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 VirusInfection 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.