Patient description: A 67-year-old healthy woman came to the clinic to have her flu shot in early October. She works at a rehab center and has no underlying disease. It is her first vaccination this year. she is menopausal and has 4 children. She does not some. She takes daily multivitamins and anti-hypertensive drugs. She exercises regularly for 30 minutes a day at least 5 days a week. She has no allergies to any food or drugs.

Clinical trial eligibility criteria: eligible ages (years): 18.0-999.0,  COVID-19 Inclusion/  for INDEX CASES: 1. Adults aged 18 years or older who are newly diagnosed with COVID-19 infection within 7 days of testing. 2. Ability and willingness to understand and provide informed consent Known current pregnancy. 2. Current hospitalization. 3. Unable to complete online questionnaires or adhere to study requirements. 4. Consume more than 1000 IU per day of vitamin D from all supplemental sources combined (individual vitamin D supplements, calcium plus vitamin D supplements, medications with vitamin D [e.g., Fosamax Plus D], and multivitamins) in the past 4 weeks. 5. Use of prescription vitamin D treatments (Calcitriol [Rocaltrol, Calcitrol, Vectical, Calcijex] or Paricalcitol [Zemplar]). 6. Consume supplements with more than 1200 mg calcium per day. 7. Known diagnosis of hypercalcemia or a condition associated with vitamin D hypersensitivity. 8. Prior diagnosis of cancer *AND* currently undergoing radiation, chemotherapy, or immunotherapy. 9. Kidney failure or dialysis; severe liver disease or cirrhosis. 10. Any parathyroid conditions. 11. Use of medications for seizures or epilepsy. Examples: Carbamazepine (Carbatrol, Tegretol), Phenytoin (Dilantin, Phenytek), Valproic acid (Depakene), Oxcarbazepine (Oxtellar, Trileptal), Phenobarbital, Topiramate (Topamax). 12. Use of digoxin. 13. Inability to receive an overnight express mail shipment of study pills at a home address. 14. Participation in other COVID-19 trials. Inclusion/ for 1. Persons aged 18 years or older who live in the same household with an index individual and have been identified as the closest household contact within the same household (limited to 1 per household). 2. Persons aged 18 years or older who live in household with someone who tested positive for COVID-19 within past 7 days but not participating in VIVID. 3. Ability and willingness to understand and provide informed consent Known current pregnancy. 2. History of SARS-CoV-2 infection. 3. Receipt of a SARS-CoV-2 vaccination or monoclonal antibody. 4. Unable to complete online questionnaires or adhere to study requirements. 5. Consume more than 1000 IU per day of vitamin D from all supplemental sources combined (individual vitamin D supplements, calcium plus vitamin D supplements, medications with vitamin D [e.g., Fosamax Plus D], and multivitamins) in the past 4 weeks. 6. Use of prescription vitamin D treatments (Calcitriol [Rocaltrol, Calcitrol, Vectical, Calcijex] or Paricalcitol [Zemplar]). 7. Consume supplements with more than 1200 mg calcium per day. 8. Known diagnosis of hypercalcemia or a condition associated with vitamin D hypersensitivity. 9. Prior diagnosis of cancer *AND* currently undergoing radiation, chemotherapy, or immunotherapy. 10. Kidney failure or dialysis; severe liver disease or cirrhosis. 11. Any parathyroid condition. 12. Use of medications for seizures or epilepsy. Examples: Carbamazepine (Carbatrol, Tegretol), Phenytoin (Dilantin, Phenytek), Valproic acid (Depakene), Oxcarbazepine (Oxtellar, Trileptal), Phenobarbital, Topiramate (Topamax). 13. Use of digoxin. 14. Inability to receive an overnight express mail shipment of study pills at a home address. 15. Participation in other COVID-19 trials