Geert Leroux-Roels

The development and harmonization of assays and models to evaluate vaccine immunogenicity and define correlates of protection remain critical challenges in the field of vaccinology, particularly for pathogens causing respiratory infections. This second volume of the Community Series in Development and Harmonization of Assays and Models to Assess Immunogenicity and Correlates of Protection of Vaccines Against Pathogens Causing Respiratory Infections showcases recent efforts to address these challenges through innovative scientific and regulatory approaches. The contributions presented in this edition explore advanced in vitro models, systems immunology, and biomarker discovery, as well as offer regulatory perspectives and novel vaccine strategies—all aimed at enhancing our understanding of immune protection and supporting the rational design and evaluation of next-generation respiratory vaccines. One publication from the Inno4Vac consortium - a public-private partnership addressing scientific bottlenecks in vaccine development (www.inno4vac.eu) - discusses the development cell-based human in vitro 3D models simulating mucosal infection to evaluate vaccine efficacy and immune protection. Ekanger et al., representing the respiratory models team, compared three human respiratory epithelium models - a primary human bronchial epithelial cell derived air-liquid interface transwell model, and two airway organoid models from human airway-and lung-derived adults stem cells - to study infection with influenza A virus, antiviral cytokine responses, and serum-mediated inhibition of infection (1). Despite differences in cell source, culture methods, and infection protocols, these models consistently produced similar infection and neutralization results. This study demonstrates that standardization enhances comparability across different complex in vitro 3D culture systems. Two further articles explore immune responses to SARS-CoV-2 mRNA vaccination and their association with inflammatory biomarkers. In one, Leno-Duran et al. analyzed 92 serum biomarkers in 147 healthcare workers post-BNT162b2 vaccination, uncovering increases in biomarkers related to T cell function (ADA, CD8a, IL-17C, and CCL25) and decreases in markers associated with inflammation and cancer (uPA, IL-18R1, EN-RAGE, CASP-8, MCP-2, TNFb, CD5, and CXCL10) five weeks after vaccination. Higher MCP-2 levels were associated with higher mean anti-S antibody titers, while HGF and CD6 levels showed negative associations with antibody titers. In another, Lucchesi et al. performed transcriptomic analysis on peripheral blood mononuclear cells from 20 hemodialysis patients and 9 healthy controls before and 7 days after each of two doses of mRNA BNT162b2 vaccine, given three weeks apart (3). Significant gene expression differences related to B cell abundance and regulation, CD4 T cell proliferation, and inflammation pathways were observed at baseline and day 7 between low-responder hemodialysis patients and healthy controls, while high-responder hemodialysis patients displayed an intermediate expression profile for these genes. Although these studies did not directly focus on assay development or correlates of protection, they highlight the importance of identifying predictive biomarkers and understanding dysregulated pathways in vaccine responses across diverse populations. Additionally, Buoninfante and Cavaleri provide a regulatory perspective on the importance of eliciting T cell-mediated immunity (CMI) in vaccination, emphasizing the need for standardized methods to measure CMI responses for regulatory submissions (4). Despite assay complexity and variability, the authors advocate for the systematic consideration of T cell responses in vaccine development, especially when CMI is considered to play a role in protection. Advances in technology and ongoing efforts to harmonize assay protocols (5-7) are expected to enable reliable, reproducible measurements of T cell responses to better inform vaccine efficacy, provided the data are validated in clinical studies. Finally, Victor Cnossen and colleagues introduce the EU-funded FLUniversal consortium, aiming to develop a universal, intranasal live-attenuated influenza vaccine – DeltaFLU-that protects against all influenza strains (8). Preclinical studies and clinical trials, including a controlled human infection model (CHIM), will evaluate safety, immunogenicity and protective efficacy. Comprehensive analyses of blood and mucosal immune responses are anticipated to uncover novel correlates of protection (CoPs). Together, the studies featured in this Research Topic underscore the importance of integrating multidisciplinary approaches—from advanced in vitro models and biomarker analysis to regulatory science and novel vaccine platforms—to better assess immunogenicity and define meaningful correlates of protection. As respiratory pathogens continue to pose significant public health threats, collaborative efforts such as those presented here are essential to drive innovation, standardization, and translational impact in vaccine development.

Influenza vaccines are the primary strategy to prevent severe influenza disease; however, their efficacy is often suboptimal, particularly in older adults (OAs). LiteVax Adjuvant (LVA), a novel adjuvant containing carbohydrate fatty-acid monosulphate ester (CMS) as the active ingredient, has demonstrated a favourable safety profile and enhanced immunogenicity when combined with a low-dose seasonal influenza vaccine in adults aged 18 to 50 years in a first-in-human phase 1 study. The present study investigates the reactogenicity and immunogenicity of CMS-based adjuvanted seasonal influenza vaccine in OAs, with a comparison to responses in younger adults (YAs). In this phase 1b, double-blind, active-controlled clinical trial, 36 YAs (18–50 years) and 48 OAs (≥60 years) were randomized (1:1:1) to receive either 0.5 mg or 1 mg LVA combined with VaxigripTetra, or VaxigripTetra alone. Solicited adverse events (AEs) were recorded using an electronic diary for 7 days following vaccination. Hemagglutination inhibition (HI) titers against four influenza strains were measured at baseline (pre-vaccination) and at 7-, 28-, and 180-days post-vaccination. All 24 YAs and 31 out of 32 OAs receiving CMS-based adjuvanted vaccines reported pain post-vaccination, compared to 8/12 YAs and 4/16 OAs receiving VaxigripTetra. Systemic AEs were more frequently reported among YAs receiving CMS-based adjuvanted vaccines (22/24) compared to those receiving VaxigripTetra (8/12). In OAs, the number of systemic AEs was similar regardless of CMS-based adjuvant administration. Most AEs were mild to moderate and resolved within 3 days. Both CMS-based adjuvanted formulations elicited increased HI titers at Day 7, peaking at Day 28, with a decline thereafter that remained above baseline at Day 180. In YAs, HI titers were comparable between the CMS-based adjuvanted and non-adjuvanted vaccines across all strains and timepoints. In contrast, CMS-based adjuvanted vaccination in OAs induced higher HI titers at Days 28 and 180 for all influenza strains tested. LVA shows an acceptable safety profile in both age cohorts and enhances humoral immune responses in older adults. The 1 mg dose of LVA was more immunogenic, highlighting its potential utility in this target population. Future research will focus on elucidating the mechanisms underlying the immunostimulatory effect of the CMS-based adjuvant.

Background: Major global economic and health burdens due to norovirus gastroenteritis could be addressed by an effective vaccine. Methods: In this study, 428 adult recipients of various compositions of the norovirus vaccine candidate, HIL-214, were followed for 5 years, to assess immune responses to its virus-like particle antigens, GI.1 and GII.4c. Serum antibodies and peripheral-blood antibody-secreting cells (ASCs) were measured. This report focuses on the single-dose 15/50 (µg GI.1/GII.4c) composition, which had been selected for further clinical development. Results: For single-dose 15/50 recipients (N = 105), GI.1-specific and GII.4c-specific histoblood-group antigen-blocking (HBGA) antibodies appeared to have persisted to 5 years, waning from a peak at 4 to 8 weeks, and plateauing above baseline after 3 years. From 3 to 5 years, GI.1-specific GMTs ranged between 53 (95%CI, 40–71) and 60 (95%CI, 46–77; N = 69–97) and were approximately 2-fold above the baseline GMT (24 (95%CI, 20–28); N = 105). GII.4c-specific GMTs ranged between 103 (95%CI, 77–138) and 114 (95%CI, 86–152; N = 70–97) and were above baseline, but by less than 2-fold (70 (95%CI, 53–92); N = 105). Similar kinetics were observed for pan-Ig titers and ASCs in a subset. Similar kinetics were also observed for HBGA and pan-Ig titers in recipients of other 15/50 dosages. Conclusions: Immune responses to HIL-214 in adults appear to persist for five years.

Background/Objectives: In a Phase 2a, double-blind, placebo-controlled study including healthy participants aged 18–55 years, OVX836, a nucleoprotein (NP)-based candidate vaccine, previously showed a good safety profile, a robust immune response (both humoral and cellular) and a preliminary signal of protection (VE = 84%) against PCR-confirmed symptomatic influenza after a single intramuscular dose of 180 µg, 300 µg or 480 µg. Methods: Using the same methodology, we confirmed the good safety and strong immunogenicity of OVX836 at the same doses in older adults (≥65 years), a key target population for influenza vaccination. Results: Significant humoral (anti-NP IgG) and cellular (interferon gamma (IFNγ) spot-forming cells per million peripheral blood mononuclear cells and specific CD4⁺ IFNγ⁺ T-cells) immune responses were observed at the three dose levels, without clear dose–response relationship. T-cell responses were shown to be highly cross-reactive against various influenza A strains, both seasonal and highly pathogenic avian strains. We also evaluated the effect of sex (stronger immune response in females) and age (stronger immune response in young adults) on the immune response to OVX836 after adjustment based on the pre-vaccination immune status. Conclusions: The results obtained with OVX836 lay the groundwork for a future placebo-controlled, field proof of concept efficacy Phase 2b trial.

Background: Major global economic and health burdens due to norovirus gastroenteritis could be addressed by an effective vaccine. Methods: 428 adult recipients of various compositions of the norovirus-vaccine candidate, HIL-214, were followed for 5 years, to assess immune responses to its virus-like–particle antigens, GI.1 and GII.4c. Serum antibodies and peripheral-blood antibody-secreting cells (ASCs) were measured. This report focuses on the single-dose 15/50 (µg GI.1/GII.4c) composition, which had been selected for further clinical development. Results: For single-dose 15/50 recipients (N=105), GI.1-specific and GII.4c-specific histoblood-group-antigen–blocking (HBGA) antibodies appeared to have persisted to 5 years, waning from a peak at 4-to-8 weeks, and plateauing above baseline after 3 years. From 3-to-5 years, GI.1-specific GMTs ranged between 53 (95%CI, 40–71) and 60 (95%CI, 46–77; N=69–97) and were approximately 2-fold above the baseline GMT (24 [95%CI, 20–28]; N=105). GII.4c-specific GMTs ranged between 103 (95%CI, 77–138) and 114 (95%CI, 86–152; N=70–97) and were above baseline, but by less than 2-fold (70 [95%CI, 53–92]; N=105). Similar kinetics were observed for pan-Ig titers and ASCs in a subset. Similar kinetics were also observed for HBGA and pan-Ig titers in recipients of other 15/50 dosages. Conclusion: Immune responses to HIL-214 in adults appear to persist for 5 years.

Seasonal influenza vaccine effectiveness is low. Carbohydrate fatty acid monosulphate ester (CMS), a new oil-in-water adjuvant, has proven potency in animal models with suggested capacity for dose-sparing. The objective was to evaluate safety and immunogenicity of CMS when added to a low-dose influenza vaccine (QIV) in humans. In a randomised, double-blind, active-controlled, first-in-human study, sixty participants (18–50 years) received either 0.5 mg CMS or 2 mg CMS with 1/5th dose QIV, or a full dose QIV without CMS. Adverse events (AE) were monitored until 7 days post-vaccination. Haemagglutinin inhibition (HI) titres in serum and CD4+ T cells in PBMCs were determined at day 0, 7, 28, and 180. Mean age was 37.6 (±10.1) years and 42/60 (70.0%) were female. Pain at injection site (42/60, 86.7%) and headache (34/60, 56.7%) were reported most and more frequently in the 2 mg CMS group. HI titres and the frequency of influenza specific CD4+ T cells were equal across strains for the three cohorts on all visits, increased until day 28 and decreased at day 180 to values higher than baseline. CMS was safe in humans. Humoral and cell-mediated immunogenicity was similar across vaccines, even with 1/5th antigen dose. CMS can have beneficial implications in low-resource settings or in a pandemic context.

Humans display substantial interindividual clinical variability after SARS-CoV-2 infection^1–3, the genetic and immunological basis of which has begun to be deciphered⁴. However, the extent and drivers of population differences in immune responses to SARS-CoV-2 remain unclear. Here we report single-cell RNA-sequencing data for peripheral blood mononuclear cells—from 222 healthy donors of diverse ancestries—that were stimulated with SARS-CoV-2 or influenza A virus. We show that SARS-CoV-2 induces weaker, but more heterogeneous, interferon-stimulated gene activity compared with influenza A virus, and a unique pro-inflammatory signature in myeloid cells. Transcriptional responses to viruses display marked population differences, primarily driven by changes in cell abundance including increased lymphoid differentiation associated with latent cytomegalovirus infection. Expression quantitative trait loci and mediation analyses reveal a broad effect of cell composition on population disparities in immune responses, with genetic variants exerting a strong effect on specific loci. Furthermore, we show that natural selection has increased population differences in immune responses, particularly for variants associated with SARS-CoV-2 response in East Asians, and document the cellular and molecular mechanisms by which Neanderthal introgression has altered immune functions, such as the response of myeloid cells to viruses. Finally, colocalization and transcriptome-wide association analyses reveal an overlap between the genetic basis of immune responses to SARS-CoV-2 and COVID-19 severity, providing insights into the factors contributing to current disparities in COVID-19 risk.

Background: V-306 is a virus-like particle-based vaccine candidate displaying respiratory syncytial virus (RSV) F site II protein mimetics (FsIIm) as an antigenic epitope. Methods: This was a randomized, placebo-controlled, double-blind, dose-escalating, first-in-human study, conducted in 60 women aged 18–45 years. Twenty subjects per cohort (15 vaccine and five placebo) received two V-306 intramuscular administrations on Days 0 and 56 at 15 µg, 50 µg, or 150 µg. Safety and immunogenicity were assessed after each vaccination and for 1 year in total. Results: V-306 was safe and well tolerated at all dose levels, with no increase in reactogenicity and unsolicited adverse events between the first and second administrations. At 50 µg and 150 µg, V-306 induced an increase in FsIIm-specific immunoglobulin G (IgG) titers, which lasted at least 4 months. This did not translate into an increase in RSV-neutralizing antibody titers, which were already high at baseline. No increase in the anti-F protein-specific IgG titers was observed, which were also high in most subjects at baseline due to past natural infections. Conclusions: V-306 was safe and well-tolerated. Future modifications of the vaccine and assay conditions will likely improve the results of vaccination.