Scarlet fever's return: what the strep A evidence shows

Scarlet fever, a childhood illness that had faded to a historical footnote, came back sharply in England after 2014, and the group A streptococcus (strep A) bacterium behind it evolved a more toxin-producing lineage over the same years [s1][s2]. That trajectory ended in a dangerous winter: in late 2022 England saw an exceptional, out-of-season surge in invasive strep A infections, with an unusual number of seriously ill children [s3].

The resurgence

Scarlet fever — a sore throat, fever and sandpapery rash caused by strep A, mostly in young children — had been declining for decades when the trend abruptly reversed. A population-based surveillance study found that notification rates in England tripled between 2013 and 2014, rising from 8.2 to 27.2 per 100,000 (rate ratio 3.34, 95 percent confidence interval 3.23 to 3.45), then climbed further to 30.6 per 100,000 in 2015 and 33.2 in 2016 [s1]. The 2016 total of 19,206 cases was the highest annual count since 1967 [s1].

The burden fell on the young. The median age of cases in 2014 was 4 years, and incidence reached 186 per 100,000 among children under 10 [s1]. This was not a paperwork artefact of more testing: hospital admissions for scarlet fever rose by 97 percent between 2013 and 2016, and about 1 in 40 notified cases were admitted for the illness or its complications [s1]. The rise was national, with 620 outbreaks reported in 2016, and it ran well above the roughly four-yearly natural cycle the disease had long shown [s1].

Notably, the resurgence was not driven by a single strain. Typing of throat isolates during the upsurge found a diversity of strep A emm types, with emm3 the commonest at 43 percent, followed by emm12 (15 percent), emm1 (11 percent) and emm4 (9 percent) [s1]. That the surge in scarlet fever cut across strain types, while the parallel rise in invasive disease later concentrated in one, is a clue that more than one thing was happening at once — a general increase in transmission, and, layered on top, the emergence of a nastier lineage [s1][s2].

A changed bacterium

Part of what made the resurgence worrying was a shift in the bacterium itself. Investigators tracking strep A genotypes found that as scarlet fever rose, so did a particular strain type, emm1, and that increase coincided with a jump in invasive disease [s2]. Among throat isolates in northwest London during the spring season, emm1 rose from 5 of 96 isolates (5 percent) in 2014 to 28 of 147 (19 percent) in 2015 and 47 of 144 (33 percent) in 2016, while nationally the invasive emm1 share climbed from 183 of 587 isolates (31 percent) in 2015 to 267 of 637 (42 percent) in 2016 [s2].

Sequencing showed these were not the old emm1 strains but a new, dominant lineage the authors named M1UK, which produced markedly more of the streptococcal pyrogenic exotoxin A — the "scarlet fever" toxin that gives the disease its rash and can drive severe illness [s2]. A more transmissible, more toxigenic strain circulating in a susceptible child population is a plausible engine for both the scarlet fever counts and the invasive cases that shadowed them [s2].

The 2022 wave

The most severe chapter came in late 2022. UK Health Security Agency surveillance recorded an increase in invasive group A streptococcal (iGAS) infections and associated deaths above seasonally expected levels, particularly in children — 772 iGAS notifications in weeks 37 to 48 of 2022 across England [s3]. The clinical pattern was unusual: the share of paediatric iGAS diagnoses coming from lower respiratory tract specimens in children under 15 rose to 28 percent in November 2022, and clinicians were alerted to unusual numbers of children presenting with pulmonary empyema, a pus-filled infection around the lungs [s3].

Invasive strep A is the rare but dangerous end of the same spectrum as scarlet fever, and the 2022 episode is why parents were urged to seek prompt care for a child who seemed to be deteriorating rather than recovering [s3]. Scarlet fever itself is a notifiable disease in England, treated with antibiotics, and remains far commoner than its invasive form — the same recognise-and-treat logic our pieces on impetigo and ear infections describe [s1].

How to read this

Scarlet fever's return is a story of a common bug behaving unusually, not a new or exotic disease, and the tools against it — recognition, antibiotics, and surveillance for the invasive minority — are unchanged. Judging when a feverish child needs urgent review is the hard part for families, a question we take up in the febrile child and the traffic-light system. This article is informational and is not medical advice.