Patients presenting high fever with lymphadenopathy after COVID-19 vaccination were diagnosed with hemophagocytic lymphohistiocytosis

Background The coronavirus disease 2019 (COVID-19) pandemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is still continuing worldwide. Currently, two mRNA-based vaccines and two DNA vaccines using an adenovirus vector are representative vaccines. Since the SARS-CoV-2 vaccines began to be administered, a significant decrease in new infections and COVID-19-associated death has been reported. However, various adverse events from mild symptoms to death have also been described after vaccination. Case description Patients with high fever and lymphadenopathy who are diagnosed with hemophagocytic lymphohistiocytosis (HLH) after COVID-19 vaccination are very rare, and there is no standard management guideline for these patients thus far. Herein, we described two cases of HLH after the administration of an mRNA-based vaccine and adenovirus vector vaccine. Discussion HLH is a life-threatening hyperinflammatory syndrome that occurs due to persistent stimulation of lymphocytes and histiocytes in various underlying conditions at all ages. Although the exact mechanisms and risk factors of COVID-19 vaccination-related HLH are still unknown, vigorous immune stimulation may trigger a huge cytokine storm, rarely resulting in HLH. It is important to note that early suspicion by clinicians can lower the mortality rate.