The threat posed by influenza viruses, particularly the Influenza A virus (H5 subtype), remains a major global public health concern. Primarily circulating in avian populations, this virus can cross the species barrier to infect humans, causing severe disease. World Health Organization (WHO) data indicates that since the first human case was identified in 1997, a staggering 1,085 confirmed cases and 530 deaths were reported globally as of July 2025, yielding an exceptionally high case fatality rate of 48.8%. Beyond human health, the widespread transmission of A(H5) among poultry causes enormous economic losses to the global poultry industry.
Faced with this severe challenge, the development of a highly effective vaccine against the A(H5) influenza virus is paramount. A recent study, published in the prestigious international journal Nature under the title “A vaccine central in A(H5) influenza antigenic space confers broad immunity,” details a significant breakthrough. Researchers from institutions including the Erasmus University Rotterdam Medical Center in the Netherlands have successfully designed a vaccine antigen positioned at the center of the A(H5) influenza antigenic space, which promises to offer broad immune protection.
Overcoming the Limitation of Traditional Vaccines
Traditional vaccine design necessitates continuous updating of vaccine strains to combat viral mutation, a process that is both time-consuming and often slow to react to newly emerging strains. To overcome this issue, the researchers focused on the antigenic evolution and diversity of the A(H5) influenza virus. They aimed to identify a vaccine antigen capable of inducing a wide-ranging immune response against various antigenic types by constructing a high-resolution antigenic map. An antigen located centrally within this antigenic space is theoretically positioned to provide broader protection and reduce the risk of immune escape caused by viral variation.
Mapping the Antigenic Space
The study began by collecting 127 representative hemagglutinin (HA) genes of the A(H5) influenza virus from around the world, spanning viral strains from 1959 to 2022. Using reverse genetics technology, these HA genes were cloned into the genetic backbone of an attenuated influenza strain (A/Puerto Rico/8/1934, PR/8) to generate a series of recombinant viruses. Ferrets were then immunized with these viruses, and their collected sera were used in subsequent hemagglutination inhibition (HI) assays.
The HI assays provided cross-reactivity data between the 127 antigens and 33 ferret sera, which was then used to construct a three-dimensional antigenic map. On this map, distance between antigens is inversely correlated with the HI titer: the higher the reaction titer between an antigen and serum, the closer their distance on the map. This visualization provided researchers with an intuitive view of the A(H5) influenza virus’s antigenic evolution and diversity.
Rational Design of a Broad-Spectrum Antigen
Analysis of the 12 wild-type antigens near the map center revealed they possessed higher immunogenicity, reacting with more sera and exhibiting a higher geometric mean titer (GMT). A key finding was that most of these central antigens lacked a potential glycosylation site at position 154. Additionally, some central antigens contained amino acid residues associated with altered HA receptor-binding properties.
These observations suggested that enhancing vaccine immunogenicity and cross-reactivity could be achieved by modifying receptor-binding properties and removing the glycosylation site. Based on this, the team designed a series of candidate vaccine antigens (CVAs). These CVAs were based on non-central HA from different genetic lineages, but the introduction of specific mutations (Q222L and G224S) altered the receptor-binding properties and the removal of the glycosylation site at position 154 (T156A) enabled these antigens to bind to both α2,3- and α2,6-linked sialic acids, whereas their parental HA only bound to α2,3-linked sialic acids.
The resulting CVAs demonstrated heightened reactivity in HI assays and were positioned closer to the map center, with an average distance of 1.33 antigenic units (AU). Critically, this distance was 4.15 AU closer to the center than their respective wild-type antigens, confirming that rational design can create a vaccine antigen positioned at the center of the antigenic space to offer broader immune protection.
Proof of Broad Protection: The AC-Anhui Vaccine
The researchers evaluated the immunogenicity and protective efficacy of the designed vaccine, named AC-Anhui, in the ferret model. The ferret sera elicited by the whole-virus inactivated vaccine containing the mutated HA demonstrated wide cross-reactivity within the A(H5) antigenic space.
Fig.1 Ferret sera: broad cross-reactivity to A(H5) via mutated HA vaccine.1
The AC-Anhui vaccine induced high levels of neutralizing antibodies in ferrets, exhibiting broad reactivity against multiple genetic lineages and antigenic types of A(H5) influenza viruses. In challenge experiments, vaccinated ferrets showed significantly reduced disease symptoms and lower viral loads compared to the unvaccinated control group after infection with H5N1 Giza or H5N6 Sichuan viruses. This clearly indicates that the AC-Anhui vaccine not only protects against specific strains but also offers robust cross-protection against other related strains, demonstrating an excellent broad-spectrum immunity effect.
A New Paradigm for Vaccine Development
In conclusion, this research successfully designed the AC-Anhui vaccine antigen by mapping the A(H5) influenza virus’s antigenic landscape and positioning the antigen at the center of the antigenic space, validating its broad immune protection in animal models. This achievement provides a novel concept and methodology for influenza vaccine research, holding promise to overcome the challenges of viral variation inherent in traditional vaccine design. While the AC-Anhui vaccine has shown excellent immunogenicity and protective efficacy in animal studies, researchers note that its application in humans will require further clinical trials for validation.
The findings offer vital theoretical basis and practical guidance for A(H5) influenza virus vaccine development, while also providing valuable reference for designing vaccines against other highly variable viruses in the future.
Reference
- Kok, Adinda, et al. “A vaccine central in A (H5) influenza antigenic space confers broad immunity.” Nature(2025): 1-9. CC BY 4.0. https://doi.org/10.1038/s41586-025-09626-3