Comparison of hemagglutination inhibition and hemagglutinin pseudovirus neutralization titers in relation to protection against influenza in a mouse model

The hemagglutination inhibition (HI) test has long been used as a standard measure of antibody response for inactivated influenza vaccines. However, the HI test has limitations, such as insensitivity when using some H3N2 virus strains and failure to detect neutralizing antibodies that target regions distant from the receptor binding site. We therefore examined a hemagglutinin pseudovirus neutralization (PVN) test as a possible supplement or alternative to the HI test. We evaluated the association of HI or PVN titers with protection against influenza infection in mice based on morbidity (where the illness was defined as 25% body weight loss). We assessed this relationship using dose-response models incorporating HI or PVN titers as a variable. The morbidity was correlated with the pre-exposure titers and such a correlation was well described by a modified dose-response model. The mathematical modeling suggests PVN titers consistently show a stronger association with in vivo protection as compared to HI titers in mice. Given our findings, the PVN test warrants further investigation as a tool for evaluating antibody responses to influenza vaccines containing hemagglutinin. The resulting models may also be useful for analyzing human clinical data to identify potentially protective antibody titers against influenza illness.