In the present study, adults exhibited a lower correlation (vs. that among young and older children), which indicates that the high seroprevalence among adults might be related to natural infection with B. pertussis. This concept is supported by our observation that some adults had non-IgG anti-PT immunoglobulins (e.g., anti-PT IgM and IgA) (Fig 3). Here, F2 is linked to infection.