Table 2 Key findings on vitamin D deficiency on ARI incidence

Incidence of ARI/pneumonia

Laaksi, 2007 [19]

Healthy

Clinically diagnosed episodes of sinusitis, tonsillitis, otitis, bronchitis, pneumonia, pharyngitis, and laryngitis

1.03 (0.90, 1.18)^a,c; 88.9% (24/27)

–

1.00 (reference); 86.1% (628/729)

–

–

He, 2013 [17]

Healthy

Symptom diary; URTI was deemed present when (i) total symptom score was ≥15 on any two consecutive days and (ii) when a subject positively indicated suffering from a common cold on ≥3 days

NR; 66.7% (12/18)

NR; 39.7% (27/68)

NR; 43.8% (56/128)

NR; 27.3% (3/11)

–

Jovanonich, 2014 [18]

Mixed^b

Laboratory confirmed CAP with chest radiograph

2.57 (1.08, 6.08)^d; NR

–

0.96 (0.35, 2.61)^d; NR

1.03 (0.51, .09)^d; NR

–

Rafiq, 2018 [20]

Mixed^b

Self-reported episodes of cold in the month prior to questionnaire

–

–

–

–

1.00 (0.96, 1.05)^e; N/A

Sabetta, 2010 [24]

Healthy

Self-reported, followed by clinical diagnosis of acute viral RTI, which may or may not be laboratory confirmed

1.00 (reference); 45.0% (81/180, 32 laboratory-confirmed cases)

–

–

0.52 (0.25, 0.84)^a; 16.7% (3/18; 1 laboratory-confirmed case)

–

Berry, 2011 [21]

Supposedly healthy

Self-reported episodes of respiratory infections (influenza, pneumonia, bronchitis, severe cold) in past 3 weeks prior to questionnaire

1.00 (reference)

0.94 (0.7, 1.27)^f

0.74 (0.54, 1.02)^f

75–99.9 nmol/L: 0.66 (0.46, 0.96)^f; NR

≥100 nmol/L: 0.57 (0.34, 0.94)^e; NR

0.93, (0.89, 0.97)^f; N/A

Nanri, 2017 [23]

Supposedly healthy

Self-reported to be clinically diagnosed with influenza, which may or may not be laboratory confirmed

1.00 (reference)^g; 32.8% (59/180)

1.11 (0.74, 1.68)^g; '28.6% (106/303)

0.77 (0.37,1.59)^g; 28.6% (14/49)

–

–

Lee, 2018 [22]

Healthy

Clinical diagnosis of ILI (retrospective retrieval from system)

1.43 (0.77, 2.67)^h; NR

1.55 (0.84, 2.86)^h; NR

1.00 (reference); NR

–

–