F2 and pertussis: In the present study, the antibody titers against pertussis antigens, DT and TT tended to increase after booster vaccination even in the Groups 2 and 3, in which PBS or Td vaccine was administered as the booster vaccination, although the anti-PT and anti-FHA IgG titers against pertussis antigens in the Groups 2 and 3 were lower than those in the Groups 4 and 5.