Immunodominance is recognized as a key factor in the antigenic drift of seasonal influenza viruses. In the immunodominance model, each individual in a population predominantly responds to a single epitope among the five antigenic epitopes of the viral hemagglutinin (HA), driving escape mutations one at a time, and sequential mutations in multiple individuals who respond to different epitopes eventually generate a drifted strain with mutations in epitopes that are targeted by a majority of the population. A focused antibody response to the Sa epitope in people born between 1965 and 1979 was believed to contribute to a mutation at HA residue 163 and the first antigenic drift of the 2009 pandemic influenza A H1N1 virus. A serine-to-threonine mutation at HA residue 185 in the Sb epitope emerged in 2010 even before the 163 mutation. We show here that a large fraction of the population in 2010-2011 had responses to the Sb epitope, as shown by 47% of tested sera having altered titers to the S185T mutant. Responses to the Sb epitope showed an age-specific trend similar to that found for the response to Sa epitope in these subjects. Together, the focused responses to Sa and Sb epitopes may have driven the first antigenic drift of the 2009 pandemic H1N1 virus.

免疫优势被认为是季节性流感病毒抗原漂移的关键因素。在免疫优势模型中，种群中的每个个体主要对病毒血凝素（HA）的五个抗原表位中的一个表位作出反应，一次驱动一个逃逸突变，最终对多个人做出不同反应的连续突变被大多数人群所针对的表位突变的漂移菌株。据信，在1965年至1979年之间出生的人群中，针对Sa表位的聚焦抗体反应可导致HA残基163发生突变，以及2009年大流行性甲型H1N1流感病毒的首次抗原性漂移。Sb表位的HA残基185处的丝氨酸到苏氨酸突变出现在2010年，甚至在163个突变之前。我们在这里显示，2010-2011年的大部分人口都对Sb表位有反应，如47％的被测血清对S185T突变体的滴度发生了改变。对Sb表位的反应显示出与年龄相关的趋势，类似于在这些受试者中对Sa表位的反应所发现的趋势。总之，对Sa和Sb表位的集中反应可能是2009年大流行H1N1病毒的第一个抗原性漂移。