Conclusions Vitamin D supplementation  was safe and it protected against acute respiratory tract infection overall .
Patients who were very vitamin D deficient and those not receiving bolus doses experienced the most benefit .
Objectives To assess the overall effect of vitamin D supplementation on  risk of acute respiratory tract infection  , and to identify factors modifying this effect .
Effects of vitamin D supplements on influenza A illness during the 2009 H1N1 pandemic: a randomized controlled trial.
In a prior r and omized trial , we found that the incidence of influenza A was less in the vitamin D3 group than among those on placebo , but the total incidence of either influenza A or B did not differ between groups . In this trial , the incidence of influenza A or B was less in the vitamin D3 group than in the placebo group only during the first half of the study . To eluci date whether vitamin D3 has preventive actions against influenza A , we conducted another trial during the 2009 p and emic of the H1N1 subtype of influenza A. Students ( n = 247 ) of a Japanese high school were r and omly assigned to receive vitamin D3 supplements ( n = 148 ; 2000 IU per day ) or a placebo ( n = 99 ) in a double-blind study for 2 months . The primary outcome was incidence of influenza A diagnosed by a rapid influenza diagnostic test by medical doctors . Influenza A was equally likely in the vitamin D3 group ( 20/148 : 13.5 % ) compared with the placebo group ( 12/99 : 12.1 % ) . By post hoc analysis , influenza A occurred significantly less in the vitamin D3 group ( 2/148 : 1.4 % ) compared with the placebo group ( 8/99 : 8.1 % ) ( risk ratio , 0.17 ; 95 % confidence interval , 0.04 to 0.77 ; P = 0.009 ) in the first month . However , during the second month , the vitamin D3 group experienced more events and effectively caught up with the placebo group . Vitamin D3 supplementation did not lower the overall incidence of influenza A during the 2009 H1N1 p and emic . A post hoc analysis suggests that the initial benefit during the first month of treatment was lost during the second month
Conclusions Vitamin D supplementation  was safe and it protected against acute respiratory tract infection overall .
Patients who were very vitamin D deficient and those not receiving bolus doses experienced the most benefit .
Objectives To assess the overall effect of vitamin D supplementation on  risk of acute respiratory tract infection  , and to identify factors modifying this effect .