Subjects are able to participate if they:  Are 18 years or older. Live or work in the Chicagloland area (Illinois counties: Cook, Lake, McHenry, DuPage, Kane, Kendall, Grundy, Will, and Kankakee; Indiana counties: Lake and Porter). Are interested in vitamin D as a potential preventive measure against COVID-19 in which they self-administer a daily dose of vitamin D during the 9-month study period. Are willing to attend the laboratory for drop-in appointments at UChicago Medicine or Rush University Medical Center every 3 months at 4 time points over a 9-month period for blood draws measuring COVID-19 antibodies, calcium, vitamin D and PTH levels. Are willing to complete self-report measures at 4 time points over the course of 9 months by completing a 15-minute survey at intake by telephone or via web and 10-minute web-based follow-up surveys.  Subjects are excluded from study participation if they:  Report ever having a positive COVID-19 PCR test result Report being pregnant, planning to become pregnant, and/or report breastfeeding during the study period. Report a history of chronic kidney disease, including a history of abnormal GFR and/or creatinine. Report a history of hyperparathyroidism. Report a history of increased falls. Report a history of hypercalcemia. Report a history of gastrointestinal absorptive disorders, including having undergone bariatric surgery. Report a history of kidney stones (1 in past year or 2 in lifetime). Report already taking more than 400 IU of vitamin D daily as recommended by their health care provider, excluding multivitamins and excluding supplements that include vitamin D and calcium together. Report taking D2. Report a history of sarcoidosis. Screen positive for hypercalcemia during the initial blood test or follow-up blood tests. Screen positive for primary hyperparathyroidism during the initial blood test. Screen positive for COVID-19 antibodies during the initial blood test. Have vitamin D levels of >100ng/mL at study start, or >250ng/mL during follow-up labs. Are unwilling to provide blood samples during quarterly blood tests. Are unwilling to take daily vitamin D.