Rapid expansion and international spread of M1UK in the post-pandemic UK upsurge of Streptococcus pyogenes

Abstract

The UK observed a marked increase in scarlet fever and invasive group A streptococcal infection in 2022 with severe outcomes in children and similar trends worldwide. Here we report lineage M1_UK to be the dominant source of invasive infections in this upsurge. Compared with ancestral M1_global strains, invasive M1_UK strains exhibit reduced genomic diversity and fewer mutations in two-component regulator genes covRS. The emergence of M1_UK is dated to 2008. Following a bottleneck coinciding with the COVID-19 pandemic, three emergent M1_UK clades underwent rapid nationwide expansion, despite lack of detection in previous years. All M1_UK isolates thus-far sequenced globally 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 M1_UK point to a fitness advantage in pathogenicity, and a striking ability to persist through population bottlenecks.