High frequency transmission, asymptomatic shedding, and airborne spread of Streptococcus pyogenes among schoolchildren exposed to scarlet fever: a longitudinal multi-cohort moleculo-epidemiological contact tracing study

Abstract Background Despite recommendations regarding prompt treatment of cases and enhanced hygiene measures, scarlet fever outbreaks increased in England between 2014-2018. Methods We undertook a prospective, intensive contact tracing study in schools with consecutive scarlet fever cases to assess the impact of standard interventions on transmission of Streptococcus pyogenes between cases, classroom contacts, households, and classroom environments over 4 weeks using genome sequencing. Findings Six classes, comprising 12 scarlet fever cases, 17 household contacts, and 278 classroom contacts were recruited. Prevalence of the outbreak strain in asymptomatic classroom contacts was high, increasing from 9.6% in week 1, to 26.9% in week 2, 23.9% in week 3, then 14.3% in week 4. Colonisation with non-outbreak strains was 0 - 7.5%. Genome sequencing showed clonality of isolates within each of six classes, confirming recent transmission accounted for high carriage. Of asymptomatic classroom contacts with S. pyogenes-positive throat swabs who were tested for transmissibility, 6/28 (21%) had positive cough plates and/or hand swabs, of whom three remained S. pyogenes-positive for 3 weeks. Only 1/60 surface swabs taken in 3 classrooms yielded S. pyogenes. In contrast, settle plates placed in elevated locations were S. pyogenes-positive in both classrooms tested. Interpretation S. pyogenes transmission in schools is intense and may occur prior to, or in spite of reported treatment of cases, underlining a need for rapid case management. Despite guideline adherence, heavy shedding of S. pyogenes by small numbers of classroom contacts may perpetuate outbreaks, and airborne transmission has a plausible role in spread. Funding Action Medical Research, UKRI, NIHR