Recent studies, summarized in this report, showed that the attack rate of primary and secondary rheumatic fever was conditioned by the virulence and epidemicity of infecting strains of Group A streptococci and was related to the magnitude of the streptococcal immune response. Rheumatic fever occurred only twice (0.3%) following untreated mild streptococcal pharyngitis in 608 children in a Chicago pediatric clinic. Only 14% of the infections observed were comparable clinically, bacteriologically and immunologically to streptococcal pharyngitis observed in military epidemics. The frequency of recurrences of rheumatic fever was also related to the magnitude of the streptococcal immune response but was considerably greater in patients with rheumatic heart disease than in rheumatic subjects with normal hearts.
Stollerman GH. Factors Determining the Attack Rate of Rheumatic Fever. JAMA. 1961;177(12):823-828. doi:10.1001/jama.1961.03040380001001