Association of Vitamin D Deficiency with COVID-19 Susceptibility and Severity in Hospitalized Children: A Case–Control Study

Background: Vitamin D exerts immunomodulatory and anti-inflammatory effects that may influence outcomes of viral respiratory infections, including COVID-19. However, pediatric data remain limited. This study examined the relationship between serum 25-hydroxy-vitamin D [25(OH)D] levels and susceptibility to and severity of COVID-19 among hospitalized children in Iran. Methods: In this retrospective case–control study, 63 children (0–16 years) with RT-PCR-confirmed COVID-19 admitted to Firouzabadi Hospital, Tehran (2019–2021) were compared with 68 age- and sex-matched historical controls hospitalized two years before the pandemic for non-respiratory conditions. Moreover, historical controls were used to avoid pandemic-era confounding due to widespread supplementation and asymptomatic infection. Demographic, clinical, and laboratory data including 25(OH)D, C-reactive protein (CRP), lymphocyte count, and need for pediatric intensive-care-unit (PICU) admission-were also extracted. Statistical analysis employed non-parametric tests with P-value < 0.05 as significant. Results: Vitamin D deficiency (<20 ng/ml) occurred in 61% of cases versus 14% of controls (P<0.001). Mean 25(OH)D levels were lower in cases (18.6±9.6 ng/ml) than controls (43.4±25.3 ng/ml, P<0.001). PICU-admitted patients showed lower vitamin D than non-PICU patients (13.8±6.4 vs 19.7±9.9 ng/ml, P=0.035). Conclusions: Vitamin D deficiency was strongly associated with COVID-19 susceptibility but not with in-hospital severity. Therefore, routine screening and correction of vitamin D deficiency may be a cost-effective preventive strategy for pediatric populations.