Table 4.

Studies on the association between low levels of 25-hydroxyvitamin D and increased susceptibility to lower respiratory tract infections (LRTI) in childhood.

Study	Author	Country	Study Population	Results
Case-control study of the role of nutritional rickets in the risk of developing pneumonia in Ethiopian children.	Muhe et al., 1997 [124]	Ethiopia	500 children with pneumonia vs. 500 healthy controls	Higher incidence of rickets in children with pneumonia.
Association of subclinical vitamin D deficiency with severe acute lower respiratory infection in Indian children under 5 years.	Wayse et al., 2004 [125]	India	Children with severe ALRI vs. controls	Vitamin D levels >22.5 nmol/L associated with lower risk of severe ALRI.
The frequency of nutritional rickets among hospitalized infants and its relation to respiratory diseases.	Najada et al., 2004 [126]	Jordan	443 children hospitalized due to different causes	Higher risk of being admitted due to LRTI and significantly more prolonged hospital stay in children with rickets.
Vitamin D deficiency in young children with severe acute lower respiratory infection.	McNally et al., 2009 [127]	Canada	105 children <5 years with ALRI vs. healthy controls	Significantly lower vitamin D levels in children admitted to PICU.
Nutritional rickets and vitamin D deficiency–association with the outcomes of childhood very severe pneumonia: a prospective cohort study.	Banajeh et al., 2009 [128]	Yemen	152 children aged 2–59 months with pneumonia	Significantly more frequent treatment failure in rachitic children; vitamin D deficiency associated with day 5 hypoxemia <88%.
Vitamin D status is not associated with the risk of hospitalization for acute bronchiolitis in early childhood.	Roth et al., 2009 [129]	Canada	64 children aged 1–25 months with ALRI vs. healthy controls	Similar vitamin D concentrations among cases and controls.
Vitamin D status and acute lower respiratory infection in early childhood in Sylhet, Bangladesh.	Roth et al., 2010 [130]	Bangladesh	25 children aged 1–18 months with ALRI vs. 25 healthy controls	Significantly lower vitamin D in ALRI cases than in controls.
Frequency of nutritional rickets in children admitted with severe pneumonia.	Haider et al., 2010 [131]	Pakistan	137 children with severe pneumonia	High frequency of rickets (74% of cases).
Relationship between vitamin D levels and outcome of pneumonia in children.	Oduwole et al., 2010 [132]	Nigeria	24 children with pneumonia vs. healthy controls	Lower vitamin D levels in cases than in controls; increased complications frequency when lower vitamin D levels.
Low serum 25-hydroxyvitamin D levels are associated with increased risk of viral coinfections in wheezing children.	Jartti et al., 2010 [133]	Finland	children hospitalized for wheezing	Lower vitamin D level linked to higher risk of having a viral infection.
Serum vitamin D concentrations and associated severity of acute lower respiratory tract infections in Japanese hospitalized children.	Inamo et al., 2011 [134]	Japan	28 children with ALRI	Vitamin D deficiency (<15 ng/mL) correlates to the need for supplementary oxygen and ventilator management.
Vitamin D intake in young children with acute lower respiratory infection.	Leis et al., 2012 [135]	Canada	children with ALRI vs. controls	Children reporting a lower vitamin D intake were more likely to have ALRI.s
Correlation between serum vitamin D level and severity of community acquired pneumonia in young children.	Ren et al., 2013 [136]	China	103 children with CAP vs. healthy controls	Lower vitamin D levels in severe CAP cases than in mild CAP and controls.
The association between 25-dehydroxy vitamin D and lower respiratory infection in children aged less than 5 years in Imam Reza hospital, Bojnurd, Iran.	Khakshour et al., 2015 [137]	Iran	90 children hospitalized either for acute LRTI or for other reasons	Not significantly different vitamin D levels between the two groups.
Vitamin D Levels Are Unrelated to the Severity of Respiratory Syncytial Virus Bronchiolitis Among Hospitalized Infants.	Beigelman et al., 2015 [138]	USA	Children hospitalized with bronchiolitis	Similar duration of hospitalization and severity of the disease in deficient and non-deficient children.
Association of Vitamin D Deficiency with Acute Lower Respiratory Infection in Toddlers.	Narang et al., 2016 [139]	India	50 children hospitalized with ALRI vs. 50 healthy controls	Lower vitamin D levels in cases than in controls (mean level 20.4 ng/mL).
Serum 25-Hydroxyvitamin D Was Not Associated with Influenza Virus Infection in Children and Adults in Hong Kong, 2009–2010.	Xu et al., 2016 [140]	Hong Kong	Over 3000 children and adults	Vitamin D levels not significantly associated with frequency of influenza infections.
Evaluation of serum 25-hydroxy vitamin D levels in children with acute bronchiolitis.	Mahyar et al., 2017 [141]	Iran	57 children with bronchiolitis vs. 57 healthy controls	No significant difference between the 2 groups.
The effect of vitamin D deficency on the severity of bronchiolitis in infants.	Erol et al., 2017 [142]	Turkey	Children with bronchiolitis	Higher incidence of vitamin D deficiency in children with moderate or severe bronchiolitis.
Vitamin D Status at the Time of Hospitalization for Bronchiolitis and Its Association with Disease Severity.	Vo et al., 2018 [143]	USA	Over 1000 children hospitalized with bronchiolitis	Vitamin D deficiency correlates to increased risk of intensive care admission and longer hospital stay.
Association between serum 25-hydroxyvitamin D concentration and pulmonary infection in children.	Li et al., 2018 [144]	China	Children with pneumonia vs. healthy controls	Lower vitamin D levels in the pneumonia group (mean 19 ng/mL), especially in the pneumonia induced sepsis subgroup.

ALRI, acute lower respiratory tract infection; CAP, community-acquired pneumonia; LRTI, lower respiratory tract infection; PICU, pediatric intensive care unit.