Science advances in two modes: incremental refinement and paradigm disruption. Most vaccine research falls into the first category — optimising adjuvants, improving delivery platforms, extending cold chain stability. Occasionally, however, a study emerges that does not merely refine the existing framework but challenges its foundational assumptions. The March 2026 publication from KU Leuven's Rega Institute, demonstrating that CD4-T cells rather than antibodies mediate vaccine-induced protection against Sudan virus, belongs firmly in the second category.
The implications of this finding extend far beyond the Sudan virus itself. They touch the core of how vaccines are evaluated by regulatory agencies, how clinical trials are designed, how correlates of protection are defined, and how the global scientific community conceptualises immune memory and protection. Understanding these implications requires a careful examination of the current paradigm and why it has persisted — and why the KU Leuven findings may finally be the catalyst for change.
The Antibody Paradigm: Origins, Strengths, and Limitations
The centrality of antibodies in vaccine science is not arbitrary. Antibodies are measurable, mechanistically well-understood, and in many cases genuinely protective. For vaccines against pathogens that establish infection at mucosal surfaces — influenza, SARS-CoV-2, poliovirus — neutralising antibodies that prevent viral entry into host cells are a logical and effective correlate of protection. The success of antibody-based vaccines against these pathogens has reinforced the paradigm to the point where it has become, in many contexts, an unquestioned assumption.
Regulatory agencies including the FDA, EMA, and WHO have incorporated antibody titres into their frameworks for vaccine approval and lot release. Immunogenicity studies — which measure the magnitude and quality of antibody responses — are standard components of vaccine clinical trials. Correlates of protection studies, which seek to identify the immune response parameter that best predicts vaccine efficacy, have overwhelmingly focused on antibody metrics. This is not because cellular immunity is considered unimportant, but because antibody assays are technically simpler, more standardised, and more amenable to high-throughput screening than cellular immunity assays.
The limitation of this approach becomes apparent when vaccines are developed against pathogens for which antibody-mediated protection is not the primary mechanism — or, as the KU Leuven study suggests, not even a necessary mechanism. In such cases, the antibody paradigm does not merely fail to capture the full picture; it actively misleads vaccine developers and evaluators.
The KU Leuven Experiment: A Methodological Masterclass
The elegance of the KU Leuven study lies in its systematic dismantling of alternative hypotheses. Rather than simply demonstrating that CD4-T cells are present after vaccination — a finding that would be expected and unremarkable — the researchers used a series of depletion experiments to prove that CD4-T cells are necessary for protection. They eliminated NK cells, CD8-T cells, and CD4-T cells sequentially, testing vaccine efficacy after each depletion. Only the removal of CD4-T cells abrogated protection.
This approach — sometimes called "loss-of-function" or "depletion" immunology — is methodologically rigorous because it establishes necessity, not merely correlation. The finding that serum transfer (passive antibody transfer) did not confer protection further strengthened the conclusion that antibodies are not sufficient for protection in this model, even when present at high concentrations. Lara Kelchtermans summarised the key finding: "The protection relied on other mechanisms" — a statement that is both scientifically precise and paradigmatically significant.
Professor Kai Dallmeier added a crucial conceptual dimension: "Sometimes, it is not sufficient for a vaccine to block the initial infection. It is also important to temper the mechanism by which a particular pathogen drives disease." This observation points to a broader principle: vaccine efficacy is not a single-dimensional concept. A vaccine that prevents infection is different from one that prevents severe disease, which is different again from one that prevents death. The CD4-T cell mechanism identified in the KU Leuven study appears to operate at multiple levels simultaneously — blocking viral replication and modulating the immunopathological response that drives haemorrhagic disease.
Regulatory and Clinical Trial Design Implications
If the CD4-T cell mechanism of protection identified in mice translates to humans — a critical caveat that the researchers themselves acknowledge — the regulatory implications are substantial. Current vaccine approval pathways are designed around antibody-based correlates of protection. A vaccine that protects through cellular immunity but produces modest antibody responses might fail to meet regulatory benchmarks, even if it is genuinely efficacious. Conversely, a vaccine that produces high antibody titres but weak cellular immunity might be approved despite offering fragile protection.
Several concrete changes to vaccine evaluation frameworks would be warranted. First, cellular immunity assays — including intracellular cytokine staining, ELISpot assays for CD4-T cell cytokine production, and T cell proliferation assays — should be incorporated as standard endpoints in vaccine clinical trials for pathogens where the correlates of protection are unknown. Second, correlates of protection studies should be designed to capture multi-dimensional immune responses, using machine learning and multivariate statistical methods to identify the combination of immune parameters that best predicts protection. Third, regulatory agencies should develop guidance on how to evaluate vaccines that protect through cellular rather than humoral mechanisms, providing a clear pathway to approval for this class of vaccine.
Pandemic Preparedness: A Systemic Lesson
The COVID-19 pandemic demonstrated, with devastating clarity, the consequences of inadequate vaccine preparedness for emerging pathogens. Sudan virus is not currently a pandemic-scale threat — its outbreaks are geographically contained and relatively infrequent. But the same was true of SARS-CoV-2 before December 2019. The filoviral family, which includes Sudan virus, Ebola virus, Bundibugyo virus, and Taï Forest virus, represents a persistent and evolving threat to global health security, particularly in the context of climate change, deforestation, and increasing human-wildlife interfaces in Central and West Africa.
The KU Leuven findings have direct implications for pandemic preparedness. If CD4-T cell-mediated immunity is a general feature of filoviral vaccine protection — a hypothesis that requires further investigation — then the entire approach to filoviral vaccine stockpiling, clinical trial design, and regulatory approval needs to be reconsidered. Vaccine candidates that have been deprioritised because of modest antibody responses may warrant re-evaluation. New candidates should be designed with CD4-T cell induction as an explicit goal, using adjuvants and delivery platforms known to promote T helper responses.
A Biosafety and Biosecurity Perspective
From a biosafety and biosecurity standpoint, the KU Leuven study highlights the importance of investing in fundamental immunological research for BSL-4 pathogens, even in the absence of immediate outbreak pressure. The discovery that CD4-T cells mediate protection against Sudan virus was made possible by years of basic research infrastructure at the Rega Institute — including the development of a safe surrogate virus (VSV-SUD) that could be used in lower-containment settings, enabling the depletion experiments that were central to the study's conclusions.
This kind of translational research infrastructure — combining BSL-4 capability with innovative surrogate virus systems and rigorous immunological methodology — is precisely what is needed to accelerate vaccine development for neglected pathogens. It requires sustained investment, international collaboration, and a willingness to challenge established paradigms rather than simply refining them. The KU Leuven study is a model of what this investment can produce.
As Professor Dallmeier noted: "The protective role of CD4-T cells is known in cancer and chronic infections, but these cells have received little attention as direct antiviral protagonists in vaccine responses. Our new study suggests that they might play a far more important role than previously thought." This is not merely a scientific observation — it is an invitation to the global vaccine research community to expand its conceptual framework, invest in cellular immunity research, and build the regulatory infrastructure needed to evaluate and approve the next generation of vaccines.
The antibody is not dead. But it is no longer enough.