Increasing susceptibility in vaccinated children and adolescents has been linked with waning immunity, especially a decline in circulating antibodies after immunization with aP vaccines.3–5 Evasion of antibody responses induced by aP vaccines may also have resulted through emergence of B. pertussis strains with mutations or deletions in key antigens, especially PT or PRN.6–10 However, these aspects alone are not enough to explain the rising incidence of pertussis cases in infants. Here, CIAO3 is linked to pertussis.