4 level not within normal limits - Active infection (for COVID-19 infection and other infections, participants may be rescreened six weeks after resolution of infection) 17. Known diagnosis of chronic Lyme disease with persistent symptoms, sequelae, or related therapy. 18. Any marijuana illicit drug use within 30 days of informed consent 19. Inability to discontinue symptomatic medications for the identified time periods 20. Moderate or severe immunocompromised patients, such as those described in the NIH COVID-Treatment Guidelines 21. Are scheduled for a surgery during the period of study participation, had minor surgery within three months prior to screening, or had major surgery within 6 months prior to screening 22. Participating in any interventional (including social-behavioral therapy) clinical trial of an investigational therapy within 6 weeks prior to consent, or planning to participate in another interventional clinical trial of an investigational therapy during the course of this study 23. COVID Vaccination within 90 days prior to entry and for the duration of the study 24. Pregnancy is excluded. Women of childbearing age will be given a pregnancy test.