Hemagglutination Inhibition (HAI) assay

The HAI assay is the reference method for assessing anti‑hemagglutinin (HA) antibody responses following influenza vaccination or natural infection and is applicable to viruses with hemagglutinating activity, primarily influenza A, B, and C viruses, and less commonly paramyxoviruses (e.g., parainfluenza and mumps viruses) and certain alphaviruses (e.g., chikungunya virus). The assay measures the ability of anti‑HA antibodies to inhibit virus‑induced red blood cell agglutination following incubation of RDE‑treated, serially diluted serum samples with a standardized viral antigen. The HAI titer is defined as the reciprocal of the highest serum dilution that completely inhibits agglutination.

Recognized by the European Medicines Agency (EMA) as a key assay for influenza vaccine evaluation, it is considered the gold-standard correlate of protection and has been established, validated, and routinely performed at VisMederi since the company’s inception (Haemagglutination inhibition and virus microneutralisation serology assays: use of harmonised protocols and biological standards in seasonal influenza serology testing and their impact on inter-laboratory variation and assay correlation: A FLUCOP collaborative study; Effect of Repeated Freeze-Thaw Cycles on Influenza Virus Antibodies; Comparison of influenza-specific neutralizing antibody titers determined using different assay readouts and hemagglutination inhibition titers: good correlation but poor agreement ; Impact of erythrocyte species on assays for influenza serology ).