Increase in H5N1 vaccine antibodies confers cross-neutralization of highly pathogenic avian influenza H5N1

Abstract

H5N1, a highly pathogenic avian influenza virus, presents pandemic risks due to its ability to adapt and spread among mammalian species. Vaccination may control its spread, but the effectiveness of existing H5N1 vaccines against circulating strains, especially clade 2.3.4.4b, remains uncertain. In this study, we assess neutralizing antibody responses to global circulating H5N1 strains, using sera from individuals vaccinated with an inactivated H5N1 vaccine (NCT00535665). Neutralization is measured against 17 pseudoviruses, representing circulating and vaccine H5 strains. Our results indicate that broad protective effects are observed only when high antibody titers are achieved by vaccination. Correlation analysis estimates that a pseudovirus-based neutralization titer of at least 1:980 is required to achieve a cross-protection rate above 60%. The findings suggest that the current H5N1 vaccine can elicit cross-neutralization of circulating H5N1 strains, if high antibody titers are achieved. Until updated H5N1 vaccines are developed, this vaccine may serve as a bridging measure.

Fig. 1. The genetic evolutionary relationships among H5Nx circulating strains and vaccine viruses were analyzed.

Phylogenetic inference was conducted using a maximum likelihood approach based on amino acid sequences of the hemagglutinin (HA) protein. These strains were categorized into various clades, with approved vaccine strains or candidate vaccine viruses marked with red triangles, and selected epidemic representative strains highlighted in bold. A/Darwin/9/2021(H3N2) served as the out-group. Source data are provided as a source data file.

Fig. 2. Neutralizing antibody levels against H5 influenza pseudoviruses and H5N1 vaccine strain A/Vietnam/1194/2004 before and after vaccination, and correlation analysis among hemagglutination inhibition (HI), microneutralization (MN), and pseudovirus-based neutralizing assays (PBNA).

Shown are the ID₅₀ values representing neutralizing antibody levels in human sera before and after vaccination against HA of various H5 strains, as measured by PBNA. Based on post-vaccination neutralizing antibody levels against the vaccine strain, subjects were classified into high, medium, and low response groups, as represented in (a, b, c). Orange dots represent pre-vaccination, while light blue dots represent post-vaccination. The thick black line indicates the pre-vaccination geometric mean titer adding a 95% confidence interval. Geometric mean increase (GMI) and seroconversion rates (SCR) between pre- and post-vaccination are shown above each group. The strains were classified into clades in (c). d, e Pearson correlation analysis (two-tailed) was used to evaluate the association between PBNA and HI or MN. f shows the relationship between PBNA and overall neutralizing antibody SCR, with simple linear regression equation. Statistical significance was assessed using multiple unpaired two-sided t tests, assuming individual variances for each row. The False Discovery Rate was controlled at 1% using the two-stage step-up method of Benjamini, Krieger, and Yekutieli. Source data are provided as a source data file.