F2 and infection: For pertussis, the number of years since vaccination was significantly associated with decreased odds for a seropositive anti-PT titer (≥5 IU/mL) (OR: 0.8; 0.7–0.9; p = 0.002), past infection or vaccination (anti-PT ≥ 50 IU/mL) (OR: 0.7; 0.4–0.9; p = 0.03).