Fatal Post-Influenza Streptococcal Toxic Shock Syndrome Caused by M1UK Streptococcus pyogenes in South Korea: A Case Report

Streptococcus pyogenes (S. pyogenes) M1UK is an emergent emm1 lineage associated with increased production of the superantigen SpeA and has spread internationally. We report fatal post-influenza streptococcal toxic shock syndrome (STSS) caused by M1UK S. pyogenes in a 49-year-old man in South Korea. Following a diagnosis of influenza A, the patient developed rapidly progressive shock and respiratory failure and died approximately 22 h after hospital presentation despite antimicrobial therapy and intensive organ support. S. pyogenes was isolated from blood cultures. Short-read whole-genome sequencing identified the isolate as emm1 and demonstrated all 27 M1UK lineage-defining single-nucleotide polymorphisms. The isolate carried speA, and no acquired macrolide–lincosamide resistance determinants were detected. Comparative core-genome phylogenetic analysis placed the isolate within the international M1UK lineage, whereas Korean pre-COVID-19 emm1 comparator genomes belonged to the M1global lineage. This case documents a rapidly fatal post-influenza STSS associated with genomically confirmed M1UK S. pyogenes. It highlights the need to consider STSS and adjunctive antitoxin therapy in patients with shock after influenza, even when the presentation is pulmonary, and supports continued genomic surveillance of invasive S. pyogenes in Korea.