Background:
High quality surveillance data will be required to evaluate future Strep A vaccine introduction, particularly any impacts on strain-specific disease prevalence and herd immunity. To inform future surveillance design, we investigated Strep A density and diversity in a cohort of remote-living children in Western Australia (WA) who experience high rates of Strep A infection.
Methods:
We utilised samples from the Missing Piece Surveillance Study (based in two school sites in the Kimberley, WA) which comprised of cross-sectional screening twice per year and weekly active surveillance over 9 months. Children were assessed for Strep A pharyngitis and impetigo. Throat and skin swabs were collected, Strep A were isolated, and subject to whole genome sequencing to derive emm-types, resistance and virulence profile. speB quantitative PCR was performed directly on swab specimens to determine Strep A density. Densities were compared within clinical sources and inferred to be due to single emm-types.
Results:
Eighty-two Strep A isolates were derived from 470 swab specimens. Sixteen unique emm-types were identified, with 12, 1, 44 and 70 being most prevalent. Six of 16 emm-types were found in pharyngitis, carriage, or skin only. emm-type densities were not stable across clinical sources or within individuals over time, with wide ranges (0.02pg/µL->1000pg/µL) evident in most prevalent types.
Conclusion:
Dynamic transmission between host niches and within schools was evident. In settings where Strep A throat and skin infections are prevalent, sampling pharyngitis cases only would not provide adequate coverage to inform epidemiological burden. Density measures add complexity yet is a secondary metric for routine surveillance.