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Antigen-specific, MHC-restricted αβ T cells are necessary for protective immunity against Mycobacterium tuberculosis, but the ability to broadly study these responses has been limited. In the present study, we used single-cell and bulk T cell receptor (TCR) sequencing and the GLIPH2 algorithm to analyze M. tuberculosis-specific sequences in two longitudinal cohorts, comprising 166 individuals with M. tuberculosis infection who progressed to either tuberculosis (n = 48) or controlled infection (n = 118). We found 24 T cell groups with similar TCR-β sequences, predicted by GLIPH2 to have common TCR specificities, which were associated with control of infection (n = 17), and others that were associated with progression to disease (n = 7). Using a genome-wide M. tuberculosis antigen screen, we identified peptides targeted by T cell similarity groups enriched either in controllers or in progressors. We propose that antigens recognized by T cell similarity groups associated with control of infection can be considered as high-priority targets for future vaccine development.

Background Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection may be associated with worse clinical outcomes in people with human immunodeficiency virus (HIV) (PWH). We report anti–SARS-CoV-2 antibody responses in patients hospitalized with coronavirus disease 2019 in Durban, South Africa, during the second SARS-CoV-2 infection wave dominated by the Beta (B.1.351) variant. Methods Thirty-four participants with confirmed SARS-CoV-2 infection were followed up with weekly blood sampling to examine antibody levels and neutralization potency against SARS-CoV-2 variants. Participants included 18 PWH, of whom 11 were HIV viremic. Results SARS-CoV-2–specific antibody concentrations were generally lower in viremic PWH than in virologically suppressed PWH and HIV-negative participants, and neutralization of the Beta variant was 4.9-fold lower in viremic PWH. Most HIV-negative participants and antiretroviral therapy–suppressed PWH also neutralized the Delta (B.1.617.2) variant, whereas the majority of viremic PWH did not. CD4 cell counts Conclusions HIV viremia was associated with reduced Beta variant neutralization. This highlights the importance of HIV suppression in maintaining an effective SARS-CoV-2 neutralization response.

Background:HIV infection dysregulates the B cell compartment, affecting memory B cell formation and the antibody response to infection and vaccination. Understanding the B cell response to SARS-CoV-2 in people living with HIV (PLWH) may explain the increased morbidity, reduced vaccine efficacy, reduced clearance, and intra-host evolution of SARS-CoV-2 observed in some HIV-1 coinfections.Methods:We compared B cell responses to COVID-19 in PLWH and HIV negative (HIV-ve) patients in a cohort recruited in Durban, South Africa, during the first pandemic wave in July 2020 using detailed flow cytometry phenotyping of longitudinal samples with markers of B cell maturation, homing, and regulatory features.Results:This revealed a coordinated B cell response to COVID-19 that differed significantly between HIV-ve and PLWH. Memory B cells in PLWH displayed evidence of reduced germinal centre (GC) activity, homing capacity, and class-switching responses, with increased PD-L1 expression, and decreased Tfh frequency. This was mirrored by increased extrafollicular (EF) activity, with dynamic changes in activated double negative (DN2) and activated naïve B cells, which correlated with anti-RBD-titres in these individuals. An elevated SARS-CoV-2-specific EF response in PLWH was confirmed using viral spike and RBD bait proteins.Conclusions:Despite similar disease severity, these trends were highest in participants with uncontrolled HIV, implicating HIV in driving these changes. EF B cell responses are rapid but give rise to lower affinity antibodies, less durable long-term memory, and reduced capacity to adapt to new variants. Further work is needed to determine the long-term effects of HIV on SARS-CoV-2 immunity, particularly as new variants emerge.Funding:This work was supported by a grant from the Wellcome Trust to the Africa Health Research Institute (Wellcome Trust Strategic Core Award [grant number 201433/Z/16/Z]). Additional funding was received from the South African Department of Science and Innovation through the National Research Foundation (South African Research Chairs Initiative [grant number 64809]), and the Victor Daitz Foundation.

SARS-CoV-2 Omicron (B.1.1.529) BA.4 and BA.5 sub-lineages, first detected in South Africa, have changes relative to Omicron BA.1 including substitutions in the spike receptor binding domain. Here we isolated live BA.4 and BA.5 viruses and measured BA.4/BA.5 neutralization elicited by BA.1 infection either in the absence or presence of previous vaccination as well as from vaccination without BA.1 infection. In BA.1-infected unvaccinated individuals, neutralization relative to BA.1 declines 7.6-fold for BA.4 and 7.5-fold for BA.5. In vaccinated individuals with subsequent BA.1 infection, neutralization relative to BA.1 decreases 3.2-fold for BA.4 and 2.6-fold for BA.5. The fold-drop versus ancestral virus neutralization in this group is 4.0-fold for BA.1, 12.9-fold for BA.4, and 10.3-fold for BA.5. In contrast, BA.4/BA.5 escape is similar to BA.1 in the absence of BA.1 elicited immunity: fold-drop relative to ancestral virus neutralization is 19.8-fold for BA.1, 19.6-fold for BA.4, and 20.9-fold for BA.5. These results show considerable escape of BA.4/BA.5 from BA.1 elicited immunity which is moderated with vaccination and may indicate that BA.4/BA.5 may have the strongest selective advantage in evading neutralization relative to BA.1 in unvaccinated, BA.1 infected individuals.

BACKGROUND: Universal test-and-treat trials increased population-level virological suppression across trial sites in sub-Saharan Africa. We followed the ANRS 12249 TasP trial population for 6 years to determine whether the intervention had longer-term survival benefits.METHODS: The TasP trial was a cluster-randomised trial implemented in 22 communities in rural South Africa, from 2012’2016. Households were offered six-monthly home-based HIV testing. Immediate antiretroviral therapy (ART) was offered in trial clinics to all people living with HIV (PLHIV) in the intervention clusters and according to national guidelines in the control clusters. At trial end, individuals attending the intervention clinics were transferred to the public ART programme, with a ’treat-all’ strategy adopted in September 2016. Deaths during and two years after trial end were ascertained through annual demographic surveillance. Random effects Poisson regression was used to estimate rate ratios (RR) and 95%CI for the effect of trial arm on mortality among i) all PLHIV regardless of serostatus awareness, ii) PLHIV aware of their status, iii) those not on ART at entry to trial clinics. An interaction term between period and treatment arm was included, to allow the effect of trial arm to differ between periods.RESULTS: Amongst all PLHIV and those aware of their serostatus, there was no effect of immediate ART on mortality (Table). Among individuals who started ART during the trial, there was evidence that the intervention decreased mortality (aRR=0.69, 95%CI=0.45-1.04, p=0.08), although the effect was primarily during the trial (aRR=0.49, 95%CI=0.28-0.85, p=0.01), but not after the trial ended (aRR=1.15, 95%CI=0.59-2.21, p=0.69).CONCLUSIONS: The ’treat-all’ strategy resulted in a mortality benefit amongst individuals who started ART within the trial but not in all PLHIV over 6 years of follow-up. To achieve maximum benefit of immediate ART in South Africa, barriers to ART uptake and retention in care need to be addressed.

A coordinated system of disease surveillance will be critical to effectively control the coronavirus disease 2019 (Covid-19) pandemic. Such systems enable rapid detection and mapping of epidemics and inform allocation of scarce prevention and intervention resources. Although many lower- and middle-income settings lack infrastructure for optimal disease surveillance, health and demographic surveillance systems (HDSS) provide a unique opportunity for epidemic monitoring. This protocol describes a surveillance program at the Africa Health Research Institute’s Population Intervention Platform site in northern KwaZulu-Natal. The program leverages a longstanding HDSS in a rural, resource-limited setting with very high prevalence of HIV and tuberculosis to perform Covid-19 surveillance. Our primary aims include: describing the epidemiology of the Covid-19 epidemic in rural KwaZulu-Natal; determining the impact of the Covid-19 outbreak and non-pharmaceutical control interventions (NPI) on behaviour and wellbeing; determining the impact of HIV and tuberculosis on Covid-19 susceptibility; and using collected data to support the local public-sector health response. The program involves telephone-based interviews with over 20,000 households every four months, plus a sub-study calling 750 households every two weeks. Each call asks a household representative how the epidemic and NPI are affecting the household and conducts a Covid-19 risk screen for all resident members. Any individuals screening positive are invited to a clinical screen, potential test and referral to necessary care – conducted in-person near their home following careful risk minimization procedures. In this protocol we report the details of our cohort design, questionnaires, data and reporting structures, and standard operating procedures in hopes that our project can inform similar efforts elsewhere.

Characterizing SARS-CoV-2 evolution in specific geographies may help predict properties of the variants that come from these regions. We mapped neutralization of a SARS-CoV-2 strain that evolved over 6 months from ancestral virus in a person with advanced HIV disease in South Africa; this person was infected prior to emergence of the Beta and Delta variants. We longitudinally tracked the evolved virus and tested it against self-plasma and convalescent plasma from ancestral, Beta, and Delta infections. Early virus was similar to ancestral, but it evolved a multitude of mutations found in Omicron and other variants. It showed substantial but incomplete Pfizer BNT162b2 escape, weak neutralization by self-plasma, and despite pre-dating Delta, it also showed extensive escape of Delta infection-elicited neutralization. This example is consistent with the notion that SARS-CoV-2 evolving in individual immune-compromised hosts, including those with advanced HIV disease, may gain immune escape of vaccines and enhanced escape of Delta immunity, and this has implications for vaccine breakthrough and reinfections., Graphical abstract, Cele et al. examine a SARS-CoV-2 infection persisting over 6 months, starting as ancestral virus but evolving various mutations found in Omicron and other variants. The evolved virus substantially but incompletely escaped BNT162b2-elicited immunity as well as neutralization by self-plasma and showed extensive escape from neutralization elicited by Delta infections.

HIV-1 infection substantially increases the risk of developing tuberculosis (TB). There is extensive depletion of Mycobacterium tuberculosis (M.tuberculosis)-specific CD4+ T cells in blood in early HIV infection, but little is known about responses in the lungs at this stage. Given that mucosal organs are a principal target for HIV-mediated CD4 destruction, we investigated M.tuberculosis-specific responses in bronchoalveolar lavage (BAL), in persons with latent TB infection and untreated HIV-1 co-infection with preserved CD4 counts. M.tuberculosis-specific CD4+ cytokine responses (IFN-, TNF- and IL-2) were discordant in frequency and function between BAL and blood. Responses in BAL were 15-fold lower in HIV-infected compared to uninfected persons (p=0.048), whilst blood responses were 2-fold lower (p=0.006). However, an increase in T cells in the airways in HIV-infected persons resulted in the overall number of M.tuberculosis-specific CD4+ cells in BAL being similar. Our study highlights the important insights gained from studying TB immunity at the site of disease during HIV infection.

Background: Childhood tuberculosis (TB) is usually Mycobacterium tuberculosis (MTB) culture negative. Furthermore, clinical presentation may be altered by active case finding, isoniazid prophylaxis and early treatment. We aimed to establish the value of presenting symptoms for intrathoracic TB case diagnosis among young children. Methods: Healthy, HIV-uninfected, South African infants in an efficacy trial of a novel TB vaccine (MVA85A) were followed for 2 years for suspected TB. When suspected, investigation followed a standardized algorithm comprising symptom history, QuantiFERON Gold-in-Tube, chest radiography (CXR), MTB culture and Xpert MTB/RIF from paired gastric lavage and induced sputa. Adjusted odds ratios and 95% confidence intervals describe the associations between symptoms and positive MTB culture or Xpert MTB/RIF, and CXR compatible with intrathoracic TB. Results: Persistent cough was present in 172/1017 (16.9%) of the children investigated for TB. MTB culture/Xpert MTB/RIF was positive in 38/1017 children (3.7%); and CXR was positive, that is, compatible with intrathoracic TB, in 131/1017 children (12.9%). Children with persistent cough had more than triple the odds of a positive MTB culture/Xpert MTB/RIF (adjusted odds ratios: 3.3, 95% confidence interval: 1.5–7.0) and positive CXR (adjusted odds ratios: 3.5, 95% confidence interval: 2.2–5.5). Persistent cough was the only symptom that differentiated children with severe (56.5%) from nonsevere intrathoracic TB disease (28.2%; P = 0.001). Conclusion: Persistent cough was the cardinal diagnostic symptom associated with microbiologic and radiologic evidence, and disease severity, of intrathoracic TB. Symptom-based definitions of TB disease for diagnostic, preventive and therapeutic studies should prioritize persistent cough above other symptoms compatible with childhood TB.

Summary Background BCG vaccination provides incomplete protection against tuberculosis in infants. A new vaccine, modified Vaccinia Ankara virus expressing antigen 85A (MVA85A), was designed to enhance the protective efficacy of BCG. We aimed to assess safety, immunogenicity, and efficacy of MVA85A against tuberculosis and Mycobacterium tuberculosis infection in infants. Methods In our double-blind, randomised, placebo-controlled phase 2b trial, we enrolled healthy infants (aged 4–6 months) without HIV infection who had previously received BCG vaccination. We randomly allocated infants (1:1), according to an independently generated sequence with block sizes of four, to receive one intradermal dose of MVA85A or an equal volume of Candida skin test antigen as placebo at a clinical facility in a rural region near Cape Town, South Africa. We actively followed up infants every 3 months for up to 37 months. The primary study outcome was safety (incidence of adverse and serious adverse events) in all vaccinated participants, but we also assessed efficacy in a protocol-defined group of participants who received at least one dose of allocated vaccine. The primary efficacy endpoint was incident tuberculosis incorporating microbiological, radiological, and clinical criteria, and the secondary efficacy endpoint was M tuberculosis infection according to QuantiFERON TB Gold In-tube conversion (Cellestis, Australia). This trial was registered with the South African National Clinical Trials Register (DOH-27-0109-2654) and with ClinicalTrials.gov on July 31, 2009, number NCT00953927 Findings Between July 15, 2009, and May 4, 2011, we enrolled 2797 infants (1399 allocated MVA85A and 1398 allocated placebo). Median follow-up in the per-protocol population was 24·6 months (IQR 19·2–28·1), and did not differ between groups. More infants who received MVA85A than controls had at least one local adverse event (1251 [89%] of 1399 MVA85A recipients and 628 [45%] of 1396 controls who received the allocated intervention) but the numbers of infants with systemic adverse events (1120 [80%] and 1059 [76%]) or serious adverse events (257 [18%] and 258 (18%) did not differ between groups. None of the 648 serious adverse events in these 515 infants was related to MVA85A. 32 (2%) of 1399 MVA85A recipients met the primary efficacy endpoint (tuberculosis incidence of 1·15 per 100 person-years [95% CI 0·79 to 1·62]; with conversion in 178 [13%] of 1398 infants [95% CI 11·0 to 14·6]) as did 39 (3%) of 1395 controls (1·39 per 100 person-years [1·00 to 1·91]; with conversion in 171 [12%] of 1394 infants [10·6 to 14·1]). Efficacy against tuberculosis was 17·3% (95% CI −31·9 to 48·2) and against M tuberculosis infection was −3·8% (–28·1 to 15·9). Interpretation MVA85A was well tolerated and induced modest cell-mediated immune responses. Reasons for the absence of MVA85A efficacy against tuberculosis or M tuberculosis infection in infants need exploration. Funding Aeras, Wellcome Trust, and Oxford-Emergent Tuberculosis Consortium (OETC).

We write in response to the article by Kashangura et al. who conducted a systematic review of the animal efficacy data concerning MVA85A, a candidate TB vaccine.

Vaccines to protect against tuberculosis (TB) are urgently needed. We performed a case–control analysis to identify immune correlates of TB disease risk in Bacille Calmette–Guerin (BCG) immunized infants from the MVA85A efficacy trial. Among 53 TB case infants and 205 matched controls, the frequency of activated HLA-DR+ CD4+ T cells associates with increased TB disease risk (OR=1.828, 95% CI=1.25–2.68, P=0.002, FDR=0.04, conditional logistic regression). In an independent study of Mycobacterium tuberculosis-infected adolescents, activated HLA-DR+ CD4+ T cells also associate with increased TB disease risk (OR=1.387, 95% CI=1.068–1.801, P=0.014, conditional logistic regression). In infants, BCG-specific T cells secreting IFN-γ associate with reduced risk of TB (OR=0.502, 95% CI=0.29–0.86, P=0.013, FDR=0.14). The causes and impact of T-cell activation on disease risk should be considered when designing and testing TB vaccine candidates for these populations., BCG vaccine confers only partial protection against tuberculosis. Here the authors show that the risk of tuberculosis infection and progression to disease in BCG-immunized children positively correlates with the frequency of activated HLA-DR+CD4+ T cells.

HIV-1-infected people have an increased risk of developing extrapulmonary tuberculosis (TB), the immunopathogenesis of which is poorly understood. Here, we conducted a detailed immunological analysis of human pericardial TB, to determine the effect of HIV-1 co-infection on the phenotype of Mycobacterium tuberculosis (MTB)-specific memory T cells and the role of polyfunctional T cells at the disease site, using cells from pericardial fluid and blood of 74 patients with (n = 50) and without (n = 24) HIV-1 co-infection. The MTB antigen-induced IFN-γ response was elevated at the disease site, irrespective of HIV-1 status or antigenic stimulant. However, the IFN-γ ELISpot showed no clear evidence of increased numbers of antigen-specific cells at the disease site except for ESAT-6 in HIV-1 uninfected individuals (p = 0.009). Flow cytometric analysis showed that CD4+ memory T cells in the pericardial fluid of HIV-1-infected patients were of a less differentiated phenotype, with the presence of polyfunctional CD4+ T cells expressing TNF, IL-2 and IFN-γ. These results indicate that HIV-1 infection results in altered phenotype and function of MTB-specific CD4+ T cells at the disease site, which may contribute to the increased risk of developing TB at all stages of HIV-1 infection.

Background: Childhood tuberculosis (TB) is usually Mycobacterium tuberculosis (MTB) culture negative. Furthermore, clinical presentation may be altered by active case finding, isoniazid prophylaxis and early treatment. We aimed to establish the value of presenting symptoms for intrathoracic TB case diagnosis among young children. Methods: Healthy, HIV-uninfected, South African infants in an efficacy trial of a novel TB vaccine (MVA85A) were followed for 2 years for suspected TB. When suspected, investigation followed a standardized algorithm comprising symptom history, QuantiFERON Gold-in-Tube, chest radiography (CXR), MTB culture and Xpert MTB/RIF from paired gastric lavage and induced sputa. Adjusted odds ratios and 95% confidence intervals describe the associations between symptoms and positive MTB culture or Xpert MTB/RIF, and CXR compatible with intrathoracic TB. Results: Persistent cough was present in 172/1017 (16.9%) of the children investigated for TB. MTB culture/Xpert MTB/RIF was positive in 38/1017 children (3.7%); and CXR was positive, that is, compatible with intrathoracic TB, in 131/1017 children (12.9%). Children with persistent cough had more than triple the odds of a positive MTB culture/Xpert MTB/RIF (adjusted odds ratios: 3.3, 95% confidence interval: 1.5–7.0) and positive CXR (adjusted odds ratios: 3.5, 95% confidence interval: 2.2–5.5). Persistent cough was the only symptom that differentiated children with severe (56.5%) from nonsevere intrathoracic TB disease (28.2%; P = 0.001). Conclusion: Persistent cough was the cardinal diagnostic symptom associated with microbiologic and radiologic evidence, and disease severity, of intrathoracic TB. Symptom-based definitions of TB disease for diagnostic, preventive and therapeutic studies should prioritize persistent cough above other symptoms compatible with childhood TB.

Heterologous prime-boost strategies hold promise for vaccination against tuberculosis. However, the T-cell characteristics required for protection are not known. We proposed that boost vaccines should induce long-lived functional and phenotypic changes to T cells primed by Bacille Calmette Guerin (BCG) and/or natural exposure to mycobacteria. We characterized changes among specific CD4(+) T cells after vaccination with the MVA85A vaccine in adults, adolescents, and children. CD4(+) T cells identified with Ag85A peptide-bearing HLA class II tetramers were characterized by flow cytometry. We also measured proliferative potential and cytokine expression of Ag85A-specific CD4(+) T cells. During the effector phase, MVA85A-induced specific CD4(+) T cells coexpressed IFN-γ and IL-2, skin homing integrins, and the activation marker CD38. This was followed by contraction and a transition to predominantly IL-2-expressing, CD45RA(-) CCR7(+) CD27(+) or CD45RA(+) CCR7(+) CD27(+) specific CD4(+) T cells. These surface phenotypes were similar to Ag85A-specific T cells prior to MVA85A. However, functional differences were observed postvaccination: specific proliferative capacity was markedly higher after 6-12 months than before vaccination. Our data suggest that MVA85A vaccination may modulate Ag85A-specific CD4(+) T-cell function, resulting in greater recall potential. Importantly, surface phenotypes commonly used as proxies for memory T-cell function did not associate with functional effects of vaccination.

The extent of immune reactivity measured by the tuberculin skin test (TST) and interferon-gamma (IFN-gamma) T-cell assays is usually not analysed.To determine the impact of age and sex on assay positivity and on the extent of reactivity of both TST and T-cell assays in young persons in an area of South Africa with high TB transmission.Age had a strong impact on assay positivity for all seven immune phenotypes tested (P0.0007). Among positive responders, the extent of purified protein derivative (PPD) triggered IFN-gamma release (P0.003) was sensitive to age. ESAT-6 triggered IFN-gamma release (day 7, P = 0.03) and the frequency of PPD-specific IFN-gamma(+)CD4(+) (P = 0.03) and IFN-gamma(+)CD8(+) cells (P = 0.04) were weakly dependent on age. By contrast, the extent of TST induration was insensitive to age (P0.05), and sex had no significant impact on any phenotype measured (P0.05). The high proportion of positive responders in the 1-10 year age-group observed with long-term whole blood assays, but not with 3-day assays and TST, suggests that long-term whole blood assays may be confounded by bacille Calmette-Guérin vaccination in this age group.There is a significant impact of age, but not sex, on different assays of immune reactivity in this high TB transmission setting.

A high tuberculosis (TB) burden area in South Africa (notification rate for all TB cases 1400 per 100 000 population). To determine the prevalence of and predictive factors associated with latent TB infection in adolescents. Adolescents aged 12-18 years were recruited from high schools, clinical and demographic data were collected, and a tuberculin skin test (TST) and a QuantiFERON®-TB Gold In-Tube (QFT) assay performed. A total of 6363 (58.2%) of 10 942 adolescents at the schools were enrolled. After exclusions, of 5244 participants, 55.2% (95%CI 53.8-56.5) had TST ≥ 5 mm, while 50.9% (49.5-52.2) were QFT-positive. On multivariate analysis, Black/mixed race racial groups, male sex, older age, household TB contact, low income and low education level were predictive factors for both TST- and QFT-positive results. About half of the adolescents were found to be latently infected with TB in a high TB burden area with demographic and poverty-related socio-economic factors predicting the risk of TB infection. Adolescents from deprived communities should be considered an important target group for educational interventions by TB control programmes in high-burden settings

Limited data are available on the characteristics of tuberculosis (TB) disease in young children, especially in high-burden countries. To assess the incidence and severity of TB in children aged or =1 TB clinical feature) TB in children aged 1 lobe involved). The under 5 years incidence of disseminated TB was 0.33%. Of 239 (15%) cases that were bacteriologically confirmed, clinical features typical of TB disease were individually present in 2 weeks and weight loss, occurring in 43/239 (18%). TB incidence was high, and peaked in children aged 12-23 months. Many children experienced severe disease. A fifth of children with microbiologically confirmed disease presented with only one feature typically associated with TB

Background There is an urgent need for improved vaccines to protect against tuberculosis. The currently available vaccine Bacille Calmette-Guerin (BCG) has varying immunogenicity and efficacy across different populations for reasons not clearly understood. MVA85A is a modified vaccinia virus expressing antigen 85A from Mycobacterium tuberculosis which has been in clinical development since 2002 as a candidate vaccine to boost BCG-induced protection. A recent efficacy trial in South African infants failed to demonstrate enhancement of protection over BCG alone. The immunogenicity was lower than that seen in UK trials. The enzyme Indoleamine 2,3-dioxygenase (IDO) catalyses the first and rate-limiting step in the breakdown of the essential amino acid tryptophan. T cells are dependent on tryptophan and IDO activity suppresses T-cell proliferation and function. Methods Using samples collected during phase I trials with MVA85A across the UK and South Africa we have investigated the relationship between vaccine immunogenicity and IDO using IFN-γ ELISPOT, qPCR and liquid chromatography mass spectrometry. Results We demonstrate an IFN-γ dependent increase in IDO mRNA expression in peripheral blood mononuclear cells (PBMC) following MVA85A vaccination in UK subjects. IDO mRNA correlates positively with the IFN-γ ELISPOT response indicating that vaccine specific induction of IDO in PBMC is unlikely to limit the development of vaccine specific immunity. IDO activity in the serum of volunteers from the UK and South Africa was also assessed. There was no change in serum IDO activity following MVA85A vaccination. However, we observed higher baseline IDO activity in South African volunteers when compared to UK volunteers. In both UK and South African serum samples, baseline IDO activity negatively correlated with vaccine-specific IFN-γ responses, suggesting that IDO activity may impair the generation of a CD4+ T cell memory response. Conclusions Baseline IDO activity was higher in South African volunteers when compared to UK volunteers, which may represent a potential mechanism for the observed variation in vaccine immunogenicity in South African and UK populations and may have important implications for future vaccination strategies. Trial registration Trials are registered at ClinicalTrials.gov; UK cohort NCT00427830 , UK LTBI cohort NCT00456183 , South African cohort NCT00460590 , South African LTBI cohort NCT00480558 .