An outbreak of genomically clustered group A Streptococcus in a school community, Victoria, 2023

In 2023, a global increase in invasive group A streptococcal disease (GAS) caused serious illness and death. The North Eastern Public Health Unit (NEPHU) in Victoria, Australia, identified an outbreak of GAS in a school in July 2023; we investigated to describe the epidemiology, to identify risk factors for severe disease, and to implement control measures. We defined confirmed cases as those with laboratory, clinical and epidemiological evidence. Probable cases had clinical evidence with an epidemiological link to a confirmed case but no laboratory evidence. Absentee data were collated for students preceding the onset of the index case. We developed an online, self-administered survey for all students to identify contacts with clinically compatible illnesses. Cultures were genomically sequenced. We identified 11 cases (five confirmed, six probable) among the cohort of 38 (a 29% attack rate), with onset dates from 24 July to 27 August 2023. The index case had a severe invasive GAS infection requiring hospitalisation; eight of 11 cases (73%) reported sore throat and one reported scarlet fever as their primary syndrome. Fifteen of 28 students (54%) were absent from the school during the period preceding the index case's onset. We monitored for two incubation periods following the onset of the last case to 5 September 2023 (six days), with no further cases identified. Isolates all typed as emm1, with genomic clustering consistent with localised transmission. This outbreak demonstrated group A Streptococcus (GAS) transmissibility in a school with multiple clinical manifestations.