Abstract Background Antibodies directed against haemagglutinin. measured by the haemagglutination inhibition (HI) assay are essential to protective immunity against influenza infection. An HI titre of 1:40 is generally accepted to correspond to a 50% reduction in the risk of contracting influenza in a susceptible population. but limited attempts have been made to further quantify the ... https://unitedssports.shop/product-category/goalie-goalie-mask-youth/