Vaccination of swine with an inactivated porcine parvovirus vaccine in the presence of passive immunity.

A study was conducted to determine whether low hemagglutination inhibiting (HI) titers (1:5) for porcine parvovirus (PPV) block the development of immune response to a PPV vaccine. Pigs with low (1:5), medium (1:10 or 1:20), or high (1:40 or 1:80) titers were obtained by IV injections with various amounts of PPV immune serum. Pigs were inoculated with 1 or 2 doses of vaccine and were monitored for serum HI antibodies to PPV. Pigs with low titers responded to vaccine just as well as did the seronegative pigs. The HI titers of pigs with medium titers did not increase after first vaccination. After the second vaccination, however, their titers increased and were similar to those of pigs with low titers. High titers blocked the response to vaccination. The pigs that received 2 doses of vaccine had higher titers than did those of pigs that received 1 dose of vaccine. The results indicated that low titers, which would be expected in gilts at the time of vaccination, do not interfere with immunization by the inactivated PPV vaccine, and that 2 doses of vaccine may provide better and longer lasting immune response to inactivated PPV vaccine and probably longer lasting immunity against PPV-induced reproductive failure.