Low vitamin D status (25OHD <20ng/mL) is associated with increased mortality in critical illness [14]. Forty to 70% of septic patients have low vitamin D status [15-17], yet little is known about the impact of vitamin D3 (vitD3) supplementation in this patient population. As such, the investigators propose a randomized, double-blinded, placebo-controlled trial to test the hypothesis that early, rapid correction of low vitamin D status, as an adjunct to established treatment guidelines, will improve clinical outcomes and measurably alter immune profile in patients with severe sepsis. Patients will be assigned to a single dose of vitD3 (400,000 IU) or placebo (n=300/arm) within 24 hours of new-onset severe sepsis, followed by weekly doses of vitD3 (25,000 IU) or placebo, respectively, up to 90 days to assess clinical outcomes and key biomarkers.