Vaccination of influenza a virus decreases transmission rates in pigs

Romagosa, Anna; Allerson, Matt; Gramer, Marie; Joo, Han Soo; Deen, John; Detmer, Susan; Torremorell, Montserrat

Antibodies against the hemagglutinin protein have been correlated with strong immune response to influenza and with a decrease of the likelihood of becoming infected. However, the levels of protective HI titers against virus replication are not easily established due to the continuous antigenic drift of the virus and due to that reciprocal HI titers alone may not guarantee immunity or predict susceptibility [13, 52, 53] . Kyriakis et al. [52] , in an experimental study with pigs vaccinated with four different commercial vaccines and challenged with a heterologous H1N1 field isolate, found that pigs with reciprocal HI antibodies titers ≥ 20 against the field strain were virologicaly protected [52] . Van Reeth et al. determined that for complete virological protection against a heterologous strain, HI titers as high as 160 are required [13] . In those studies the viral protection was referred to individual virus titers from lung lobes at 3 to 4 days post experimental challenge and related to clinical signs, but not to transmission of the virus. In our study, two weeks after the second vaccine and before exposure to seeder pigs, pigs in the HO group had geometric mean HI titers of 295, while the geometric mean in the HE group was 14 against the challenge strain (maximum reciprocal titer was 80). The challenge virus demonstrated low serologic cross-reactivity with the antiserum induced by the heterologous vaccine and only nine of the forty pigs from the HE group had reciprocal HI titers of greater than 40 against the challenge strain before exposure to the seeder pigs. Of those, two pigs became infected and seven remained uninfected for the duration of the study. Therefore, HI titers against the challenge strain in HE pigs were not able to predict the infectious status of the pigs at the end of the study. The low immunogenicity against the challenge virus in the HE pigs was expected and is in agreement with other studies where pigs vaccinated with licensed inactivated vaccines became infected when challenged with heterologous influenza viruses [11, 12, 54, 55] . At the end of our study not all pigs that had seroconverted to the challenge strain shed virus. Eighteen of the 40 pigs in the HE group had positive HI titers against the challenge strain at necropsy, and in 8 of the 18 pigs with HI titers ≥ 40 the virus could not be detected from nasal swabs samples. Lack of detection could be due to protection and lack of virus replication, or due to the amount of virus excretion being below detectable levels.