Subjects enrolled in the trial must meet all of the following criteria. - Confirmed COVID-19 pneumonia - Hypoxia as defined by room air oxygen saturation less than 92% or supplemental oxygen requirement - Presence of at least one additional biomarker that has been shown to predict poor prognosis: a) serum ferritin ≥500ug/l, b) LDH ≥250U/L, c) d-dimer ≥1ug/L, or d) lymphopenia as defined by absolute lymphocyte count <1,000/uL - Age ≥ 18 years - Completed informed consent

Subjects who meet ANY of the following criteria are not eligible for enrollment as study participants: - Known allergy or hypersensitivity to sirolimus - Inability or refusal to provide informed consent - Advanced respiratory support (high flow oxygen ≥ 15 L/min, CPAP, non-invasive or invasive mechanical ventilation) - Active enrollment in other interventional clinical drug trials. Co-enrollment in observational studies and biorepositories is allowed. - Pregnant women - Breast feeding - On chronic immunosuppression for other medical conditions such as rheumatological disorders, inflammatory bowel disease, or in patients with organ transplants. A list of these medications is provided in Section 12.3.4 - Any clinically significant medical disease which in the opinion of the investigator precludes the patient from enrolling in the trial, including (but not limited to): - History of liver cirrhosis - End stage renal disease or need for renal replacement therapy - Decompensated heart failure - Known active tuberculosis or history of incompletely treated tuberculosis - Uncontrolled systemic bacterial or fungal infections - Active viral infection other than COVID-19