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The induction of systemic antibody titers against hemagglutinin has long been the main focus of influenza vaccination strategies, but mucosal immunity has also been shown to play a key role in the protection against respiratory viruses. By vaccinating and challenging healthy volunteers, we demonstrated that inactivated influenza vaccine (IIV) modestly reduced the rate of influenza while predominantly boosting serum antibody titers against hemagglutinin (HA) and HA stalk, a consequence of the low neuraminidase (NA) content of IIV and the intramuscular route of administration. The viral challenge induced nasal and serum responses against both HA and NA. Correlations between mucosal IgA and serum IgG against specific antigens were low, whether before or after challenge, suggesting a compartmentalization of immune responses. Even so, volunteers who developed viral shedding for multiple days had lower baseline titers across both systemic and mucosal compartments as compared to those with no shedding or a single day of shedding. Regression analysis showed that pre-challenge HA inhibition titers were the most consistent correlate of protection across clinical outcomes combining shedding and symptoms, with NA inhibition titers and HA IgG levels only predicting the duration of shedding. Despite the inclusion of data from multiple binding and functional antibody assays against HA and NA performed on both serum and nasal samples, multivariate models were unable to account for the variability in outcomes, emphasizing our imperfect understanding of immune correlates in influenza and the importance of refining models with assessments of innate and cellular immune responses.IMPORTANCEThe devastating potential of influenza has been well known for over 100 years. Despite the development of vaccines since the middle of the 20th century, influenza continues to be responsible for substantial global morbidity and mortality. To develop next-generation vaccines with enhanced effectiveness, we must synthesize our understanding of the complex immune mechanisms culminating in protection. Our study outlines the differences in immune responses to influenza vaccine and influenza infection, identifying potential gaps in vaccine-induced immunity, particularly at the level of the nasal mucosa. Furthermore, this research underscores the need to refine our imperfect models while recognizing potential pitfalls in past and future attempts to identify and measure correlates of protection., Competing Interests: The authors declare no conflict of interest.

Background: It is imperative to identify new targets for improved vaccines and therapeutics against influenza. One such target is the relatively conserved stalk region of the influenza A hemagglutinin (HA) surface protein., Methods: We conducted a randomized, double-blind, phase 2, placebo-controlled trial of a monoclonal antibody that targets the HA stalk (CR6261) in a H1N1pdm09 healthy volunteer human challenge model. A single 50 mg/kg dose of CR6261 was infused 24 hours after challenge. The primary efficacy outcome was area under the curve (AUC) of viral RNA detection over time., Results: Ninety-one healthy volunteers were randomized and underwent influenza challenge; 49 received CR6261 and 42 received placebo. CR6261 had no statistically significant effect on AUC (AUC, 48.56 log [copies/mL] × days, interquartile range [IQR], 202 vs AUC, 25.53 log [copies/mL] × days, IQR, 155; P = .315) and no clinically significant effect on influenza disease measures including number of symptoms, duration of symptoms, or inFLUenza Patient-Reported Outcome (FLU-PRO) scores. Preexisting anti-NA antibody titers were most predictive of reduced influenza disease. CR6261 reached a mean peak serum concentration of 1 × 106 ng/mL 15 minutes after infusion and a mean peak of 5.97 × 102 ng/mL in the nasal mucosa 2-3 days after infusion., Conclusions: The results of this study suggest that a monoclonal anti-stalk approach to prevent or treat influenza infection may be limited in efficacy. Future approaches should consider including and evaluating anti-stalk antibodies as part of a multifaceted strategy rather than as a stand-alone therapeutic., Clinical Trials Registration: NCT02371668., (Published by Oxford University Press for the Infectious Diseases Society of America 2020.)