Tehran University of Medical Sciences Journal of Pharmaceutical Care 2322-4630 10 1 2022 03 28 11 16 Nafiseh Alizadeh Department of Pharmaceutical Care, Baharloo Hospital, Tehran University of Medical Sciences, Tehran, Iran. Fatemeh Sadat Tabatabaei School of Medicine, Tehran University of Medical Sciences, Tehran, Iran. Mina Borran Department of Internal Medicine, School of Medicine, Shahrekord University of Medical Sciences, Shahrekord, Iran. Mehrnoush Dianatkhah Department of Clinical Pharmacy and Pharmacy Practice, School of Pharmacy and Pharmaceutical Sciences, Isfahan University of Medical Sciences, Isfahan, Iran. Amirali Azimi School of Medicine, Tehran University of Medical Sciences, Tehran, Iran. Seyedeh Nafise Forghani School of Medicine, Tehran University of Medical Sciences, Tehran, Iran. Samaneh Akbarpour Occupational Sleep Research Center, Baharloo Hospital, Tehran University of Medical Sciences, Tehran, Iran. Mahboubeh Alizadeh Department of Infectious Diseases, Baharloo Hospital, Tehran University of Medical Sciences, Tehran, Iran. Sara Torabi School of Medicine, Tehran University of Medical Sciences, Tehran, Iran. Neda Faraji Department of Internal Medicine, Baharloo Hospital, Tehran University of Medical Sciences, Tehran, Iran. 2021 11 12 2022 01 03 Background: Based upon WHO (World Health Organization) Coronavirus Dashboard more than 5 million deaths worldwide have been attributed to the COVID-19 (Coronavirus Disease 2019) caused by the SARS-Cov-2 virus (Severe Acute Respiratory Syndrome Coronavirus) till November 2021. The annual flu vaccination has raised many questions about the vaccine's effects on Covid-19 outcomes. Several possible mechanisms including cross-reactivity and cross-protection have been reported to be responsible for the potential protective effect of the flu vaccine on the COVID-19 infection. This study was performed to evaluate the possible effect of the influenza vaccine on the disease severity, the mortality rate, and the length of hospitalization in COVID-19 patients. Methods: The data of 1300 patients were recorded from May 2020 to October 2020. Patients with a previous history of COVID-19, patients under 18 years old, and patients who did not have accurate information about their influenza vaccination history were excluded. 498 hospitalized unvaccinated COVID-19 patients with typical clinical manifestations and a positive PCR (Polymerase Chain Reaction) test for COVID-19 were included in this observational, cross-sectional study. The participants were divided into two groups (vaccinated and unvaccinated) based on the history of influenza vaccination at the time of admission.Results: The length of hospital stay was lower in the vaccinated compared to the unvaccinated group (p < 0.05). However, there was no significant difference between the mortality rate, the need for ICU (Intensive Care Unit) admission, and the severity of the disease between the two groups (p> 0.05).Conclusion: Since the patients studied in this article did not receive any of the Covid-19 vaccines; Therefore, the effect of influenza vaccination on the clinical course of Covid-19 can be evaluated using the results of this study. A longer length of hospital stay was observed in the unvaccinated patients in our study, which may suggest the possible protective effect of the influenza vaccine against COVID-19.  https://jpc.tums.ac.ir/index.php/jpc/article/view/459