Age influences the emm type distribution of pediatric group A streptococcal pharyngeal isolates

Background: emm types 12, 1, 28, 3, 4, 2 and 6 (in that order) are the types most commonly associated with uncomplicated group A streptococcal (GAS) pharyngitis in the United States, together accounting for ∼78% of isolates. Objective: To determine whether the distribution of common pharyngeal group A streptococcal GAS types differs at various ages throughout childhood. Study Design: We emm typed 3356 GAS isolates collected from the United States and Canada during 3 streptococcal seasons (2000-2003). Variations in prevalence by age for the 7 most prevalent emm types and the "uncommon" category (all types accounting for <5% of the total number of isolates) were analyzed and assessed for significance by χ². Results: The proportion of uncommon isolates increased significantly with increasing age from 18% in group 1 to 37% in group 4 (P = 0.001). We found a significant decrease in the proportion of the common pharyngeal emm types, specifically emm 12 and emm 4 type isolates, with increasing age (P = 0.001 and P = 0.003, respectively); there was no significant decline in the prevalence of other common pharyngeal types (emm 1, 2, 3, 6 and 28) with increasing age. Conclusion: Age-related changes in emm type distribution of pharyngeal GAS are present in childhood; these changes may reflect acquisition of immunity to more common types as a consequence of exposure early in life, but this remains to be demonstrated.