* vitamin d supplementation \>400 iu/day or \>12,000 iu/month in past 3 months; * intention to take \>400 iu per day during the study period; * suspected or previously documented covid-19 infection; * history of nephrolithiasis, hypercalcemia, hyperphosphatemia, hyperparathyroidism, granulomatosis disease (e.g., tuberculosis, sarcoidosis), renal impairment/failure, or active cancer; * use of any of the following medications: lithium, teriparatide, or digoxin; * anticipated prolonged absence from work during the study period (i.e., pregnancy); * enrolment in a concurrent randomized trial; * has received a vaccine against covid-19.

* vitamin d supplementation \>400 iu/day or \>12,000 iu/month in past 3 months; * intention to take \>400 iu per day during the study period; * suspected or previously documented covid-19 infection; * history of nephrolithiasis, hypercalcemia, hyperphosphatemia, hyperparathyroidism, granulomatosis disease (e.g., tuberculosis, sarcoidosis), renal impairment/failure, or active cancer; * use of any of the following medications: lithium, teriparatide, or digoxin; * anticipated prolonged absence from work during the study period (i.e., pregnancy); * enrolment in a concurrent randomized trial; * has received a vaccine against covid-19.

- vitamin d supplementation >400 iu/day or >12,000 iu/month in past 3 months; - intention to take >400 iu per day during the study period; - suspected or previously documented covid-19 infection; - history of nephrolithiasis, hypercalcemia, hyperphosphatemia, hyperparathyroidism, granulomatosis disease (e.g., tuberculosis, sarcoidosis), renal impairment/failure, or active cancer; - use of any of the following medications: lithium, teriparatide, or digoxin; - anticipated prolonged absence from work during the study period (i.e., pregnancy); - enrolment in a concurrent randomized trial; - has received a vaccine against covid-19.

- vitamin d supplementation >400 iu/day or >12,000 iu/month in past 3 months; - intention to take >400 iu per day during the study period; - suspected or previously documented covid-19 infection; - history of nephrolithiasis, hypercalcemia, hyperphosphatemia, hyperparathyroidism, granulomatosis disease (e.g., tuberculosis, sarcoidosis), renal impairment/failure, or active cancer; - use of any of the following medications: lithium, teriparatide, or digoxin; - anticipated prolonged absence from work during the study period (i.e., pregnancy); - enrolment in a concurrent randomized trial; - has received a vaccine against covid-19.

- vitamin d supplementation >400 iu/day or >12,000 iu/month in past 3 months; - suspected or previously documented covid-19 infection; - history of nephrolithiasis, hypercalcemia, hyperphosphatemia, hyperparathyroidism, granulomatosis disease (e.g., tuberculosis, sarcoidosis), renal impairment/failure, or active cancer; - use of any of the following medications: lithium, teriparatide, or digoxin; - anticipated prolonged absence from work during the study period (i.e., pregnancy); - enrolment in a concurrent randomized trial; - has received a vaccine against covid-19.

- vitamin d supplementation >400 iu/day or >12,000 iu/month in past 3 months; - suspected or previously documented covid-19 infection; - history of nephrolithiasis, hypercalcemia, hyperphosphatemia, hyperparathyroidism, granulomatosis disease (e.g., tuberculosis, sarcoidosis), renal impairment/failure, or active cancer; - use of any of the following medications: lithium, teriparatide, or digoxin; - anticipated prolonged absence from work during the study period (i.e., pregnancy); - enrolment in a concurrent randomized trial; - has received a vaccine against covid-19.

- vitamin d supplementation ≥400 iu/day or ≥ 12,000 iu/month in past 3 months; - suspected or previously documented covid-19 infection; - history of nephrolithiasis, hypercalcemia, hyperphosphatemia, hyperparathyroidism, granulomatosis disease (e.g., tuberculosis, sarcoidosis), renal impairment/failure, active cancer, or malabsorption syndromes; - use of any of the following medications: lithium, thiazide or thiazide-like medication, tamoxifen, amiloride, teriparatide, theophyllin, or digoxin; - anticipated prolonged absence from work during the study period (i.e., pregnancy); - enrolment in a concurrent randomized trial; - has received a vaccine against covid-19.

- vitamin d supplementation ≥400 iu/day or ≥ 12,000 iu/month in past 3 months; - suspected or previously documented covid-19 infection; - history of nephrolithiasis, hypercalcemia, hyperphosphatemia, hyperparathyroidism, granulomatosis disease (e.g., tuberculosis, sarcoidosis), renal impairment/failure, active cancer, or malabsorption syndromes; - use of any of the following medications: lithium, thiazide or thiazide-like medication, tamoxifen, amiloride, teriparatide, theophyllin, or digoxin; - anticipated prolonged absence from work during the study period (i.e., pregnancy); - enrolment in a concurrent randomized trial; - has received a vaccine against covid-19.

- vitamin d supplementation ≥400 iu/day or ≥ 12,000 iu/month in past 3 months; - intention to take >400 iu per day during the study period; - suspected or previously documented covid-19 infection; - history of nephrolithiasis, sarcoidosis, or hypercalcemia; - nticipated prolonged absence from work during the study period (i.e., pregnancy); - enrollment in n a concurrent randomised trial; - has received a vaccine against covid-19.

- vitamin d supplementation ≥400 iu/day or ≥ 12,000 iu/month in past 3 months; - intention to take >400 iu per day during the study period; - suspected or previously documented covid-19 infection; - history of nephrolithiasis, sarcoidosis, or hypercalcemia; - nticipated prolonged absence from work during the study period (i.e., pregnancy); - enrollment in n a concurrent randomised trial; - has received a vaccine against covid-19.