Vitamin D 3 treatment was associated with a significantly lower risk for laboratory confirmed URTI ( RR : 0.54 , CI 95 : 0.34-0.84 , p = 0.007 ) and with a significantly lower mean viral load measured as log 10 viral copies/mL ( mean difference : − 0.89 , CI 95 : -1.7 , − 0.06 , p = 0.04 ).