Background: Despite the past 15 years of heightened awareness of the disease-causing potential of group A streptococci, the possible epidemiological influence of rapid changes in prevalent serotypes has not been fully appreciated.
Methods: We analysed throat cultures collected as part of routine medical care in a semi-closed community of nearly 500 children and adults between January, 1999, and April, 2000. beta-haemolytic streptococci from all positive cultures were characterised by serological grouping, T-agglutination pattern, and serotyping for M protein or opacity factor.
Findings: We saw an increase in the number of symptomatic individuals with pharyngitis beginning in mid-1999. Between July 1 and Dec 31, 1999, 111 (29%) of 378 throat cultures yielded group A streptococci, 102 (92%) of which were serotype M1. Between Jan 1 and Mar 31, 2000, 126 (45%) of 277 throat cultures yielded group A streptococci. Unexpectedly, 106 (84%) of these throat isolates were serotype M6, and only 16 (13%) were M1. 20 (28%) of the 71 individuals with M1 infection subsequently acquired infection with M6.
Interpretation: This rapid and almost complete shift in predominance of group A streptococcal serotype in this community draws attention to the dynamic epidemiology of these organisms. This change has important implications for further understanding the epidemiology of group A streptococcal infections, and for the development and use of a vaccine.