Genetic and Antigenic Characterization of an Expanding H3 Influenza A Virus Clade in U.S. Swine Visualized by Nextstrain

Genetically distinct clades of influenza A virus (IAV) in swine undermine efforts to control the disease. Swine producers commonly use vaccines, and vaccine strains are selected by identifying the most common hemagglutinin (HA) gene from viruses detected in a farm or a region. In 2019, we identified an increase in the detection frequency of an H3 phylogenetic clade, C-IVA, which was previously circulating at much lower levels in U.S. swine. Our study identified genetic and antigenic factors contributing to its resurgence by linking comprehensive phylodynamic analyses with empirical wet-lab experiments and visualized these evolutionary analyses in a Nextstrain implementation. The contemporary C-IVA HA genes did not demonstrate an increase in genetic diversity or significant antigenic changes. N2 genes did demonstrate antigenic diversity, and the expanding C-IVA clade acquired a nucleoprotein (NP) gene segment via reassortment. Virus phenotype and vaccination targeting prior dominant HA clades likely contributed to the clade's success.