Rapid expansion and international spread of M1UK in the post-pandemic upsurge of Streptococcus pyogenes infections in United Kingdom

England and Wales reported a marked increase in scarlet fever and invasive group A streptococcal infection in 2022, with severe outcomes in children, similar to other countries. Here we report M1UK to be the dominant source of invasive infections in this upsurge and associated with empyema. Compared with ancestral M1global strains, invasive M1UK strains exhibited reduced genomic diversity and fewer mutations in the covRS two-component regulator genes. The emergence of M1UK was dated to 2008. Following a bottleneck coinciding with the COVID-19 pandemic, three emergent M1UK clades underwent rapid national expansion, despite lack of detection in previous years. All M1UK isolates reported in three continents so far have a phylogenetic origin in the UK, with dispersal of the new clades in Europe. While waning immunity may promote streptococcal epidemics, the genetic features of M1UK point to a fitness advantage in pathogenicity, and a striking ability to persist through population bottlenecks.