Background. Clade 6B H1N1 pdm09 influenza viruses cause substantial morbidity and mortality worldwide. Human antibody profiles elicited upon vaccination against the clade 6B virus are largely unclear before viral emergence. Methods. Healthy volunteers, including children aged 3-8 years, adolescents aged 9-17 years, and adults, were enrolled before the clade 6B H1N1 outbreak and received the 2013-2014 inactivated influenza vaccine. We determined antibody responses before and after vaccination. Vaccine-induced plasmablast-derived antibodies were tested against H1N1 pdm09 reference and clade 6B viruses. Results. The majority of the subjects generated robust hemagglutination inhibition and neutralizing antibody responses upon vaccination across the different age groups. Nevertheless, a subset of young adults preferentially produced antibodies that failed to neutralize clade 6B viruses that emerged and circulated in 2014-2016. The hemagglutinin K163Q change at the Sa antigenic site, one of the substitutions that define clade 6B viruses, was responsible for resistance to neutralization by both postvaccination sera and vaccine-induced plasmablast-derived antibodies. Conclusions. Vaccine-induced antibody immunity is compromised by the antigenic change of H1N1 pdm09 virus in a subset of adults, and this may warrant the incorporation of human serology in the antigenic characterization of virus and vaccine strain selection.
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