In conclusion, there is evidence on the role of vitamin D in regulating the immune response to viral infections, and data from most observational studies confirm an association between lower vitamin D levels and increased susceptibility to respiratory infections. Clinical trials overall show that daily or weekly supplementation of vitamin D is more beneficial in preventing LRTI than bolus or short-term administration, as confirmed by a 2017 meta-analysis by Martineau and colleagues [171], though more research will be needed to fully determine when and how vitamin D should be supplemented. Vitamin D supplementation did not appear to be effective in treating existing infections in pediatric trials, as also described in a 2018 review from Das and colleagues [173]. The different results reached in the above-mentioned studies might be due to the heterogeneity in the baseline vitamin D status of the observed populations; it is also possible that vitamin D receptor’s polymorphisms affect the daily vitamin D requirements of different individuals. Future studies might better clarify which patients will benefit from vitamin D supplementation and which ones will not, which is the best dose to administer in each case, and whether vitamin D status should always be tested before intervention.