Abstract: RESEARCH PAPER Association of Serum Vitamin D level and COVID-19 infection: A Case-control Study Najat Abdrabbo AlYafei1*, Bushra Naaz Fathima Jaleel1, Abdel-Salam G. Abdel-Salam2, Hamda Ali Al-Saadi1, Samya Ahmad Al Abdulla1 ABSTRACT Address for Correspondence: Najat Abdrabbo AlYafei1* 1 Primary Health Care Corporation, Qatar 2 Department of Mathmatics, Statistics and Physics, College of Art and Sciences, Qatar unitversity, Doha, Box. 2713, Qatar Email & ORCID ID: nalyafei@phcc.gov.qa & https://orcid. org/0000-0002-8071-3646 http://doi.org/10.5339/qmj.2022.48 Submitted: 31 May 2022 Accepted: 22 August 2022 © 2022 AlYafei, Jaleel, Abdel-Salam, Al-Saadi, Al Abdulla, licensee HBKU Press. This is an open access article distributed under the terms of the Creative Commons Attribution license CC BY 4.0, which permits unrestricted use, distribution and reproduction in any medium, provided the original work is properly cited. Cite this article as: AlYafei NA, Jaleel BNF, Abdel-Salam ASG, Al-Saadi HA, Al Abdulla SA. Association of Serum Vitamin D level and COVID-19 infection: A Case-control Study, Qatar Medical Journal 2022(4):48 http://doi. org/10.5339/qmj.2022.48 Background: Vitamin D is considered a potent modulator of the immune system, albeit its role in COVID-19 infection is a matter of debate. The present study aimed to estimate the association between serum vitamin D levels and COVID-19 among people in Qatar. Methods: This case-control study, approved by the Institutional Review Board of Primary Health Care Corporation (PHCC) Qatar, retrospectively evaluated the principal public healthcare sector population data repository retrieved from the cloud-based Electronic Health Record (EHR) software-Cerner, during April 2020–2021. The health records of all adult patients aged .18 years who had undergone the reverse transcription-polymerase chain reaction (RT-PCR) test and whose medical records had documented serum 25-hydroxyvitamin D [25 (OH)D] levels were analyzed. Results: A total of 924,173 EHRs were extracted, of which 62,451 EHR comprised of 16,446 (26.3%) COVID-19 patients and 46,005 (73.7%) negativecontrol group patients met the inclusion criteria. The odds ratio (OR) among different categories of vitamin D deficiency (VDD) revealed that people with mild/ moderate VDD were 1.18 times (95% CI 1.126– 1.258) and those with severe VDD were 1.90 times (95% CI 1.116–1.251) more likely to have COVID-19 infection when compared to the people with optimal serum vitamin D level. On applying multiple logistic regression, the odds of having COVID-19 infection were found to be 1.27 times (95% CI 1.184–1.371) higher among those with mild/moderate VDD and 1.32 times (95% CI 1.206–1.405) higher among those with severe VDD when compared to people with optimal vitamin D level ( p , 0.001). QATAR MEDICAL JOURNAL 1 VOL. 2022 / ART. 48 Association of Serum Vitamin D level and COVID-19 infection: A Case-control Study AlYafei et al. Conclusion: Our findings demonstrated a significant association between the suboptimal serum vitamin D level and COVID-19 infection. Further studies are required to determine the effects of VDD on the severity and outcomes of COVID-19 infections. undertaken to estimate the association between serum vitamin D levels and COVID-19 among people who undertook reverse transcription-polymerase chain reaction (RT-PCR) test for COVID-19 infection during the study period of April 2020–2021. Keywords: COVID-19, SARS-CoV-2, RT-PCR,..