Abstract: European Journal of Pediatrics (2021) 180:2699–2705 https://doi.org/10.1007/s00431-021-04030-1 ORIGINAL ARTICLE The association between vitamin D levels and the clinical severity and inflammation markers in pediatric COVID-19 patients: single-center experience from a pandemic hospital Elvan Bayramoğlu 1 & Gülşen Akkoç 2 & Ayşe Ağbaş 3 Hatice Nilgün Selçuk Duru 4 & Murat Elevli 4 & Özlem Akgün 4 & Kamer Yurdakul 5 & Received: 22 November 2020 / Revised: 8 March 2021 / Accepted: 12 March 2021 / Published online: 31 March 2021 # Springer-Verlag GmbH Germany, part of Springer Nature 2021 Abstract Vitamin D has an immunomodulating property that regulates the inflammatory response. In this study, the aim was to evaluate the relationship between vitamin D levels and clinical severity and inflammation markers in children and adolescents with COVID-19. The clinical and laboratory records of 103 pediatric cases with COVID-19, whose vitamin D levels had been measured, were retrospectively reviewed. The cases were divided into groups according to their clinical severity (asymptomatic, mild, and moderate-to-severe) and vitamin D levels. The moderate-to-severe clinical group had significantly higher inflammation markers (CRP, procalcitonin, fibrinogen, Ddimer) and a lower lymphocyte count compared to both the mild and asymptomatic groups. The 25 OH vitamin D levels were also significantly lower (p < 0.001), and the ratio of vitamin D deficiency was 70.6% in the moderate-to-severe group. The vitamin D–deficient group had a significantly higher age and fibrinogen levels while also having a lower lymphocyte count compared to the insufficient and normal groups. The 25 OH vitamin D level was correlated positively with the lymphocyte count (r = 0.375, p = <0.001), and negatively with age (r = −0.496, p = <0.001), CRP (r = −0.309, p = 0.002) and fibrinogen levels (r = −0.381, p = <0.001). In a logistic regression analysis, vitamin D deficiency, D-dimer, and fibrinogen levels on admission were independent predictors of severe clinical course. Conclusion: This study revealed an association between vitamin D deficiency and clinical severity, in addition to inflammation markers in pediatric COVID-19 cases. Prophylactic vitamin D supplementation may be considered, especially in the adolescent age group. What is Known: • • The pathology of COVID-19 involves a complex interaction between the SARS-CoV-2 and the immune system. Hyperinflammation/cytokine storm is held responsible for the severity of the disease. • Vitamin D has multiple roles in the immune system that can modulate the body reaction to an infection. What is New: • • Clinically more severe group had significantly lower vit D levels and significantly higher inflammation markers. • Lower 25 OH vit D levels were associated with higher inflammation markers, suggesting an important role of vitamin D in the clinical course of COVID-19 in children and adolescents probably by regulating the systemic inflammatory response. Keywords COVID-19 . Vitamin D . Inflammation . Children . Pediatric Communicated by Peter de Winter * Elvan Bayramoğlu elvanbayramoglu@gmail.com Gülşen Akkoç agulsenakkoc@gmail.com Ayşe Ağbaş yurtayse@hotmail.com Özlem Akgün drozlemakgun@hotmail.com Kamer Yurdakul kamer_dogan@yahoo.com Hatice Nilgün Selçuk Duru nilgunduru@yahoo.com Murat Elevli muratelevli@gmail.com Extended author information available on the last page of the article 2700 Eur J Pediatr (2021)..