M1UK Lineage Streptococcus pyogenes Identified in Korean Adolescents with Toxic Shock Syndrome

Dear Editor:

Following the coronavirus disease 2019 (COVID-19) pandemic, invasive group A streptococcal (iGAS) infections have resurged globally, particularly in children, with the hypervirulent M1UK lineage of Streptococcus pyogenes closely associated with this increase [1,2]. However, the epidemiology of M1UK in Korea remains poorly characterized, and clinical reports are extremely limited.

We encountered two fatal cases of M1UK streptococcal toxic shock syndrome in Korean adolescents in 2025. In both patients, blood cultures yielded S. pyogenes susceptible to all tested antibiotics. Whole-genome sequencing confirmed emm1/ST28 strains belonging to M1UK lineage, based on characteristic single-nucleotide polymorphisms and lineage-specific genomic markers [3]. Neither patient had a recent history of international travel, suggesting community-acquired life-threatening iGAS infections.

The first case occurred in April 2025 in a healthy 14-year-old boy who presented with cardiopulmonary arrest. Four days earlier, he had influenza B infection. He subsequently developed worsening respiratory symptoms and sudden loss of consciousness at home. Imaging revealed bilateral consolidations and necrotizing pneumonia with a large pleural effusion (Fig. 1A). Despite intensive care, he remained in refractory shock with severe metabolic acidosis and died approximately nine hours after arrival. The second case occurred in September 2025 in a 15-year-old boy with well-controlled epilepsy. He presented with progressive pain and swelling of the left lower limb and a diffuse rash following a febrile illness. Laboratory evaluation revealed acute kidney injury, hepatic dysfunction, and disseminated intravascular coagulation. Several bullae, skin necrosis, and purpura fulminans developed (Fig. 1B), and he died on hospital day 3 despite aggressive management.

Figure 1. (A) Chest computed tomography of Case 1. Chest computed tomography revealing necrotizing consolidation in the left lower lung field, along with multifocal consolidations and centrilobular nodules in both lungs. (B) Skin lesions in Case 2. Development of bullae with hemorrhagic skin necrosis on the left lower leg on hospital day 2.

Recent surveillance indicate that complicated pneumonia has become a major contributor to pediatric iGAS-associated mortality [2,3,4,5,6,7]. This resurgence has coincided with the re-emergence of respiratory viral infections, with influenza as a particularly important predisposing factor for pediatric iGAS through its effects on bacterial invasion and host immune responses [2]. The expansion of hypervirulent emm1 lineages, including M1UK, may further contribute to severe clinical outcomes [1,6,7]. M1UK has a fitness advantage and has become dominant among invasive emm1 isolates in several countries [1,5,6]. In the UK, emm1 accounted for 16–28% of iGAS cases before early 2020 and over 50% by early 2023, with nearly all emm1 isolates identified as M1UK [6]. Li et al. reported a 7-day case fatality rate of 12.8% among children under 15 years with M1UK iGAS [8]. Most deaths occurred rapidly, with 56.3% before diagnostic sampling and 95.6% within one day after sampling. These findings underscore the fulminant nature of severe iGAS and the importance of early recognition and aggressive management when GAS is strongly suspected.

Although iGAS is not a notifiable disease in Korea and its true incidence remains unclear, a pediatric study from 2012 to 2019 found that emm1 was the most prevalent among invasive isolates [9], and a recent policy research report identified two GAS isolates from 2020 and 2024 as M1UK [10]. Together with our cases, these findings suggest that M1UK is circulating in Korea, underscoring the need for enhanced molecular surveillance to define its spread and impact.