Background
England is experiencing scarlet fever activity unprecedented in modern times. In 2016, England’s scarlet fever seasonal rise coincided with an unexpected elevation in invasive Streptococcus pyogenes infections. We describe the molecular-epidemiological investigation of these events and emergence of a new emm1 lineage.

Methods
We analysed changes in S. pyogenes emm-genotypes, and notifications of scarlet fever and invasive disease 2014-2016 using regional (North-West London) and national (England and Wales) data. We analysed genomes of 135 non-invasive and 552 invasive emm1 isolates from 2009-2016, and compared 2800 global emm1 sequences. Expression of Streptococcal pyrogenic exotoxin (Spe)A by sequenced non-invasive emm1 isolates was quantified.

Findings
Coincident with national increases in scarlet fever and invasive disease notifications, emm1 S. pyogenes increased significantly, from 19% (28/147) of upper respiratory tract isolates in MarchMay 2015, to 32·6% (47/144) in the same period 2016 in North-West London (χ2 (1df)=7·024, p=0·008), and from 31% (183/587) of invasive isolates in March-May 2015, to 41·9% (267/637)in the same period 2016 nationally (χ2 (1df)=15·16, p=0·0001). Sequences of emm1 isolates from 2009-2016 demonstrated emergence of a new emm1 lineage (M1UK), with overlap of pharyngitis, scarlet fever, and invasive M1UK strains, that could be genotypically distinguished from pandemic emm1 isolates (M1global). Compared with M1global, median expression of SpeA increased 9-fold in M1UK isolates (M1global, median=20·9 ng/ml, IQ range=21.3; (M1UK, median=190·2 ng/ml, IQ
3 range 31.5; Mann-Whitney p<0·001). M1UK expanded nationally to represent 84% (252/299) of all emm1 genomes in 2016; phylogenetic analysis of published datasets identified single M1UK isolates in Denmark and USA.