The effectiveness of the annual influenza vaccine has declined in recent years, especially for the H3N2 component, and is a concern for global public health. A major cause for this lack in effectiveness has been attributed to the egg-based vaccine production process. Substitutions on the hemagglutinin glycoprotein (HA) often arise during virus passaging that change its antigenicity and hence vaccine effectiveness. Here, we structurally characterize the effect of a prevalent substitution, L194P, in egg-passaged H3N2 viruses. X-ray structural analysis reveals that this substitution surprisingly increases the mobility of the 190-helix and neighboring regions in antigenic site B, which forms one side of the receptor binding site (RBS) and is immunodominant in recent human H3N2 viruses. Importantly, the L194P substitution decreases binding and neutralization by an RBS-targeted broadly neutralizing antibody by three orders of magnitude and significantly changes the HA antigenicity as measured by binding of human serum antibodies. The receptor binding mode and specificity are also altered to adapt to avian receptors during egg passaging. Overall, these findings help explain the low effectiveness of the seasonal vaccine against H3N2 viruses, and suggest that alternative approaches should be accelerated for producing influenza vaccines as well as isolating clinical isolates.

近年来，年度流感疫苗的有效性下降，尤其是对于H3N2成分，这已成为全球公共卫生关注的问题。缺乏有效性的主要原因是基于鸡蛋的疫苗生产过程。血凝素糖蛋白（HA）的取代通常在病毒传代过程中发生，从而改变其抗原性并因此改变疫苗效力。在这里，我们在结构上表征了在卵传代的H3N2病毒中普遍存在的取代L194P的作用。X射线结构分析表明，这种取代令人惊讶地增加了抗原性位点B中190螺旋和相邻区域的移动性，抗原性位点B形成了受体结合位点（RBS）的一侧，并且在最近的人类H3N2病毒中具有免疫优势。重要的，L194P取代将针对RBS的广泛中和抗体的结合和中和作用降低了三个数量级，并显着改变了HA抗原性（通过结合人血清抗体进行测量）。在卵传代过程中，受体结合模式和特异性也被改变以适应禽类受体。总体而言，这些发现有助于解释季节性疫苗对抗H3N2病毒的有效性低，并建议应加快生产流感疫苗以及分离临床分离株的替代方法。在卵传代过程中，受体结合模式和特异性也被改变以适应禽类受体。总体而言，这些发现有助于解释季节性疫苗对抗H3N2病毒的有效性低，并建议应加快生产流感疫苗以及分离临床分离株的替代方法。在卵传代过程中，受体结合模式和特异性也被改变以适应禽类受体。总体而言，这些发现有助于解释季节性疫苗对抗H3N2病毒的有效性低，并建议应加快生产流感疫苗以及分离临床分离株的替代方法。