For the purpose of this review, we focused on studies on LRTIs, even though similar studies were also conducted on the prevention of upper airways infections; a large study conducted on the TARGet kids! research network in Toronto (Canada) led to different results, reporting that a high dose (2000 IU/day) was not more effective than a standard dose (400 IU/day) in preventing upper respiratory tract infections in children [172]. 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]. In this population, the authors reported a mean baseline vitamin D of 65 nmol/L (26 ng/mL), which might be one of the reasons why a further vitamin D supplementation did not lead to the expected results. In the previously cited studies, the baseline vitamin D status is not always reported; where it is known, it is usually lower, from 7 ng/mL, equal to 17.5 nmol/L [153], to 43 nmol/L, equal to 17.2 nmol/L [167].