An additional measurement of antibody blockade ability uses RBCs as the VLP binding substrate. Previous work has demonstrated that Norwalk virus VLPs hemagglutinate (HA) O+ RBCs, that this interaction can be disrupted by antibodies found in polyclonal serum (hemagglutination inhibition; HAI), and that the HAI titer of serum correlates with antibody blockade of VLP-Bi-HBGA interaction [38, 39, 45] . To determine if these findings could be extended to study GII. 4 