Antigenic analysis
The viruses were investigated for their antigenic properties by hemagglutination inhibition (HI) assay with ferret antisera against the vaccine strains of the influenza season 2019-2020 and A(H1N1)pdm09 vaccine viruses of the influenza season 2020-2021. HI assays employed turkey and guinea pig red blood cells (the latter for H3N2); viruses were adjusted to 8 hemagglutinating units for analysis. In order to generate ferret antisera, intranasal infection of ferrets was performed with 105 TCID50 of each of the vaccine viruses and blood was sampled 14 days after infection.