Cross-reactive H1N1 antibody responses to a live attenuated influenza vaccine in children: implication for selection of vaccine strains.

Cross-reactive antibody responses of 3 trivalent, live attenuated intranasal influenza vaccine (FluMist) formulations containing 3 different H1N1 strains (A/Texas/36/91, A/Shenzhen/227/95, and A/Beijing/262/95) were evaluated in initially seronegative children. FluMist containing A/Shenzhen/227/95 was more likely to induce cross-reactive hemagglutination inhibition (HAI) antibody against A/Texas/36/91 than against A/Beijing/262/95, and FluMist containing A/Beijing/262/95 induced low levels of cross-reactive HAI antibody against A/Shenzhen/227/95 and A/New Caledonia/20/99. The observed differences in HAI cross-reactivity seem to be partly related to the number of amino acid (aa) differences on the hemagglutinin 1 domain (328 aa residues) rather than the hemagglutinin protein (550 aa residues).