Clade-defining mutations in human H1N1 hemagglutinin protein from 2021-2023 have opposing effects on in vitro fitness and antigenic drift

Seasonal influenza viruses frequently acquire mutations that have the potential to alter both virus replication and antigenic profile. Recent seasonal H1N1 viruses have acquired mutations to their hemagglutinin (HA) protein receptor binding site (RBS) and antigenic sites, and have branched into the clades 5a.2a and 5a.2a.1. Both clades demonstrated improved in vitro fitness compared with the parental 5a.2 clade as measured through plaque formation, infectious virus production in human nasal epithelial cells, and receptor binding diversity. Both clades also showed reduced neutralization by serum from healthcare workers vaccinated in the 2022-23 Northern Hemisphere influenza season compared to the vaccine strain. To investigate the phenotypic impact of individual clade-defining mutations, recombinant viruses containing single HA mutations were generated on a 5a.2 genetic background. The 5a.2a mutation Q189E improved plaque formation and virus replication, but was more efficiently neutralized by serum from individuals vaccinated in 2022-23. In contrast, the 5a.2a mutation E224A and both 5a.2a.1 mutations P137S and K142R impaired aspects of in vitro fitness but contributed significantly to antigenic drift. Surprisingly, the E224A mutation and not Q189E caused broader receptor binding diversity seen in clinical isolates of 5a.2a and 5a.2a.1, suggesting that receptor binding diversity alone may not be responsible for the phenotypic effects of the Q189E mutation. These data document an evolutionary trade-off between mutations that improve viral fitness and those that allow for the evasion of existing host immunity. ImportanceSeasonal influenza is a significant contributor to global morbidity and mortality. To better understand how these viruses evolve, recent seasonal H1N1 viruses were characterized and the impact of individual mutational differences between the hemagglutinin proteins of these viruses was assessed. Mutations that contribute to escape from preexisting immunity were deleterious to virus replication in human nasal epithelial cell cultures, requiring the accumulation of additional mutations to compensate for this loss of fitness. This adds to our understanding of how influenza evolution balances replication fitness with antigenic drift.