Multisystem Inflammatory Syndrome in Adults or Hemophagocytic Lymphohistiocytosis: A Clinical Conundrum in Fully Vaccinated Adults With Breakthrough COVID-19 Infections.

Hyperinflammatory syndrome with breakthrough coronavirus disease 2019 (COVID-19) infection in a fully vaccinated patient is not a common finding. To the best of our knowledge, this is the first such case of a patient who received the Spikevax/Moderna (elasomeran mRNA-1273) vaccine. The patient exhibited clinical characteristics consistent with both multisystem inflammatory syndrome in adults (MIS-A) and hemophagocytic lymphohistiocytosis (HLH), thus posing a diagnostic challenge. Multi-inflammatory syndrome in COVID-19 patients is frequently seen in the pediatric population, but it is a rare entity in adults especially after receiving COVID-19 vaccination. The pathophysiology of MIS-A is not completely understood yet, but it is believed that this likely occurs due to antibody-mediated immune dysregulation. There is a possibility of enhanced serologic response in patients like ours who are vaccinated and have breakthrough COVID-19 infection, thus paving the way for overwhelming antibody-mediated immune activation. There is a significant overlap between symptoms of MIS-A and other hyperinflammatory syndromes such as HLH; hence, a high degree of clinical suspicion and thorough diagnostic workup is required to explore all differentials. Our case raises concerns regarding the lack of clear algorithms and guidelines to diagnose and manage MIS-A in adults post-COVID-19 vaccination.