Heterologous versus homologous COVID-19 booster vaccinations for adults: systematic review with meta-analysis and trial sequential analysis of randomised clinical trials.

BACKGROUND: To combat coronavirus disease 2019 (COVID-19), booster vaccination strategies are important. However, the optimal administration of booster vaccine platforms remains unclear. Herein, we aimed to assess the benefits and harms of three or four heterologous versus homologous booster regimens. METHODS: From November 3 2022 to December 21, 2023, we searched five databases for randomised clinical trials (RCT). Reviewers screened, extracted data, and assessed bias risks independently with the Cochrane risk-of-bias 2 tool. We conducted meta-analyses and trial sequential analyses (TSA) on our primary (all-cause mortality; laboratory confirmed symptomatic and severe COVID-19; serious adverse events [SAE]) and secondary outcomes (quality of life [QoL]; adverse events [AE] considered non-serious). We assessed the evidence with the GRADE approach. Subgroup analyses were stratified for trials before and after 2023, three or four boosters, immunocompromised status, follow-up, risk of bias, heterologous booster vaccine platforms, and valency of booster. RESULTS: We included 29 RCTs with 43 comparisons (12,538 participants). Heterologous booster regimens may not reduce the relative risk (RR) of all-cause mortality (11 trials; RR 0.86; 95% CI 0.33 to 2.26; I2 0%; very low certainty evidence); laboratory-confirmed symptomatic COVID-19 (14 trials; RR 0.95; 95% CI 0.72 to 1.25; I2 0%; very low certainty); or severe COVID-19 (10 trials; RR 0.51; 95% CI 0.20 to 1.33; I2 0%; very low certainty). For safety outcomes, heterologous booster regimens may have no effect on SAE (27 trials; RR 1.15; 95% CI 0.68 to 1.95; I2 0%; very low certainty) but may raise AE considered non-serious (20 trials; RR 1.19; 95% CI 1.08 to 1.32; I2 64.4%; very low certainty). No data on QoL was available. Our TSAs showed that the cumulative Z curves did not reach futility for any outcome. CONCLUSIONS: With our current sample sizes, we were not able to infer differences of effects for any outcomes, but heterologous booster regimens seem to cause more non-serious AE. Furthermore, more robust data are instrumental to update this review.

Systemic infection in aged mice causes upregulation of crystallin alpha A in the RPE/choroid.

Aging changes the responsiveness of our immune defense, and this decline in immune reactivity plays an important role in the increased susceptibility to infections that marks progressing age. Aging is also the most pronounced risk factor for development of age-related macular degeneration (AMD), a disease that is characterized by dysfunctional retinal pigment epithelial (RPE) cells and loss of central vision. We have previously shown that acute systemic viral infection has a large impact on the retina in young mice, leading to upregulation of chemokines in the RPE/choroid (RPE/c) and influx of CD8 T cells in the neuroretina. In this study, we sought to investigate the impact of systemic infection on the RPE/c in aged mice to evaluate whether infection in old age could play a role in the pathogenesis of AMD. We found that systemic infection in mice led to upregulation of genes from the crystallin family in the RPE/c from aged mice, but not in the RPE/c from young mice. Crystallin alpha A (CRYAA) was the most upregulated gene, and increased amounts of CRYAA protein were also detected in the aged RPE/c. Increased CRYAA gene and protein expression has previously been found in drusen and choroid from AMD patients, and this protein has also been linked to neovascularization. Since both drusen and neovascularization are important hallmarks of advanced AMD, it is interesting to speculate if upregulation of crystallins in response to infection in old age could be relevant for the pathogenesis of AMD.

Systemic virus infection results in CD8 T cell recruitment to the retina in the absence of local virus infection.

During recent years, evidence has emerged that immune privileged sites such as the CNS and the retina may be more integrated in the systemic response to infection than was previously believed. In line with this, it was recently shown that a systemic acute virus infection leads to infiltration of CD8 T cells in the brains of immunocompetent mice. In this study, we extend these findings to the neurological tissue of the eye, namely the retina. We show that an acute systemic virus infection in mice leads to a transient CD8 T cell infiltration in the retina that is not directed by virus infection inside the retina. CD8 T cells were found throughout the retinal tissue, and had a high expression of CXCR6 and CXCR3, as also reported for tissue residing CD8 T cells in the lung and liver. We also show that the pigment epithelium lining the retina expresses CXCL16 (the ligand for CXCR6) similar to epithelial cells of the lung. Thus, our results suggest that the retina undergoes immune surveillance during a systemic infection, and that this surveillance appears to be directed by mechanisms similar to those described for non-privileged tissues.

Thiopurine 6TG treatment increases tumor immunogenicity and response to immune checkpoint blockade.

Immune-checkpoint inhibitors (ICI) are highly effective in reinvigorating T cells to attack cancer. Nevertheless, a large subset of patients fails to benefit from ICI, partly due to lack of the cancer neoepitopes necessary to trigger an immune response. In this study, we used the thiopurine 6-thioguanine (6TG) to induce random mutations and thus increase the level of neoepitopes presented by tumor cells. Thiopurines are prodrugs which are converted into thioguanine nucleotides that are incorporated into DNA (DNA-TG), where they can induce mutation through single nucleotide mismatching. In a pre-clinical mouse model of a mutation-low melanoma cell line, we demonstrated that 6TG induced clinical-grade DNA-TG integration resulting in an improved tumor control that was strongly T cell dependent. 6TG exposure increased the tumor mutational burden, without affecting tumor cell proliferation and cell death. Moreover, 6TG treatment re-shaped the tumor microenvironment by increasing T and NK immune cells, making the tumors more responsive to immune-checkpoint blockade. We further validated that 6TG exposure improved tumor control in additional mouse models of melanoma. These findings have paved the way for a phase I/II clinical trial that explores whether treatment with thiopurines can increase the proportion of otherwise treatment-resistant cancer patients who may benefit from ICI therapy (NCT05276284).

A survey of mechanisms underlying current and potential COVID-19 vaccines.

The emergence of SARS-CoV-2 caught the world off guard resulting in a global health crisis. Even though COVID-19 have caused the death of millions of people and many countries are still battling waves of infections, the odds of the pandemic ending soon have turned significantly in our favor. The key has been the development and distribution of a broad range of vaccines in record time. In this survey, we summarize the immunology required to understand the mechanisms underlying current and potential COVID-19 vaccines. Furthermore, we provide an up to date (according to data from WHO May 27, 2022) overview of the vaccine landscape consisting of 11 approved vaccines in phase 4, and a pipeline consisting of 161 vaccine candidates in clinical development and 198 in preclinical development (World Health Organization, Draft landscape and tracker of COVID-19 candidate vaccines [Internet], WHO, 2022). Our focus is to provide an understanding of the underlying biological mode of action of different vaccine platform designs, their advantages and disadvantages, rather than a deep dive into safety and efficacy data. We further present arguments concerning why a broad range of vaccines are needed and discuss future challenges.

Vaccine induced memory CD8+ T cells efficiently prevent viral transmission from the respiratory tract.

Introduction: Mucosal immunization eliciting local T-cell memory has been suggested for improved protection against respiratory infections caused by viral variants evading pre-existing antibodies. However, it remains unclear whether T-cell targeted vaccines suffice for prevention of viral transmission and to which extent local immunity is important in this context. Methods: To study the impact of T-cell vaccination on the course of viral respiratory infection and in particular the capacity to inhibit viral transmission, we used a mouse model involving natural murine parainfluenza infection with a luciferase encoding virus and an adenovirus based nucleoprotein targeting vaccine. Results and discussion: Prior intranasal immunization inducing strong mucosal CD8+ T cell immunity provided an almost immediate shut-down of the incipient infection and completely inhibited contact based viral spreading. If this first line of defense did not operate, as in parentally immunized mice, recirculating T cells participated in accelerated viral control that reduced the intensity of inter-individual transmission. These observations underscore the importance of pursuing the development of mucosal T-cell inducing vaccines for optimal protection of the individual and inhibition of inter-individual transmission (herd immunity), while at the same time explain why induction of a strong systemic T-cell response may still impact viral transmission.

Vaccines to prevent COVID-19: A living systematic review with Trial Sequential Analysis and network meta-analysis of randomized clinical trials.

BACKGROUND: COVID-19 is rapidly spreading causing extensive burdens across the world. Effective vaccines to prevent COVID-19 are urgently needed. METHODS AND FINDINGS: Our objective was to assess the effectiveness and safety of COVID-19 vaccines through analyses of all currently available randomized clinical trials. We searched the databases CENTRAL, MEDLINE, Embase, and other sources from inception to June 17, 2021 for randomized clinical trials assessing vaccines for COVID-19. At least two independent reviewers screened studies, extracted data, and assessed risks of bias. We conducted meta-analyses, network meta-analyses, and Trial Sequential Analyses (TSA). Our primary outcomes included all-cause mortality, vaccine efficacy, and serious adverse events. We assessed the certainty of evidence with GRADE. We identified 46 trials; 35 trials randomizing 219 864 participants could be included in our analyses. Our meta-analyses showed that mRNA vaccines (efficacy, 95% [95% confidence interval (CI), 92% to 97%]; 71 514 participants; 3 trials; moderate certainty); inactivated vaccines (efficacy, 61% [95% CI, 52% to 68%]; 48 029 participants; 3 trials; moderate certainty); protein subunit vaccines (efficacy, 77% [95% CI, -5% to 95%]; 17 737 participants; 2 trials; low certainty); and viral vector vaccines (efficacy 68% [95% CI, 61% to 74%]; 71 401 participants; 5 trials; low certainty) prevented COVID-19. Viral vector vaccines decreased mortality (risk ratio, 0.25 [95% CI 0.09 to 0.67]; 67 563 participants; 3 trials, low certainty), but comparable data on inactivated, mRNA, and protein subunit vaccines were imprecise. None of the vaccines showed evidence of a difference on serious adverse events, but observational evidence suggested rare serious adverse events. All the vaccines increased the risk of non-serious adverse events. CONCLUSIONS: The evidence suggests that all the included vaccines are effective in preventing COVID-19. The mRNA vaccines seem most effective in preventing COVID-19, but viral vector vaccines seem most effective in reducing mortality. Further trials and longer follow-up are necessary to provide better insight into the safety profile of these vaccines.

Efficient Control of Zika Virus Infection Induced by a Non-Replicating Adenovector Encoding Zika Virus NS1/NS2 Antigens Fused to the MHC Class II-Associated Invariant Chain.

It is generally believed that a successful Zika virus (ZIKV) vaccine should induce neutralizing antibodies against the ZIKV envelope (E) protein to efficiently halt viral infection. However, E-specific neutralizing antibodies have been implicated in a phenomenon called antibody-dependent enhancement, which represents an ongoing concern in the flavivirus-vaccinology field. In this report, we investigated the vaccination potential of replication-deficient adenoviral vectors encoding the ZIKV non-structural proteins 1 and 2 (NS1/NS2) and employed the strategy of linking the antigens to the MHC-II associated invariant chain (li) to improve immunogenicity and by inference, the level of protection. We demonstrated that li-linkage enhanced the production of anti-NS1 antibodies and induced an accelerated and prolonged polyfunctional CD8 T cell response in mice, which ultimately resulted in a high degree of protection against ZIKV infection of the CNS.

Inter-vendor variance of enteric eukaryotic DNA viruses in specific pathogen free C57BL/6N mice.

The laboratory mouse strain C57BL/6 is widely used as an animal model for various applications. It is becoming increasingly clear that the bacterial enteric community highly influences the phenotype. Eukaryotic viruses represent a sparsely investigated member of the enteric microbiome that might also affect the phenotype. We here investigated the presence of enteric eukaryotic DNA viruses (EDVs) in specific pathogen-free (SPF) C57BL/6N mice purchased from three vendors upon arrival and after being fed a low-fat diet (LFD) or high-fat diet (HFD). We detected genetic fragments of EDVs belonging to the viral families of Herpes-, Mimi-, Baculo- and Phycodnaviridae represented by two genera; Chlorovirus and Prasinovirus. The EDVs were detected in the mice upon arrival and persisted for 13 weeks. However, these signals of EDVs were only detected at notable levels in mice fed LFD from 2 out of 3 vendors, which suggested that the enteric composition of these EDVs were affected by both vendor (p < 0.003) and different dietary regimes (p < 0.013). This highlights the need of additional studies assessing the potential function of these EDVs that may influence the mouse phenotype and the reproducibility of animal studies using this C57BL/6N substrain.

Vaccines to prevent COVID-19: a protocol for a living systematic review with network meta-analysis including individual patient data (The LIVING VACCINE Project).

BACKGROUND: Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes coronavirus disease 2019 (COVID-19) which has rapidly spread worldwide. Several human randomized clinical trials assessing potential vaccines are currently underway. There is an urgent need for a living systematic review that continuously assesses the beneficial and harmful effects of all available vaccines for COVID-19. METHODS/DESIGN: We will conduct a living systematic review based on searches of major medical databases (e.g., MEDLINE, EMBASE, CENTRAL) and clinical trial registries from their inception onwards to identify relevant randomized clinical trials. We will update the literature search once a week to continuously assess if new evidence is available. Two review authors will independently extract data and conduct risk of bias assessments. We will include randomized clinical trials comparing any vaccine aiming to prevent COVID-19 (including but not limited to messenger RNA; DNA; non-replicating viral vector; replicating viral vector; inactivated virus; protein subunit; dendritic cell; other vaccines) with any comparator (placebo; "active placebo;" no intervention; standard care; an "active" intervention; another vaccine for COVID-19) for participants in all age groups. Primary outcomes will be all-cause mortality; a diagnosis of COVID-19; and serious adverse events. Secondary outcomes will be quality of life and non-serious adverse events. The living systematic review will include aggregate data meta-analyses, trial sequential analyses, network meta-analyses, and individual patient data meta-analyses. Within-study bias will be assessed using Cochrane risk of bias tool. The Grading of Recommendations, Assessment, Development and Evaluations (GRADE) and Confidence in Network Meta-Analysis (CINeMA) approaches will be used to assess certainty of evidence. Observational studies describing harms identified during the search for trials will also be included and described and analyzed separately. DISCUSSION: COVID-19 has become a pandemic with substantial mortality. A living systematic review assessing the beneficial and harmful effects of different vaccines is urgently needed. This living systematic review will regularly inform best practice in vaccine prevention and clinical research of this highly prevalent disease. SYSTEMATIC REVIEW REGISTRATION: PROSPERO CRD42020196492.

Effector CD8 T Cell-Dependent Zika Virus Control in the CNS: A Matter of Time and Numbers.

Zika virus (ZIKV), a mosquito-borne flavivirus, came into the spotlight in 2016 when it was found to be associated with an increased rate of microcephalic newborns in Brazil. The virus has further been recognized to cause neurologic complications in children and adults in the form of myelitis, encephalitis, acute disseminated encephalomyelitis (ADEM) and Guillain Barre Syndrome in a fraction of infected individuals. With the ultimate goal of identifying correlates of protection to guide the design of an effective vaccine, the study of the immune response to ZIKV infection has become the focus of research worldwide. Both innate and adaptive immune responses seem to be essential for controlling the infection. Induction of sufficient levels of neutralizing antibodies has been strongly correlated with protection against reinfection in various models, while the role of CD8 T cells as antiviral effectors in the CNS has been controversial. In an attempt to improve our understanding regarding the role of ZIKV-induced CD8 T cells in protective immunity inside the CNS, we have expanded on previous studies in intracranially infected mice. In a recent study, we have demonstrated that, peripheral ZIKV infection in adult C57BL/6 mice induces a robust CD8 T cell response that peaks within a week. In the present study, we used B cell deficient as well as wild-type mice to show that there is a race between CXCR3-dependent recruitment of the effector CD8 T cells and local ZIKV replication, and that CD8 T cells are capable of local viral control if they arrive in the brain early after viral invasion, in appropriate numbers and differentiation state. Our data highlight the benefits of considering this subset when designing vaccines against Zika virus.

MHC class II invariant chain-adjuvanted viral vectored vaccines enhances T cell responses in humans.

Strategies to enhance the induction of high magnitude T cell responses through vaccination are urgently needed. Major histocompatibility complex (MHC) class II-associated invariant chain (Ii) plays a critical role in antigen presentation, forming MHC class II peptide complexes for the generation of CD4+ T cell responses. Preclinical studies evaluating the fusion of Ii to antigens encoded in vector delivery systems have shown that this strategy may enhance T cell immune responses to the encoded antigen. We now assess this strategy in humans, using chimpanzee adenovirus 3 and modified vaccinia Ankara vectors encoding human Ii fused to the nonstructural (NS) antigens of hepatitis C virus (HCV) in a heterologous prime/boost regimen. Vaccination was well tolerated and enhanced the peak magnitude, breadth, and proliferative capacity of anti-HCV T cell responses compared to non-Ii vaccines in humans. Very high frequencies of HCV-specific T cells were elicited in humans. Polyfunctional HCV-specific CD8+ and CD4+ responses were induced with up to 30% of CD3+CD8+ cells targeting single HCV epitopes; these were mostly effector memory cells with a high proportion expressing T cell activation and cytolytic markers. No volunteers developed anti-Ii T cell or antibody responses. Using a mouse model and in vitro experiments, we show that Ii fused to NS increases HCV immune responses through enhanced ubiquitination and proteasomal degradation. This strategy could be used to develop more potent HCV vaccines that may contribute to the HCV elimination targets and paves the way for developing class II Ii vaccines against cancer and other infections.

Functionally Competent, PD-1+ CD8+ Trm Cells Populate the Brain Following Local Antigen Encounter.

Expression of programmed cell death-1 receptor (PD-1) has traditionally been linked to T-cell exhaustion, as signaling via PD-1 dampens the functionality of T-cells upon repetitive antigen exposures during chronic infections. However, resent findings pointing to the involvement of PD-1 both in T-cell survival and in restraining immunopathology, challenge the concept of PD-1 solely as marker for T-cell exhaustion. Tissue resident memory T cells (Trms) hold unique effector qualities, but within a delicate organ like the CNS, these protective abilities could potentially be harmful. In contrast to their counterparts in many other tissues, brain derived CD8+ Trms have been found to uniformly and chronically express PD-1. In this study we utilized a recently established model system for generating CNS Trms in order to improve our understanding regarding the role of PD-1 expression by Trms inside the CNS. By intracerebral (i.c.) inoculation with a non-replicating adeno-viral vector, we induced a PD-1hi CD8+ T cell memory population within the CNS. We found that PD-1 expression lowered the severity of clinical disease associated with the i.c. inoculation. Furthermore, high levels of PD-L1 expression were found on the infiltrating monocytes and macrophages as well as on the resident microglia, oligodendrocytes and astrocytes during the acute phase of the response. Additionally, we showed that the intensity of PD-1 expression correlates with local antigen encounter and found that PD-1 expression was associated with decreased CD8+ T cell memory formation in the CNS despite an increased number of infiltrating CD8+ T cells. Most importantly, our experiments revealed that despite expression of PD-1 and several additional markers linked to T-cell exhaustion, Tim-3, Lag-3 and CD39, the cells did not show signs of limited effector capacity. Collectively, these results endorse the increasing amount of evidence pointing to an immune-modifying role for PD-1 expression within the CNS, a mechanism we found to correlate with local antigen exposure.

Chemokine Expression in Murine RPE/Choroid in Response to Systemic Viral Infection and Elevated Levels of Circulating Interferon-γ.

Purpose: To examine how circulating immune mediators in vivo may affect gene and protein expression at the RPE/choroid interface. Methods: Young mice were systemically infected with lymphocytic choriomeningitis virus (LCMV) or continuously infused with IFN-γ. RPE/choroid was isolated and analyzed with whole-transcriptome gene expression microarrays. Selected gene expression findings were validated at the protein level. Results: Both the systemic immune activation from virus infection and the sterile systemically increased level of IFN-γ resulted in increased expression of chemokine ligands, chemokine receptors, and early complement components in isolates of RPE/choroid. These findings were largely absent from LCMV-infected mice deficient in either the interferon α/ß receptor or IFN-γ. Conclusions: Together, these findings demonstrate that acute systemic immune activation results in a local response at the RPE/choroid interface that may include chemokine-dependent recruitment of inflammatory cells and engagement of the complement system. This may represent a link between the systemic low-grade inflammation and the retinal pathology observed in several multifactorial entities such as aging, AMD, and diabetes.

Lipocalin-2 Functions as Inhibitor of Innate Resistance to Mycobacterium tuberculosis.

Lipocalin-2 is a constituent of the neutrophil secondary granules and is expressed de novo by macrophages and epithelium in response to inflammation. Lipocalin-2 acts in a bacteriostatic fashion by binding iron-loaded siderophores required for bacterial growth. Mycobacterium tuberculosis (M.tb) produces siderophores that can be bound by lipocalin-2. The impact of lipocalin-2 in the innate immune response toward extracellular bacteria has been established whereas the effect on intracellular bacteria, such as M.tb, is less well-described. Here we show that lipocalin-2 surprisingly confers a growth advantage on M.tb in the early stages of infection (3 weeks post-challenge). Using mixed bone marrow chimeras, we demonstrate that lipocalin-2 derived from granulocytes, but not from epithelia and macrophages, leads to increased susceptibility to M.tb infection. In contrast, lipocalin-2 is not observed to promote mycobacterial growth at later stages of M.tb infection. We demonstrate co-localization of granulocytes and mycobacteria within the nascent granulomas at week 3 post-challenge, but not in the consolidated granulomas at week 5. We hypothesize that neutrophil-derived lipocalin-2 acts to supply a source of iron to M.tb in infected macrophages within the immature granuloma, thereby facilitating mycobacterial growth.

A New In Vivo Model to Study Protective Immunity to Zika Virus Infection in Mice With Intact Type I Interferon Signaling.

The association between recent Zika virus (ZIKV) infection and neurological complications, microcephaly in the fetus, and Guillain-Barré syndrome in adults underscores the necessity for a protective vaccine. Rational vaccine development requires an in-depth understanding of the mechanisms which could protect against infection with this virus. However, so far, such an analysis has been hampered by the absence of a suitable small animal model. Unlike the situation in humans, ZIKV only replicates effectively in the peripheral organs of mice, if type I IFN signaling is interrupted. As type I IFN also impacts the adaptive immune response, mice with such a defect are not optimal for a comprehensive immunological analysis. In this report, we show that even in wild-type (WT) mice i.c. infection with low doses of virus causes marked local virus replication and lethal encephalitis in naïve mice. Furthermore, peripheral infection of WT mice with low doses of virus induces a significant immune response, which provides long-lasting protection of WT mice from a fatal outcome of subsequent i.c. challenge. Therefore, combining peripheral priming with later i.c. challenge represents a new approach for studying the adaptive immune response to ZIKV in mice with an intact type I IFN response. In this study, we focused on the mechanisms underlying resistance to reinfection. Using a combination of adoptive transfer, antibody-based cell depletion, and gene targeting, we show that the key protective factor in type I IFN replete mice is humoral immunity. CD8 T cells are not essential in mice with preformed specific antibodies, but under conditions where initial antibody levels are low, effector CD8 T cells may play a role as a back-up system. These results have important implications for our understanding of natural immunity to ZIKV infection and for Zika vaccine design.

Analysis of adenovirus-induced immunity to infection with Listeria monocytogenes: Fading protection coincides with declining CD8 T cell numbers and phenotypic changes.

Defining correlates of T cell mediated protection is important in order to accelerate the development of efficient T cell based vaccines conferring long-term immunity. Extensive studies have provided important insight regarding the characteristics and functional properties of the effector and memory CD8 T cells induced by viral vector based vaccines. However, long-term protection has been difficult to achieve with T cell inducing vaccines, and the determinants underlying this loss in protection over time are still not fully defined. In this study we analyzed different parameters of the CD8 T cell response as a function of time after vaccination with a human serotype 5 adenovector expressing the glycoprotein (GP) of LCMV tethered to the MHC class II-associated invariant chain. Using this vector we have previously found that CD8 T cells mediate protection from challenge with GP-expressing Listeria monocytogenes at 60 days post vaccination, but only little protection after further 60 days, and we now confirm this observation. A comparison of vaccine-primed CD8 T cells early and late after vaccination revealed a minor decline in the overall numbers of antigen specific memory CD8 T cells during this interval. More importantly, we also observed phenotypic changes over time with a distinct decline in the frequency and number of KLRG1+ CD8 T cells, and, notably, adoptive transfer studies confirmed that memory CD8 T cells expressing KLRG1 are central to protection from systemic L. monocytogenes infection. Together these findings imply that multiple factors including changes in memory T cell numbers and phenotypic composition over time influence the longevity of CD8 T-cell mediated protection.

Adaptive immune responses to booster vaccination against yellow fever virus are much reduced compared to those after primary vaccination.

Outbreaks of Yellow Fever occur regularly in endemic areas of Africa and South America frequently leading to mass vaccination campaigns straining the availability of the attenuated Yellow Fever vaccine, YF-17D. The WHO has recently decided to discontinue regular booster-vaccinations since a single vaccination is deemed to confer life-long immune protection. Here, we have examined humoral (neutralizing antibody) and cellular (CD8 and CD4 T cell) immune responses in primary and booster vaccinees (the latter spanning 8 to 36 years after primary vaccination). After primary vaccination, we observed strong cellular immune responses with T cell activation peaking ≈2 weeks and subsiding to background levels ≈ 4 weeks post-vaccination. The number of antigen-specific CD8+ T cells declined over the following years. In >90% of vaccinees, in vitro expandable T cells could still be detected >10 years post-vaccination. Although most vaccinees responded to a booster vaccination, both the humoral and cellular immune responses observed following booster vaccination were strikingly reduced compared to primary responses. This suggests that pre-existing immunity efficiently controls booster inoculums of YF-17D. In a situation with epidemic outbreaks, one could argue that a more efficient use of a limited supply of the vaccine would be to focus on primary vaccinations.