Introduction:
Vitamin D (VD) deficiency has been linked to recurrent respiratory infections in recent years. However, the impact of VD supplementation on recurrent viral-induced wheezing in early childhood remains unclear. Thus, we aimed to investigate the effect of daily VD supplementation on recurrent viral-induced wheezing episodes in young children.

Material and methods:
This observational prospective cohort study involved 60 patients with recurrent viral-induced wheezing and 30 healthy children, all aged 6 months to 3 years. Patients with recurrent wheezing were assigned to receive 1000 IU of vitamin D3 daily (VD group, n = 30) or no VD supplements (control group, n = 30) for a total of 12 months. Serum 25-hydroxyvitamin D (25(OH)D) concentrations were measured in all study subjects at baseline and at the 12-month follow-up visit using electrochemiluminescence immunoassay.

Results:
VD deficiency was detected in 75% of children with recurrent wheezing. VD levels below 20 ng/ml were linked to 4-fold increased odds of developing recurrent wheezing (OR = 4.35; 95% CI: 2.75-6.86; р < 0.001). At 12 months, the median number of wheezing episodes in the control group was twice as much than the VD group (2.0 [1.0; 3.0] vs. 1.0 [0.0; 2.0], respectively, p < 0.001). VD supplementation increased the median serum 25(OH)D concentration to 25.11 (12.14; 42.47) ng/ml. The vast majority (71%) of VD-insufficient recurrent wheezers attained sufficiency over 12 months. Among VD-deficient children, 54.5% remained deficient, 31.9% achieved insufficiency, and 13.6% achieved sufficient VD status.

Conclusions:
VD supplementation of 1000 IU/day reduces the incidence of recurrent early wheezing. The supplemental dose of vitamin D3 was not optimal to achieve sufficient VD status in patients with baseline VD deficiency.