subjects are able to participate if they: 1. are 18 years or older. 2. live or work in the chicagloland area (illinois counties: cook, lake, mchenry, dupage, kane, kendall, grundy, will, and kankakee; indiana counties: lake and porter). 3. 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. 4. 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. 5. 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: 1. report ever having a positive covid-19 pcr test result 2. report being pregnant, planning to become pregnant, and/or report breastfeeding during the study period. 3. report a history of chronic kidney disease, including a history of abnormal gfr and/or creatinine. 4. report a history of hyperparathyroidism. 5. report a history of increased falls. 6. report a history of hypercalcemia. 7. report a history of gastrointestinal absorptive disorders, including having undergone bariatric surgery. 8. report a history of kidney stones (1 in past year or 2 in lifetime). 9. 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. 10. report taking d2. 11. report a history of sarcoidosis. 12. screen positive for hypercalcemia during the initial blood test or follow-up blood tests. 13. screen positive for primary hyperparathyroidism during the initial blood test. 14. screen positive for covid-19 antibodies during the initial blood test. 15. have vitamin d levels of \>100ng/ml at study start, or \>250ng/ml during follow-up labs. 16. are unwilling to provide blood samples during quarterly blood tests. 17. are unwilling to take daily vitamin d.

subjects are able to participate if they: 1. are 18 years or older. 2. live or work in the chicagloland area (illinois counties: cook, lake, mchenry, dupage, kane, kendall, grundy, will, and kankakee; indiana counties: lake and porter). 3. 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. 4. 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. 5. 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: 1. report ever having a positive covid-19 pcr test result 2. report being pregnant, planning to become pregnant, and/or report breastfeeding during the study period. 3. report a history of chronic kidney disease, including a history of abnormal gfr and/or creatinine. 4. report a history of hyperparathyroidism. 5. report a history of increased falls. 6. report a history of hypercalcemia. 7. report a history of gastrointestinal absorptive disorders, including having undergone bariatric surgery. 8. report a history of kidney stones (1 in past year or 2 in lifetime). 9. 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. 10. report taking d2. 11. report a history of sarcoidosis. 12. screen positive for hypercalcemia during the initial blood test or follow-up blood tests. 13. screen positive for primary hyperparathyroidism during the initial blood test. 14. screen positive for covid-19 antibodies during the initial blood test. 15. have vitamin d levels of \>100ng/ml at study start, or \>250ng/ml during follow-up labs. 16. are unwilling to provide blood samples during quarterly blood tests. 17. are unwilling to take daily vitamin d.

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.

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.

subjects are able to participate if they: 1. are 18 years or older. 2. live or work in the chicagloland area (illinois counties: cook, lake, mchenry, dupage, kane, kendall, grundy, will, and kankakee; indiana counties: lake and porter). 3. 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. 4. 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. 5. 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: 1. report ever having a positive covid-19 pcr test result 2. report being pregnant, planning to become pregnant, and/or report breastfeeding during the study period. 3. report a history of chronic kidney disease, including a history of abnormal gfr and/or creatinine. 4. report a history of hyperparathyroidism. 5. report a history of increased falls. 6. report a history of hypercalcemia. 7. report a history of gastrointestinal absorptive disorders, including having undergone bariatric surgery. 8. report a history of kidney stones (1 in past year or 2 in lifetime). 9. 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. 10. report taking d2. 11. report a history of sarcoidosis. 12. screen positive for hypercalcemia during the initial blood test or follow-up blood tests. 13. screen positive for primary hyperparathyroidism during the initial blood test. 14. screen positive for covid-19 antibodies during the initial blood test. 15. have vitamin d levels of >100ng/ml at study start, or >250ng/ml during follow-up labs. 16. are unwilling to provide blood samples during quarterly blood tests. 17. are unwilling to take daily vitamin d.

subjects are able to participate if they: 1. are 18 years or older. 2. live or work in the chicagloland area (illinois counties: cook, lake, mchenry, dupage, kane, kendall, grundy, will, and kankakee; indiana counties: lake and porter). 3. 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. 4. 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. 5. 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: 1. report ever having a positive covid-19 pcr test result 2. report being pregnant, planning to become pregnant, and/or report breastfeeding during the study period. 3. report a history of chronic kidney disease, including a history of abnormal gfr and/or creatinine. 4. report a history of hyperparathyroidism. 5. report a history of increased falls. 6. report a history of hypercalcemia. 7. report a history of gastrointestinal absorptive disorders, including having undergone bariatric surgery. 8. report a history of kidney stones (1 in past year or 2 in lifetime). 9. 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. 10. report taking d2. 11. report a history of sarcoidosis. 12. screen positive for hypercalcemia during the initial blood test or follow-up blood tests. 13. screen positive for primary hyperparathyroidism during the initial blood test. 14. screen positive for covid-19 antibodies during the initial blood test. 15. have vitamin d levels of >100ng/ml at study start, or >250ng/ml during follow-up labs. 16. are unwilling to provide blood samples during quarterly blood tests. 17. are unwilling to take daily vitamin d.