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

Highly pathogenic avian influenza viruses in mammals: host-range expansion and implications of milk-borne viral shedding

SUMMARY Highly pathogenic avian influenza (HPAI) viruses, especially subtype H5N1, have caused major outbreaks in poultry and serious human infections since the first description of “fowl plague” in 1878. Over time, these viruses have expanded their host range, causing huge losses in domestic poultry, spreading among wild bird populations, and occasionally infecting humans. Understanding how influenza viruses adapt and cross species barriers depends on analyzing their fundamental molecular characteristics. Key features such as the polybasic cleavage site within the hemagglutinin protein and the distinctive “1+7” ribonucleoprotein complex, which consists of a single polymerase core surrounded by seven RNA segments, enhance viral replication and broaden host range. Despite decades of intervention, including lessons from the 1997 Hong Kong outbreak and the continued global circulation of clade 2.3.4.4b, HPAI remains difficult to control. A major shift occurred in 2024, when H5N1 was detected in U.S. dairy cattle. This was the first confirmed instance of viral shedding into milk from a mammalian host, suggesting a new potential route of transmission beyond the traditional avian reservoirs. This review unites historical milestones, structural insights, epidemiological data, and recent cross-species findings to better define the current landscape of risk and transmission. We further discuss economic, public health, and agricultural impacts of these developments, particularly the 2024 cattle cases, and underscore the urgent need for an integrated One Health approach to better manage the growing risks posed by HPAI viruses.

Highly pathogenic avian influenza A(H5N1) in domestic cats: epidemiology, clinical manifestations, and One Health implication.

Risk assessment of avian influenza A(H5N5) virus from the first human case using the ferret model

ABSTRACT The incursion of Eurasian-origin genotype A6 A(H5N5) virus into North America expanded the genetic diversity among North American highly pathogenic avian influenza viruses and heightened concern about zoonotic risk. Following a fatal human infection with the A(H5N5) virus A/Washington/2148/2025, viral replication was assessed in polarized human bronchial epithelial cells, and pathogenicity, transmissibility in direct contact and respiratory droplet models, and airborne virus shedding were evaluated in ferrets to inform pandemic risk assessment. A(H5N5) displayed robust replication in Calu-3 cells at 33°C and 37°C, showing kinetics and peak titers comparable to those of contemporary genotype B3.13 and D1.1 A(H5N1) viruses. In ferrets, A(H5N5) replicated efficiently in the respiratory tract, disseminated to extrapulmonary tissues, and caused fatal disease in all inoculated animals. Airborne transmission was not observed, and infrequent, low-level detection of virus in air samples paralleled that of A(H5) viruses that are not transmissible via air in ferrets. In a direct contact model, limited transmission was detected within 4 days of exposure, with evidence of lower respiratory tract replication in contact animals. These findings indicate that the A(H5N5) virus has the capacity for robust replication in an airway epithelial cell line and can cause severe systemic infection and mortality in ferrets but has not acquired adaptations for airborne spread in mammals. Collectively, these results underscore heterogeneity among clade 2.3.4.4b A(H5Nx) viruses in North America and the need for genotype-by-genotype evaluation of newly emerged viruses to understand public health risk. IMPORTANCE The emergence of Eurasian-origin genotype A6 highly pathogenic avian influenza A(H5N5) virus in North America has increased viral diversity and raised concerns about zoonotic and pandemic risk. In this study, we evaluated the replication kinetics, pathogenesis, and transmission of A/Washington/2148/2025 A(H5N5) virus, which was isolated from the first reported human infection with this influenza virus subtype, using polarized human bronchial epithelial cells and the ferret model. The A(H5N5) virus replicated efficiently in vitro at temperatures representative of the upper and lower respiratory tracts and caused fatal systemic disease in inoculated ferrets. Limited transmission was observed during 4 days of direct contact. Airborne virus detection was infrequent and did not result in airborne transmission. These findings show that A(H5N5) virus can replicate robustly in mammalian cells and cause severe disease but lacks adaptations supporting efficient airborne spread, informing assessment of the pandemic risk posed by genotype A6 influenza viruses. The emergence of Eurasian-origin genotype A6 highly pathogenic avian influenza A(H5N5) virus in North America has increased viral diversity and raised concerns about zoonotic and pandemic risk. In this study, we evaluated the replication kinetics, pathogenesis, and transmission of A/Washington/2148/2025 A(H5N5) virus, which was isolated from the first reported human infection with this influenza virus subtype, using polarized human bronchial epithelial cells and the ferret model. The A(H5N5) virus replicated efficiently in vitro at temperatures representative of the upper and lower respiratory tracts and caused fatal systemic disease in inoculated ferrets. Limited transmission was observed during 4 days of direct contact. Airborne virus detection was infrequent and did not result in airborne transmission. These findings show that A(H5N5) virus can replicate robustly in mammalian cells and cause severe disease but lacks adaptations supporting efficient airborne spread, informing assessment of the pandemic risk posed by genotype A6 influenza viruses.

A lethal human H5N5 influenza virus isolate exhibits low pandemic risk traits

In fall of 2025, a fatal infection of highly pathogenic avian influenza (HPAI) virus H5N5 occurred. To define the risk of this emerging virus to humans, we performed a comprehensive analysis based on our established triage. Serological analysis revealed that humans across all birth years had no detectable neutralizing antibodies to this H5N5 isolate. Further characterization revealed a lack of phenotypic signatures associated with epidemiologically successful influenza viruses in humans, including reduced replication in human airway cells and an avian-like pH of inactivation. Additionally, assessment of H5N5 in ferrets revealed a lack of direct contact transmission and moderate disease severity. H5N5 infection in ferrets with prior immunity against the 2009 H1N1 pandemic strain resulted in fewer clinical signs and reduced viral shedding. Together our data suggest that the current H5N5 HPAI lineage poses a low pandemic risk. Importance HPAI H5N5 viruses have caused widespread infection and death in avian species, and characterizing their pandemic risk traits is critical to understanding the threat posed to humans. In this work we analyzed an isolate that resulted in a human fatality in 2025. We found that this strain lacks many key features of influenza viruses with epidemiological success in humans including reduced growth in human lung cultures, a pH of inactivation less than 5.0, and lack of transmission to cohoused recipient ferrets. Prior immunity with seasonal H1N1 strain also reduced the viral load and disease burden of the virus. Taken together, these data suggest that currently circulating H5N5 poses a low risk to humans but highlights the importance of phenotypic characterizations for future risk assessments as the virus evolves in wild birds.

Protective effect of A(H5N8) stockpiled vaccine against a virus genetically identical to a human isolate of bovine A(H5N1) influenza virus

Summary Background Since early 2024, highly pathogenic avian influenza A(H5N1) viruses of clade 2.3.4.4b have caused extensive outbreaks in dairy cattle in the United States, with spillover into mammalian species, including humans. A bovine-derived A(H5N1) virus isolated from a human case retains high pathogenicity and transmissibility in mammalian models, highlighting its pandemic potential. Stockpiled pre-pandemic influenza vaccines are intended to provide early protection before strain-matched vaccines are available; however, their protective efficacy against bovine A(H5N1) viruses has not been directly evaluated in vivo. Methods In this study, we assessed the protective efficacy of an AS03-adjuvanted A/Astrakhan/3212/2020 (H5N8) clade 2.3.4.4b-based influenza vaccine stockpiled in Japan using mouse and ferret models. Vaccinated and unvaccinated animals were challenged with a virus genetically identical to a human isolate of bovine A(H5N1) virus. Neutralising antibody responses, viral replication in organs, and survival were evaluated. Findings Vaccination with the AS03-adjuvanted A(H5N8)-based stockpiled vaccine induced robust neutralising antibody responses in both animal models, significantly suppressed viral replication, and conferred complete protection against lethal challenge. In contrast, all unvaccinated mice and ferrets succumbed to infection. These findings demonstrate that the AS03-adjuvanted A(H5N8)-based stockpiled vaccine provides strong cross-protective efficacy against bovine A(H5N1) viruses. Interpretation An AS03-adjuvanted A(H5N8)-based vaccine stockpiled in Japan could serve as an immediate countermeasure against bovine A(H5N1) viruses during the early phase of a pandemic. Funding This work was supported by grants from the Japan Program for Infectious Diseases Research and Infrastructure (JP20wm0125002) and the Japan Initiative for World-leading Vaccine Research and Development Centers (JP223fa627001) from the 10.13039/100009619Japan Agency for Medical Research and Development.

Genomic and Antigenic Evolution of Influenza A(H3N2) After the COVID-19 Era: A Scoping Review with Focus on J and K Subclades and Implications for Vaccine Effectiveness

A scoping review was conducted across six databases in January 2026, including studies on genomic surveillance, antigenic characterization, and vaccine effectiveness (VE), which indicated diminished VE against drifted strains, though no consistent increase in clinical severity was observed.