Similar studies were conducted worldwide, evaluating the circulating levels of vitamin D in children with LRTI and in controls: several studies found that lower vitamin D levels were associated with higher risk of developing an acute respiratory tract infection [125,130,132,133,135,139,144], or were linked to a more severe course of illness [136,142], with more frequent need for oxygen supplementation, ventilation support [134], or increased risk of intensive care unit (ICU) admission and longer hospital stay [143].