Goldilocks zone of preexisting immunity: too little or too much suppresses diverse antibody responses against influenza viruses.

Preexisting immunity against influenza viruses has long been known to regulate the magnitude and specificity of vaccine-induced humoral immunity. In this manuscript by Lu et al., the authors highlight how varying levels of preexisting antibodies against a single site on hemagglutinin impact vaccine-induced antibody responses. This commentary discusses the essential findings and implications of the study, emphasizing the importance of understanding how preexisting antibodies suppress the diversification of humoral immunity and how next generation vaccine strategies can overcome preexisting immunity to generate immunity against ever-evolving influenza viruses.