The outbreak of Coronavirus Disease 2019 (COVID-19) has led to a major concern for those who are more vulnerable to infections. Objectives: This study aimed to evaluate the most important risk factors for severe COVID-19 pneumonia. Methods: This retrospective study included information on clinical and epidemiological features of 105 patients with severe COVID-19 pneumonia hospitalized in Tajrish Hospital, Tehran, Iran. Initially, the medical records of the patients were investigated, and an

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... rview was conducted based on a pre-prepared checklist to seek information about symptoms, past medical history, medication history, and behavior before hospitalization. Results: Out of 105 participants, 76 (72.5%) cases were male, and 54 (51.4%) patients were older than 54 years old. The majority of the patients (n=18; 17.1%) had both hypertension and diabetes (n=12; 11.4%). Metformin (n=36; 34.3%) was the most used medication amongst the studied patient. In addition, 24 (22.9%) patients were recreational hookah smokers, and the majority (75%) of them were under the age of 46 years old. Eventually, 19 patients were excluded from the study, of whom 11 individuals had diabetes, and 10 cases were using metformin. Conclusion: Apparently, hookah smoking played a critical role in the spread of COVID-19 in Iran and has made younger people more susceptible. In addition to older age, the immunosuppressive effects of Metformin seem to make diabetic patients with an impaired immune system more vulnerable to severe COVID-19 pneumonia. More studies on the immune system of vulnerable individuals by identifying their differences can help to protect them.