Halim Issever, Hakan Kocoglu, Fatih Okay, Bekir Dogan, Mehmet Hursitoglu, Kadriye Kart Yaşar, Melih Filiz, Faruk Karandere, Fatma Eksi, Betul Erismis, and Şemsi Nur Karabela
Objectives: We aimed to study the effect of seasonal influenza (flu) vaccination on the susceptibility to coronavirus disease 2019 (COVID-19). Methods: A total of 203 healthcare workers of a pandemic centre of Istanbul, Turkey, were included in this retrospective study. According to the presence or absence of flu vaccination, participants were divided into group 1 and group 2. A comparison of the rate of COVID-19 was done between these two groups. Also, the mean age and the sex ratio of females/males were evaluated and compared between these two groups. Results: Group 1 participants (n = 65) were older than participants in group 2 (n = 138) (p < 0.05). Despite of this, interestingly, the COVID-19 infection rate was lower in the 1st group (in comparison to the 2nd group) (p < 0.05). Conclusion: Our study results showed that, even if low, the flu vaccination may have a protective effect on the susceptibility to COVID-19 infection. Using this beneficial adjuvant effect of the vaccine may help us in this unpredictable battle with the COVID-19 pandemic. Further studies are needed to confirm this assumption.