Objective To analyze and reveal the genetic evolution and variation of influenza viruses in cases of co-infection in Guangdong Province.

Methods Throat swab samples were collected from four cases of H1N1pdm and H3N2 co-infection for viral isolation. The isolated strains were subjected to antigen analysis and neuraminidase inhibitor susceptibility test. High-throughput sequencing was used to detect the sequences of strains in three throat swab samples and one virus strain, and then genetic variations were analyzed.

Results Four influenza viruses were isolated with one strain of H1N1pdm and three of H3N2 subtype, and all of them were genetically similar to the vaccine strain in 2022-2023. The HA genes of H1N1pdm and H3N2 strains belonged to clade 6B.1A.5a.2a and 2a.3a.1, respectively. The isolated strains belonged to the same clade as the strains prevalent in Guangdong during the same period. No drug-resistant variations were detected in N1 or N2 gene, and the isolated strains were sensitive to oseltamivir and zanamivir.

Conclusions H1pdm subtype had stronger replication ability than H3 subtype in the influenza viruses isolated from co-infected cases. H1N1pdm and H3N2 subtype influenza viruses were genetically similar to the strains circulating in Guangdong at the same time. The isolated H1N1pdm and H3N2 strains were sensitive to both oseltamivir and zanamivir, indicating that they could continue to be used in the treatment of influenza virus infections caused by one or two genotypes.