Hemagglutination Inhibition and Alternate Serologic Responses Following Influenza A(H3N2) Virus Infection

Although the hemagglutination inhibition (HAI) titer remains the gold standard correlate of protection against influenza, it does not fully capture the broader antibody responses that contribute to immunity.

We analyzed immune responses in paired pre-infection and convalescent sera from 306 RT-PCR-confirmed A(H3N2) infections from two household studies (2014-2018) in Managua, Nicaragua. Antibody responses were measured by HAI and enzyme-linked immunosorbent assays (ELISAs) against full-length hemagglutinin (HA), the HA stalk, and neuraminidase (NA). Participants were classified as HAI responders (≥ 4-fold HAI rise), alternate responders (no HAI rise but ≥ 4-fold boost in ≥ 1 ELISA), or no-response individuals (no ≥ 4-fold rise in any assay). We compared demographic, clinical, and pre-infection antibody characteristics across these groups. We also analyzed predictors of an NA response.

Overall, 77% of participants had HAI seroconversion or a fourfold rise. Among the 23% HAI non-responders, 62% had alternate antibody responses. No-response individuals had the highest pre-infection HAI and full-length HA titers (p < 0.01), the lowest viral loads, and the lowest frequency of fever or influenza-like illness symptoms (p < 0.01). An NA response was more common among symptomatic individuals and moderate baseline titers, with both low and high extremes reducing NA response odds.

High baseline HAI titers can limit detectable fourfold rises and are associated with milder illness. Evaluating additional immune responses may capture a more complete picture of the host response to infection, thereby improving surveillance and informing vaccine development.