We also found two studies reporting negative results with vitamin D daily/weekly supplementation: the first, conducted in 2014, tested a daily supplementation of vitamin D 2000 IU/day for 2 months to Japanese high school students, and found no efficacy in lowering the overall incidence of influenza A [160]; the second, conducted in 2019 in Vietnam, analyzed a 14,000 IU/week supplementation of vitamin D to children and adolescents for 8 months, which was unable to prevent influenza infection during the flu season, but moderately reduced the incidence of other respiratory viral infections [172].