Pre-existing and Cross-Reactive Immunity to Avian Influenza H5N1 in Humans: Implications for Pandemic Risk and Vaccine Strategies.

Abstract

Due to the continuous evolution of Influenza A viruses (IAVs), novel strains with efficient human-to-human transmission may emerge and cause future pandemics. Among these, highly pathogenic avian influenza (HPAI) H5N1 remains a major concern because of its impact on wildlife, livestock, and human health. The widespread circulation of H5N1 clade 2.3.4.4b, detected in hundreds of bird species and numerous mammals worldwide, highlights important changes in viral ecology and transmission, increasing its zoonotic and pandemic potential. This review summarizes current evidence on cross-reactive and cross-protective immunity to H5N1 in humans, focusing primarily on humoral immune responses. We examine the presence of pre-existing H5N1-reactive antibodies in individuals without known exposure and discuss how previous seasonal influenza infection or vaccination may contribute to their development. Particular attention is given to antibodies targeting conserved regions of hemagglutinin (HA), especially the stalk domain, as well as neuraminidase (NA), which may provide heterosubtypic protection. We also evaluate the ability of seasonal influenza vaccines and infections to induce cross-reactive responses against H5N1 and their potential role in partial protection or immune priming. Finally, we review current and emerging H5N1 vaccination strategies, including adjuvanted and mRNA-based platforms, and identify priorities for surveillance, population immunity assessment, and the development of broadly protective influenza vaccines.