Newly emerging influenza virus strains pose a constant threat as they encounter a population lacking neutralizing antibodies against the new strain. However, cross-reactive non-neutralizing antibodies (nnABs) may be present and assist in mitigating disease symptoms via various effector mechanisms, including antibody-dependent cellular cytotoxicity (ADCC). Although nnABs to influenza virus have received more attention lately, little information is available on their age-related prevalence, steady-state levels, functional properties, and changes in these parameters over time.

Using longitudinal samples from adolescents, adults, and older adults, collected before and after the 2009 swine flu pandemic, we comprehensively characterized the specificity and functionality of nnAB responses against H1N1 pandemic 2009 (H1N1pdm09) virus. Remarkably, all participants exhibited cross-reactive antibodies to this virus before having encountered it through infection or vaccination, with the highest baseline levels observed in older adults. The levels of these IgG antibodies showed a strong correlation with engagement of fragment crystallizable γ receptor IIIa (FcγRIIIa) and ADCC activity, both of which were notably lower in adolescents compared to adults and older adults. Without infection or vaccination, average amounts of H1N1pdm09-reactive antibodies remained relatively stable on population level over the 5-year study period. However, on an individual level, substantial increases and decreases occurred. H1N1pdm09 infection or vaccination significantly enhanced specific antibody levels and the FcγRIIIa-engaging capacity of these antibodies in all age groups. ADCC-mediating antibodies increased however only in adolescents, reaching the same level as observed in the adult groups.

Taken together, our results demonstrate the presence of cross-reactive, non-neutralizing, functional, and boostable antibodies against a never-encountered influenza virus strain across all age groups. These antibodies can potentially contribute to protection from severe disease. Accordingly, in case of a newly emerging virus, their further enhancement by vaccination could be beneficial as an immediate protective measure before a strain-specific vaccine becomes available.

Author summary Nearly everyone has contracted influenza and/or has been vaccinated against influenza several times over the years. While the antibodies raised during these earlier encounters will not prevent infection by a newly emerging influenza virus strain, they can help to protect from severe disease. Therefore, it is important to determine the prevalence and quantity of these antibodies, understand their mechanisms of action, assess their persistence over time, and examine potential age-related differences in these parameters. We studied antibody responses to the H1N1pdm09 virus in blood samples of young, adult, and older adult individuals from a large cohort study. Irrespective of age, all blood samples contained antibodies that reacted with a never-before-encountered influenza virus strain. The amounts of these antibodies were initially lower in adolescents but with time increased, reaching the same levels as observed in adults. Importantly, infection with or vaccination against the new virus strengthened the responses in all age groups. We conclude that boosting such broadly-reactive antibodies through vaccination could serve as an immediate strategy when a new virus emerges, buying critical time to develop a more specific vaccine.