Abstract: Hindawi BioMed Research International Volume 2021, Article ID 1676914, 9 pages https://doi.org/10.1155/2021/1676914 Research Article Factors Affecting the Incidence, Progression, and Severity of COVID-19 in Type 1 Diabetes Mellitus Amira S. Ahmed ,1,2 Wejdan S. Alotaibi,3 Maha A. Aldubayan ,1 Ahmad H. Alhowail ,1 Amal H. Al-Najjar ,4 Sridevi Chigurupati ,5 and Rehab M. Elgharabawy 1,6 1 Department of Pharmacology & Toxicology, College of Pharmacy, Qassim University, Qassim, Saudi Arabia Hormones Department, Medical Research and Clinical Studies Institute, National Research Centre, Giza, Egypt 3 College of Pharmacy, Qassim University, Qassim, Saudi Arabia 4 Drug and Poison Information Specialist, Pharmacy Services, Security Forces Hospital, Riyadh, Saudi Arabia 5 Department of Medicinal Chemistry and Pharmacognosy, College of Pharmacy, Qassim University, Buraydah, Saudi Arabia 6 Department of Pharmacology & Toxicology, Faculty of Pharmacy, Tanta University, Tanta, Egypt 2 Correspondence should be addressed to Amira S. Ahmed; dr.amira2007@yahoo.com Received 15 August 2021; Revised 8 October 2021; Accepted 23 October 2021; Published 23 November 2021 Academic Editor: San-Ji Gao Copyright © 2021 Amira S. Ahmed et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Objectives. This study screened for factors affecting coronavirus disease 2019 (COVID-19) incidence in type 1 diabetes mellitus (T1DM) patients, appraised vitamin D’s efficacy in preventing COVID-19, and assessed the effects of clinical characteristics, glycemic status, vitamin D, and hydroxychloroquine administration on COVID-19’s progression and severity in T1DM patients. Methods. This retrospective research on 150 adults was conducted at Security Forces Hospital, Riyadh, KSA. Participants were allocated to three groups (50/group): control, T1DM, and T1DM with COVID-19. Participants’ fasting blood glucose (FBG), glycated hemoglobin (HbA1c), complete blood count, vitamin D, C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), ferritin, lactate dehydrogenase (LDH), prothrombin time, activated partial thromboplastin time, D-dimer, liver and kidney function, and hydroxychloroquine treatment were retrieved and analyzed. Results. The percentages of comorbidities and not taking hydroxychloroquine were significantly higher among T1DM patients with COVID-19 than patients with T1DM only. Mean vitamin D level was significantly lower in T1DM with COVID-19 patients than in the other two groups. Vitamin D showed a significant negative correlation with LDH, CRP, ESR, ferritin, and D-dimer, which was the most reliable predictor of COVID-19 severity in T1DM patients. Conclusion. Comorbidities and vitamin D deficiency are risk factors for COVID-19 in patients with T1DM. Patients who do not take hydroxychloroquine and have higher FBG and HbA1c levels are vulnerable to COVID-19. Vitamin D may be useful for preventing COVID-19 in T1DM patients. Comorbidities, higher FBG and HbA1c levels, not taking hydroxychloroquine, and vitamin D inadequacy elevate COVID-19 progression and severity in patients with T1DM.