In our study, the fimbrial serotype shift toward FIM2 mostly likely resulted from emergence of isolates with the prn::del(–292, 1340) mutation, suggesting a possible link between the 2 characteristics, which could provide an adaptive advantage of the isolates to escape population immunity, whether generated by vaccination or by natural infection by FIM3-producing B. pertussis. This evidence concerns the gene CIAO3 and infection.