Up to date, some clinical trial have evaluated the efficacy and safety of Vitamin D supplementation in children with RRI, with no conclusive information.

Therefore, the investigators designed a prospective, single-blind, clinical trial to evaluate whether oral supplementation with vitamin D from October to April reduces the global health burden of recurrent respiratory tract infections in a primary care setting.

The primary outcome was evaluated if Vitamin D supplementation during autumn and winter can reduce the number of respiratory tract infections in children diagnosed with recurrent respiratory tract infections the seasons before.

Secondary objectives was the assessment of Vitamin D supplementation benefits on global socioeconomic burden of recurrent respiratory tract infections in a primary care setting, according to number of visits to the primary care paediatrician and use of antibiotics due to respiratory tract infections.

The University study personnel randomly allocated patients to receive, from October to March, a Vitamin D oral supplementation (400 UI/die up to 12 month or 600 UI/die beyond 1 year) or not.

Randomization was performed using a free web-based service that offers random assignment; patients were randomized considering gender and age class (up to 12 months; beyond 1 years). The primary care practitioner was blinded regarding the study group allocation.

Inclusion criteria were: a) patients diagnosed the seasons before with: ≥ 6 respiratory infections per annum OR ≥ 1 respiratory infections per month involving the upper airways from September to April OR ≥ 3 respiratory infections per annum involving the lower airways. b) patients with no findings suggestive of an immunodeficiency on history and physical examination.

From the recruitment, parents recorded number and type of diagnosed respiratory tract infections, number of ambulatory visits, use of antibiotics and duration of symptoms in a structured diary.

Upper Respiratory Tract Infections (URTIs) comprehends diagnosis of acute otitis media, acute rhinosinusitis and acute pharyngotonsillitis. Lower Respiratory Tract Infections (LRTIs) comprehend diagnosis of bronchiolitis and pneumonia.

All parents had a close telephone contact to help them in the diary compilation. Adverse events related to the protocol were monitored. Missing data were recovered through the information system of the primary care practitioner.

Quantitative data were expressed as mean and standard deviation. To compare the mean between the two groups, Student's t-test for unpaired was used. The frequency of each visits to the pediatricians and use of antibiotics in the two group was calculated and expressed as a percentage. The χ² test was used to compare the percentages between the two groups. For all tests, P-values <0.05 were considered significant.

Since Italian National Health System covers all costs for pediatrician visits and antibiotics, cost assessment was made analysing the costs regarding the frequency of each parameters in treated and nontreated group. We considered as direct cost National Health System: Medical examination (20,66 euros for each examination, as provided by the Italian Ministry of Health) and use of Antibiotics (the cost was as in the list of the National Drug Authority).

Inclusion Criteria:

• patients diagnosed the seasons before with: ≥ 6 respiratory infections per annum
• ≥ 1 respiratory infections per month involving the upper airways from September to April
• ≥ 3 respiratory infections per annum involving the lower airways

Exclusion Criteria:

• patients with findings suggestive of an immunodeficiency on history and physical examination