To the Editor: Multisystem inflammatory syndrome in children associated with COVID-19 (MIS-C) is an inflammatory complication with recent exposure to SARS-CoV-2. Despite the increasing worldwide incidence, occurrence in siblings cannot be clearly explained. It is thought that there are personal and genetic differences in the etiology. There are genetic features defined for families with severe COVID-19, but this has not yet been possible for MIS-C. Although there are case reports on MIS-C from the same family, only one case report that has MIS-C at the same time has been identified [1]. A 16-y-old girl and her sibling a 4.5-y-old boy, who presented with the complaints of fever, abdominal pain, and rash, one day apart, also had a history of COVID one month ago. There were differences in the treatment; while the girl received pulse steroid + IVIG and inotropic treatment, her brother was treated only with IVIG.

Coexistence of Kawasaki disease (KD) in siblings has been frequently described in the literature [2, 3]. Banday et al. said that infections are considered as one of the triggers, especially in genetically susceptible hosts [2]. The KD that emerged in a short time interval between the siblings, supports this hypothesis. Gorelik noted that KD had a significantly higher incidence in eastern Asia and more common in black and Hispanic populations [4]. These differences between the populations suggest that a genetic risk factor may play a role in the pathogenesis of KD and similarly in MIS-C. Genetic tests could not be performed because they could not be studied in our hospital. Therefore, a relationship could not be established between the genetic tendency and MIS-C association, which constitutes the limitation of the study.

In conclusion, MIS-C can occur simultaneously in siblings with similar characteristics. Detailed genome-wide association studies are required for sibling pairs that develop MIS-C in close temporal proximity and to understand the cause of susceptibility to this inflammatory condition.