High frequency transmission, asymptomatic shedding, and airborne spread of Streptococcus pyogenes among schoolchildren exposed to scarlet fever cases

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, observational study in classes with two consecutive scarlet fever cases to assess the impact of standard interventions on transmission of Group A Streptococcus (GAS) in cases, classroom contacts, households, and classroom environment over 4 weeks. Results: Six classes, comprising 12 scarlet fever cases, 17 household contacts, and 278 classroom contacts were recruited. Prevalence of the outbreak GAS 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 just 0 - 7.5%. Genome sequencing showed clonality of isolates within each of the six classes, confirming recent transmission accounted for high carriage. Of 28 classroom contacts with GAS-positive throat swabs, who were tested for transmissibility, 6 (21%) had positive cough plates and/or hand swabs, of whom three remained GAS-positive for 3 weeks. Only 1/60 surface swabs taken in 3 classrooms yielded GAS. In contrast, settle plates were GAS-positive in 2/2 classrooms tested. Conclusion: GAS transmission in schools is intense and may occur prior to, or in spite of, treatment of cases with antibiotics, underlining a need for rapid case management. Despite guideline adherence, heavy shedding of GAS by small numbers of classroom contacts, may perpetuate outbreaks. Airborne transmission is a key route for transmission.