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

Differences in transmission could also be due to differences in vaccine efficacy due to different antigen concentration, adjuvant used, or both factors [30, 56] . A study with equine H3N8 strains documented the crucial role of vaccine dose and adjuvant in protection against heterologous challenge [57] . In our study, the amount of antigen was not standardized between the homologous and the heterologous vaccines because the HE group was vaccinated with a commercial licensed vaccine and information on the antigen content was not available. The improved protection provided by the autogenous vaccine was most likely due to its preparation with the virus strain homologous to the challenge virus. It is well documented that strain-specific antibody is more effective than cross reactive-antibody in conferring protection against flu infection [55] . In our study, pigs that received the homologous vaccine had no detectable virus in nasal secretions, had a robust antibody response to the challenge virus, and had significantly lower macroscopic lung lesion scores compared with the pigs that received the licensed heterologous vaccine. There were no differences between microscopic lesions between vaccinated groups, and histopathology in the NV and HE group was compatible with previous influenza infection. Histopathology for the HO group indicated that although lesions were absent in most of the homologous pigs, a few pigs had minimal bronchiolar lesions that were consistent with recovery from influenza infection. Despite the fact that all RT-PCR results in the HO group were negative, we cannot conclusively rule-out the possibility that infection took place in a few pigs in the HO group. If infection had taken place, infection appeared limited to the lower respiratory tract and virus could not be detected with the methods used in this study.