The receptor analogs Neu5Ac (A), Neu5Gc (B), Neu5,9Ac2 (C), and Neu5Gc9Ac (D) at indicated concentrations were added to IDV (D/bovine/Oklahoma/660/2013) or ICV (C/Johannesburg/1/66) containing 4 HA units each. Mixtures were incubated for 30 min at room temperature. Aliquotes of turkey RBCs were then added to the mixtures and results were read after 30 min. The data shown represents 4 independent experiments performed in duplicate. Note that “+” sign indicates no hemagglutination, while “−” denotes evident hemagglutination. DMEM serves as a mock negative control to replace the virus solution in the HA assay to determine any non-specific inhibitory effect on hemagglutination caused by indicated receptor analogs.