Background: 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.

Methods: 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.

Results: 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.

Conclusions: 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.