Influenza A virus (IAV) is an important respiratory pathogen of swine in the United States. Whole virus, inactivated vaccines are a common method used to control infection and clinical disease. However, homologous IAV vaccine antibody responses are rarely evaluated in the field and many influenza serological evaluations are lacking. New replacement gilts are an unstudied population in breeding farms and have been shown to affect viral ecology through introducing new IAV or lacking protection against resident viruses. Serological evaluations are uncommon in the current literature and could prove important in evaluating vaccine responses. This study sought to address all of these points by conducting a one-year longitudinal serological evaluation of replacement gilts on twelve different United States farms.

The study was performed by acquiring serum samples from each farm at four time points over the course of a year. The serum samples were evaluated using nucleoprotein enzyme-linked immunosorbent assay and hemagglutination inhibition assay. The hemagglutination inhibition assay was performed using homologous vaccine antigens based on each farm-specific vaccination protocol and six representative viruses, which were selected due to their predominance in the US swine population and representing the major antigenic clades and genetic clusters.

The study found that a large number of gilts were IAV antibody negative when delivered to the breeding farm or at the age when gilts are breed for production. This suggests new replacement gilts might be susceptible to endemic IAV and may help maintain endemic infections on a farm. It was also found that the levels of vaccine-induced antibody were highly variable between farms and overtime on the same farm. This variability in the number of gilts demonstrating homologous vaccine antibody responses suggests gaps in protection may occur which allows for either the maintenance of endemic viruses within a herd or creates a risk of lateral infection with new or emerging IAV from outside sources. This study supports the idea of increased serological surveillance to evaluate vaccine antibody responses that may correlate with efficacy and help determine the magnitude of protection on a farm or reasons why vaccines appear to fail if antibody responses are lacking.