Background: The USDA influenza A virus in swine national surveillance plan identified an increase in the detection frequency of a group of swine 1A.1.1.3 hemagglutinin (HA) clade viruses. This change was associated with reassortment events that resulted in new neuraminidase (NA) gene pairings. We hypothesized that the new N1 genes improved the transmission efficiency of the virus.

Methods: We assessed the pathogenesis and transmission of four H1 1A.1.1.3 swine strains paired with different NA subtypes and lineages, all of which shared similar internal gene constellations.

Results: There was little variation in the titers of viral nasal shedding across the groups, and the different surface protein pairings had no effect on transmission efficiency. All IAV strains, reflecting both pre- and post-reassortment genetic patterns, were transmitted to na?ve indirect contact pigs.

Conclusions: These data indicate that the combinations of 1A.1.1.3 HA with NA subtypes had little impact on transmission. These findings suggest that the increased detection of the H1 1A.1.1.3 clade in the United States was unlikely directly due to altered replication, transmission, or antigenic drift, but perhaps due to changes in population immunity resulting from differential vaccine use or prior exposure, variations in production practices, or ecological conditions.