A novel reassorted swine H3N2 influenza virus demonstrates an undetected human-to-swine spillover in Latin America and highlights zoonotic risks.

Influenza A virus (FLUAV) affects a wide range of hosts, including humans and animals, posing a threat to public health. In swine, H3N2 subtype is associated with human-to-swine spillovers of seasonal viruses. In Latin America, the molecular and antigenic characteristics of swine FLUAV H3N2, as well as its phylogenetic origin, are poorly understood. Therefore, the objective of this study was to characterize the first swine H3N2 detected in Colombia. The origin and lineage of the virus were estimated through phylogenetic and molecular clock analyses. Antigenic characterization was achieved by comparing the amino acid constitution of the HA with previously reported swine FLUAVs and seasonal vaccine strains using a sequence-based method. In addition to HA and NA, internal genes were also characterized. The results showed that the Colombian H3N2 corresponded to a novel phylogenetic and antigenic swine FLUAV variant that emerged due to an independent reverse zoonotic event, likely occurring in Colombia in the early 2000s. The immunodominant epitope in the virus was predominantly present in antigenic epitope A, which showed the highest amino acid variation. Some mutations that alter the N-Glycosylation of antigenic sites at the HA were detected. Internally, the virus exhibited pandemic configuration. This study provides the first evidence of a novel FLUAV in Colombia and describes its origin, variability, and persistence in geographically restricted populations, highlighting the need for strengthen molecular surveillance of the virus in animal populations.