Binding antibody titers against the hemagglutinin and neuraminidase correlate with protection against medically attended influenza A and B disease.

Human challenge and cohort studies have identified various correlates of protection (CoP) against influenza A and B viruses (IAV/IBV). However, associations with viral load, investigation of mucosal CoPs, and CoPs against IBV are limited in the context of natural infections. Plasma and nasal swabs were collected (2017-2020) from 56 adults diagnosed with IAV (n = 25 H1N1, n = 19 H3N2) or IBV (n = 9 B/Victoria, n = 3 B/Yamagata) in the emergency department. Viral load was determined in nasal swabs. Antibodies (total Ig and IgA) specific for the hemagglutinin (HA) and neuraminidase (NA) of contemporary viruses from the subtype or lineage infecting each individual were measured by enzyme-linked immunosorbent assay (ELISA). Antibodies to a non-infecting influenza strain were measured and used as "control cases" to determine associations with protection from infection. Viral load decreased with time post-symptom onset. The Ct value at which 50% of the samples were positive in viral culture was 24.75 (95% confidence intervals, 23.7-27.01). Systemic HA and NA-specific Ig titers correlated with protection from medically-attended influenza disease. Neither systemic nor mucosal antibody measurements were associated with disease severity. We observed an inverse correlation between Ig anti-NA antibodies in nasal swabs and viral load by Ct value (regression coefficient = 3.25, CI = 0.3-6.2, P = 0.031), though this analysis was not corrected for multiple comparisons. Overall, high titers of HA and NA-specific antibodies measured by ELISA were associated with protection from the development of influenza A or B disease. Further work is needed to understand immune parameters associated with viral clearance and mucosal CoPs.IMPORTANCEThere is a great need to better understand correlates of protection (CoP) against influenza A and B viruses (IAV/IBV). In our study, we analyzed paired plasma and nasal swabs from patients presenting with influenza A or B disease as well as control patients. We measured hemagglutinin (HA) and neuraminidase (NA) specific antibodies in both sample types and also determined the amount of virus in nasal swabs. We found that higher systemic binding antibodies to the hemagglutinin and neuraminidase were associated with protection from medically attended disease. These findings expand our understanding of correlates of protection against influenza viruses and identify areas of future research to further understand protection from influenza.