Objectives
Immunisation remains the most cost-effective mechanism to combat influenza infection and is widely employed against seasonal influenza viruses. Zoonotic transmission of avian influenza A viruses represents a significant threat to human health given the lack of population-level immunity. Therefore, there is a need to better understand pre-existing cross-reactive human immunity against avian influenza strains, as highlighted by the recent global spread of avian H5Nx clade 2.3.4.4b variants.

Methods
Here, we quantified the frequencies and specificities of B and T?cells recognising avian hemagglutinin (HA) within unexposed adults and characterised the ability of seasonal immunisation to boost cross-reactive immune responses to H5Nx strains, including from clade 2.3.4.4b.

Results
Low but detectable serum antibody titres against H5 and H7 avian influenza HA were observed in donors. The frequency of memory B cells with cross-reactive recognition of H5 and H7 HA was below 0.13% and two- to five-fold lower than populations of seasonal HA-specific B cells. Boosting of B-cell responses against clade 2.3.4.4b H5Nx HA following seasonal immunisation was sporadic with only three of 19 individuals showing an increased population of probe-positive cells. Cross-reactive B cells generally expressed immunoglobulins drawn from variable heavy chain genes associated with HA stem recognition. CD4+ T-cell responses towards H5 HA were weakly boosted with little increase in circulating T follicular helper cell populations.

Conclusion
These findings highlight the need for avian influenza-specific vaccine products to bolster immunity in human populations. Such vaccines could aid pre-pandemic preparedness by expanding baseline frequencies of avian influenza-specific memory lymphocytes.