Phase I Open-Label Safety Trial of Pembrolizumab for Neurological Post- Acute Sequelae of SARS-CoV-2 (PD1-PASC I)

Study Description: Despite clinical recovery from an acute SARS-CoV-2 infection, some individuals continue to experience ongoing symptoms for months to years afterwards, with many of these symptoms being neurologic. These neurologic post-acute sequelae of SARS-CoV-2 infection (Neuro-PASC) may be related to persisting viral antigen that leads to immune exhaustion. This study will primarily evaluate the safety of one dose of intravenous Pembrolizumab therapy in neuro-PASC participants with evidence of immune exhaustion. Pembrolizumab therapy may have positive clinical and laboratory effects on participants with persistent neurological symptoms, which will be measured as secondary outcomes. Objectives: Primary Objective: -To determine the safety of a single dose of intravenous Pembrolizumab in participants with neurological postacute sequalae of SARS-CoV-2 infection. Secondary objectives: * To determine if one dose of intravenous Pembrolizumab can normalize markers of immune exhaustion in neuro-PASC. * To determine if one dose of intravenous Pembrolizumab can lead to clinically relevant improvement in subjective and objective measures of ability. Exploratory objectives: * To determine if one dose of intravenous Pembrolizumab can change measures of SARS-CoV-2 antigen. * To determine if one dose of intravenous Pembrolizumab can change measures of cytokines. * To determine if one dose of intravenous Pembrolizumab can lead to improvement in subjective measures of symptoms. Endpoints: Primary endpoint: \- Number of adverse events and serious adverse events. Secondary endpoints: * Number of participants with normalization of PD-1 expression on CD8 T cells. * Number of participants achieving a minimal clinically important difference in measures of function: * Work Disability Functional Assessment Battery (WDFAB) * PROMIS Global Health * Short Form 36 (SF-36) * PROMIS Cognitive Function * Montreal Cognitive Assessment (MoCA) * Incremental shuttle walk test Exploratory endpoints: * Detection of viral antigen * Change in cytokine profile * Statistically significant positive change from baseline: * PROMIS Depression score * PROMIS Anxiety * PROMIS Fatigue * PROMIS Sleep Disturbance * WHO post COVID-19 functional scale * Patient Post-COVID-19 Functional Status (PCFS) scale (0-4). * Provider Post-COVID-19 Functional Status (PCFS) scale (0-4). * FUNCAP55 * Multidimensional Fatigue Inventory (MFI) * Pittsburgh Sleep Quality Index (PSQI) * Modified Depaul Symptom Questionnaire-Post Exertional Malaise (DSQ-PEM) * Post Exertional Malaise Visual Analogue Scale * Patient Global Impression of Change * Physician Global Impression of Change * Brief tablet-based neurocognitive testing * Near Infrared Spectroscopy of muscle * Actigraphy * Hand grip strength * Holter monitoring for 24 hrs * Orthostatic Intolerance Questionnaire/Orthostatic Intolerance Daily Activities Scale (OIQ/OIDAS) * Dysautonomia measures in response to the QSART, Valsalva maneuver or head-up tilt table testing. * Ophthalmology Examination