Does antiretroviral treatment increase the infectiousness of smear-positive pulmonary tuberculosis?

BACKGROUND: Understanding of the effects of human immunodeficiency virus (HIV) infection and antiretroviral treatment (ART) on Mycobacterium tuberculosis transmission dynamics remains limited. We undertook a cross-sectional study among household contacts of smear-positive pulmonary tuberculosis (TB) cases to assess the effect of established ART on the infectiousness of TB. METHOD: Prevalence of tuberculin skin test (TST) positivity was compared between contacts of index cases aged 2-10 years who were HIV-negative, HIV-positive but not on ART, on ART for <1 year and on ART for 1 year. Random-effects logistic regression was used to take into account clustering within households. RESULTS: Prevalence of M. tuberculosis infection in contacts of HIV-negative patients, HIV-positive patients on ART 1 year and HIV-positive patients not on ART/on ART <1 year index cases was respectively 44%, 21% and 22%. Compared to contacts of HIV-positive index cases not on ART or recently started on ART, the odds of TST positivity was similar in contacts of HIV-positive index cases on ART 1 year (adjusted OR [aOR] 1.0, 95%CI 0.3-3.7). The odds were 2.9 times higher in child contacts of HIV-negative index cases (aOR 2.9, 95%CI 1.0-8.2). CONCLUSIONS: We found no evidence that established ART increased the infectiousness of smear-positive, HIV-positive index cases.

Risk factors for Mycobacterium tuberculosis infection in 2-4 year olds in a rural HIV-prevalent setting.

BACKGROUND: Mycobacterium tuberculosis infection in children acts as a sentinel for infectious tuberculosis. OBJECTIVE: To assess risk factors associated with tuberculous infection in pre-school children. METHOD: We conducted a population-wide tuberculin skin test (TST) survey from January to December 2012 in Malawi. All children aged 2-4 years residing in a demographic surveillance area were eligible. Detailed demographic data, including adult human immunodeficiency virus (HIV) status, and clinical and sociodemographic data on all diagnosed tuberculosis (TB) patients were available. RESULTS: The prevalence of M. tuberculosis infection was 1.1% using a TST induration cut-off of 15 mm (estimated annual risk of infection of 0.3%). The main identifiable risk factors were maternal HIV infection at birth (adjusted OR [aOR] 3.6, 95%CI 1.1-12.2), having three or more adult members in the household over a lifetime (aOR 2.4, 95%CI 1.2-4.8) and living in close proximity to a known case of infectious TB (aOR 1.6, 95%CI 1.1-2.4), modelled as a linear variable across categories (>200 m, 100-200 m, <100 m, within household). Less than 20% of the infected children lived within 200 m of a known diagnosed case. CONCLUSION: Household and community risk factors identified do not explain the majority of M. tuberculosis infections in children in our setting.

Methodological challenges in measuring vaccine effectiveness using population cohorts in low resource settings.

Post-licensure real world evaluation of vaccine implementation is important for establishing evidence of vaccine effectiveness (VE) and programme impact, including indirect effects. Large cohort studies offer an important epidemiological approach for evaluating VE, but have inherent methodological challenges. Since March 2012, we have conducted an open prospective cohort study in two sites in rural Malawi to evaluate the post-introduction effectiveness of 13-valent pneumococcal conjugate vaccine (PCV13) against all-cause post-neonatal infant mortality and monovalent rotavirus vaccine (RV1) against diarrhoea-related post-neonatal infant mortality. Our study sites cover a population of 500,000, with a baseline post-neonatal infant mortality of 25 per 1000 live births. We conducted a methodological review of cohort studies for vaccine effectiveness in a developing country setting, applied to our study context. Based on published literature, we outline key considerations when defining the denominator (study population), exposure (vaccination status) and outcome ascertainment (mortality and cause of death) of such studies. We assess various definitions in these three domains, in terms of their impact on power, effect size and potential biases and their direction, using our cohort study for illustration. Based on this iterative process, we discuss the pros and cons of our final per-protocol analysis plan. Since no single set of definitions or analytical approach accounts for all possible biases, we propose sensitivity analyses to interrogate our assumptions and methodological decisions. In the poorest regions of the world where routine vital birth and death surveillance are frequently unavailable and the burden of disease and death is greatest We conclude that provided the balance between definitions and their overall assumed impact on estimated VE are acknowledged, such large scale real-world cohort studies can provide crucial information to policymakers by providing robust and compelling evidence of total benefits of newly introduced vaccines on reducing child mortality.

Large-scale whole genome sequencing of M. tuberculosis provides insights into transmission in a high prevalence area.

To improve understanding of the factors influencing tuberculosis transmission and the role of pathogen variation, we sequenced all available specimens from patients diagnosed over 15 years in a whole district in Malawi. Mycobacterium tuberculosis lineages were assigned and transmission networks constructed, allowing ≤10 single nucleotide polymorphisms (SNPs) difference. We defined disease as due to recent infection if the network-determined source was within 5 years, and assessed transmissibility from forward transmissions resulting in disease. High-quality sequences were available for 1687 disease episodes (72% of all culture-positive episodes): 66% of patients linked to at least one other patient. The between-patient mutation rate was 0.26 SNPs/year (95% CI 0.21-0.31). We showed striking differences by lineage in the proportion of disease due to recent transmission and in transmissibility (highest for lineage-2 and lowest for lineage-1) that were not confounded by immigration, HIV status or drug resistance. Transmissions resulting in disease decreased markedly over time.

Impact of changing diagnostic criteria for smear-positive tuberculosis: a cohort study in Malawi.

We assessed the impact on measured burden and outcomes of the revised World Health Organization and Malawi guidelines reclassifying people with single (including 'scanty') positive smears as smear-positive pulmonary tuberculosis cases. In a retrospective cohort in rural Malawi, 567 (34%) of 1670 smear-positive episodes were based on single positive smears (including 176 with scanty smears). Mortality rates and the proportion starting treatment were similar in those with two positive smears or single, non-scanty smears. Those with single scanty smears had higher mortality and a lower proportion starting treatment. The reclassification will increase the reported burden substantially, but should improve treatment access.

Differential gene expression of activating Fcγ receptor classifies active tuberculosis regardless of human immunodeficiency virus status or ethnicity.

New diagnostics and vaccines for tuberculosis (TB) are urgently needed, but require an understanding of the requirements for protection from/susceptibility to TB. Previous studies have used unbiased approaches to determine gene signatures in single-site populations. The present study utilized a targeted approach, reverse transcriptase multiplex ligation-dependent probe amplification (RT-MLPA), to validate these genes in a multisite study. We analysed ex vivo whole blood RNA from a total of 523 participants across four sub-Saharan countries (Ethiopia, Malawi, South Africa, and The Gambia) with differences in TB and human immunodeficiency virus (HIV) status. We found a number of genes that were expressed at significantly lower levels in participants with active disease than in those with latent TB infection (LTBI), with restoration following successful TB treatment. The most consistent classifier of active disease was FCGR1A (high-affinity IgG Fc receptor 1 (CD64)), which was the only marker expressed at significantly higher levels in participants with active TB than in those with LTBI before treatment regardless of HIV status or genetic background. This is the first study to identify a biomarker for TB that is not affected by HIV status or geo-genetic differences. These data provide valuable clues for understanding TB pathogenesis, and also provide a proof-of-concept for the use of RT-MLPA in rapid and inexpensive validation of unbiased gene expression findings.

Normal Range of CD4 Cell Counts and Temporal Changes in Two HIVNegative Malawian Populations.

Longitudinal studies were carried out to determine trends in CD4 cell counts over a four year period in healthy HIV-negative adults in a rural (134 individuals) and an urban (80 individuals) site in Malawi, using TruCountTM and FACScountTM platforms. At baseline, median counts and 95% ranges were 890 (359-1954) cells per microlitre (µl) and 725 (114-1074) cells/µl respectively. 1.5% and 6% respectively had baseline counts below 350 cells/µl and 1.5% and 2.5% below 250 cells per µl. Transient dips to below 250 cells/µl were observed in seven individuals, with two individuals having persistently low CD4 counts over more than one year. Women and individuals from the urban site were significantly more likely to have "low CD4 count" (< 500 cells/µl) even when adjusted for other factors. In common with neighbouring countries, HIV-negative populations in Malawi have CD4 counts considerably lower than European reference ranges, and healthy individuals may have persistently or transiently low counts. Within Malawi, ranges differ according to the selected population.

Human immunodeficiency virus associated tuberculosis more often due to recent infection than reactivation of latent infection.

BACKGROUND: It is unclear whether human immunodeficiency virus (HIV) increases the risk of tuberculosis (TB) mainly through reactivation or following recent Mycobacterium tuberculosis (re)infection. Within a DNA fingerprint-defined cluster of TB cases, reactivation cases are assumed to be the source of infection for subsequent secondary cases. As HIV-positive TB cases are less likely to be source cases, equal or higher clustering in HIV-positives would suggest that HIV mainly increases the risk of TB following recent infection. METHODS: A systematic review was conducted to identify all studies on TB clustering and HIV infection in HIV-endemic populations. Available individual patient data from eligible studies were pooled to analyse the association between clustering and HIV. RESULTS: Of seven eligible studies, six contributed individual patient data on 2116 patients. Clustering was as, or more, likely in the HIV-positive population, both overall (summary OR 1.26, 95%CI 1.0-1.5), and within age groups (OR 1.50, 95%CI 0.9-2.3; OR 1.00, 95%CI 0.8-1.3 and OR 2.57, 95%CI 1.4-5.7) for ages 15-25, 26-50 and >50 years, respectively. CONCLUSIONS: Our results suggest that HIV infection mainly increases the risk of TB following recent M. tuberculosis transmission, and that TB control measures in HIV-endemic settings should therefore focus on controlling M. tuberculosis transmission rather than treating individuals with latent M. tuberculosis infection.

Declining child mortality in northern Malawi despite high rates of infection with HIV.

OBJECTIVE: To determine whether routine surveys, such as the Demographic and Health Surveys (DHS), have underestimated child mortality in Malawi. METHODS: Rates and causes of child mortality were obtained from a continuous-registration demographic surveillance system (DSS) in Malawi for a population of 32 000. After initial census, births and deaths were reported by village informants and updated monthly by project enumerators. Cause of death was established by verbal autopsy whenever possible. The likely impact of human immunodeficiency virus (HIV) infection on child mortality was also estimated from antenatal clinic surveillance data. Overall and age-specific mortality rates were compared with those from the 2004 Malawi DHS. FINDINGS: Between August 2002 and February 2006, 38 617 person-years of observation were recorded for 20 388 children aged < 15 years. There were 342 deaths. Re-census data, follow-up visits at 12 months of age and the ratio of stillbirths to neonatal deaths suggested that death registration by the DSS was nearly complete. Infant mortality was 52.7 per 1000 live births, under-5 mortality was 84.8 per 1000 and under-15 mortality was 99.1 per 1000. One-fifth of deaths by age 15 were attributable to HIV infection. Child mortality rates estimated with the DSS were approximately 30% lower than those from national estimates as determined by routine surveys. CONCLUSION: The fact that child mortality rates based on the DSS were relatively low in the study population is encouraging and suggests that the low mortality rates estimated nationally are an accurate reflection of decreasing rates.

Human immunodeficiency virus increases the risk of tuberculosis due to recent re-infection in individuals with latent infection.

BACKGROUND: Human immunodeficiency virus associated tuberculosis (TB) disease can follow reactivation of latent Mycobacterium tuberculosis infection or recent (re-)infection with M. tuberculosis. If contemporary TB cases share identical M. tuberculosis strains (i.e., are 'clustered'), the episode is likely to have followed recent (re-)infection, irrespective of evidence of previous latent infection. METHODS: Individuals experiencing a first TB episode between 1996 and 2008 in Karonga District, Northern Malawi, were included if information on M. tuberculosis infection status (from tuberculin tests) before 1990 and a DNA fingerprint from the TB episode were available. We explored differences in proportion clustered by prior M. tuberculosis infection status and HIV status, adjusting for age, sex, bacille Calmette-Guérin scar status and time since tuberculin testing. RESULTS: Of 79 HIV-negative TB cases, those with previous M. tuberculosis infection were much less likely to be clustered than cases without prior infection (29% vs. 77%, adjusted OR = 0.15, 95%CI 0.04-0.59). Among 119 HIV-positive TB cases, clustering was similar in both groups (88% vs. 84%, adjusted OR = 1.85, 95%CI 0.41-8.29). DISCUSSION: HIV infection appears to increase the risk of TB following recent re-infection in patients with latent M. tuberculosis infection. Our results add to the mounting evidence that HIV-associated TB mainly follows recent M. tuberculosis infection.

Declining child mortality in northern Malawi despite high rates of infection with HIV

Objective:To determine whether routine surveys; such as the Demographic and Health Surveys (DHS); have underestimated child mortality in Malawi : Methods :Rates and causes of child mortality were obtained from a continuous-registration demographic surveillance system (DSS) in Malawi for a population of 32 000. After initial census; births and deaths were reported by village informants and updated monthly by project enumerators. Cause of death was established by verbal autopsy whenever possible. The likely impact of human immunodeficiency virus (HIV) infection on child mortality was also estimated from antenatal clinic surveillance data. Overall and age-specific mortality rates were compared with those from the 2004 Malawi DHS. Findings:Between August 2002 and February 2006; 38 617 person-years of observation were recorded for 20 388 children aged 15 years. There were 342 deaths. Re-census data; follow-up visits at 12 months of age and the ratio of stillbirths to neonatal deaths suggested that death registration by the DSS was nearly complete. Infant mortality was 52.7 per 1000 live births; under-5 mortality was 84.8 per 1000 and under-15 mortality was 99.1 per 1000. One-fifth of deaths by age 15 were attributable to HIV infection. Child mortality rates estimated with the DSS were approximately 30 lower than those from national estimates as determined by routine surveys Conclusion: The fact that child mortality rates based on the DSS were relatively low in the study population is encouraging and suggests that the low mortality rates estimated nationally are an accurate reflection of decreasing rates

Mortality and health among internally displaced persons in western Kenya following post-election Violence; 2008: novel use of demographic surveillance

Objective To evaluate mortality and morbidity among internally displaced persons (IDPs) who relocated in a demographic surveillance system (DSS) area in western Kenya following post-election violence. Methods In 2007; 204 000 individuals lived in the DSS area; where field workers visit households every 4 months to record migrations; births and deaths. We collected data on admissions among children 5 years of age in the district hospital and developed special questionnaires to record information on IDPs. Mortality; migration and hospitalization rates among IDPs and regular DSS residents were compared; and verbal autopsies were performed for deaths. Findings Between December 2007 and May 2008; 16 428 IDPs migrated into the DSS; and over half of them stayed 6 months or longer. In 2008; IDPs aged 15.49 years died at higher rates than regular residents of the DSS (relative risk; RR: 1.34; 95confidence interval; CI: 1.004.1.80). A greater percentage of deaths from human immunodeficiency virus (HIV) infection occurred among IDPs aged . 5 years (53) than among regular DSS residents (25.29) (P 0.001). Internally displaced children 5 years of age did not die at higher rates than resident children but were hospitalized at higher rates (RR: 2.95; 95CI: 2.44.3.58). Conclusion HIV-infected internally displaced adults in conflict-ridden parts of Africa are at increased risk of HIV-related death. Relief efforts should extend to IDPs who have relocated outside IDP camps; particularly if afflicted with HIV infection or other chronic conditions

What has Karonga taught us? Tuberculosis studied over three decades.

This paper summarises tuberculosis (TB) research over almost 30 years in Karonga District, northern Malawi, an area typical of much of rural Africa. The dominant factor has been the human immunodeficiency virus (HIV), which arrived in the district about 1980, leading to an increase in TB incidence to a peak of approximately 65 smear-positive pulmonary cases per 100000 population in 2000. Tuberculin surveys indicate annual risks of Mycobacterium tuberculosis infection of approximately 1%; thus, most of the population is uninfected and at risk of primary infection and disease. Molecular epidemiological studies demonstrate that about two thirds of TB arises from recent infection, but recognisable recent contact is responsible for only about 10% of disease. By 2001, 57% of TB was directly attributable to HIV, implying that it would have declined were it not for HIV. HIV infection increases the risk of TB most among young adults, and greatly increases the risk of recurrence from new infection after treatment. Mortality rates in the HIV-infected are high, but there is no association of HIV with drug resistance. Other risk factors with relatively smaller effects include age and sex, contact, several genetic polymorphisms and area. Neither one nor two doses of the bacille Calmette-Guérin (BCG) vaccine provides protection against adult pulmonary TB, despite protecting against leprosy. Skin test surveys, cohort studies and comparative immunological studies with the UK suggest that exposure to environmental mycobacteria provides some protection against TB and that BCG's failure is attributable partly to this widespread heterologous exposure masking effects of the vaccine. Drug resistance has remained constant (<10%) over more than 20 years. Immunotherapy with M. vaccae provided no benefits, but treatment of HIV-positive patients with cotrimoxazole reduced mortality. The Karonga programme illustrates the value of long-term population-based studies to investigate the natural history of TB and to influence TB control policy. Current studies focus on immunological markers of infection, disease and protection, and on elucidating the impact of antiretroviral treatment on TB incidence at population level.

Herpes simplex virus type 2 trends in relation to the HIV epidemic in northern Malawi.

OBJECTIVES: It is unclear whether the high prevalence of herpes simplex virus type 2 (HSV-2) found in much of Africa predates the HIV epidemic or is, to some extent, a consequence of it. HSV-2 prevalence trends in a rural African community were assessed over a period in which HIV prevalence rose sharply, and antenatal clinic (ANC) surveillance was explored as a method of estimating community HSV-2 prevalence. METHODS: HSV-2 seroprevalence was determined among community controls seen for case-control studies of mycobacterial disease in Karonga district, Malawi, in 1988-90, 1998-2001 and 2002-5, and in women attending ANC as part of surveillance for HIV in 1999-2000. Over this period HIV prevalence rose from 4% to 12%. RESULTS: HSV-2 prevalence in all periods increased sharply with age and was higher in women than in men. After excluding migrants, there was no evidence of change in HSV-2 prevalence in the different periods. Women in the ANC group had lower HSV-2 prevalence than those in the community, but the ANC prevalence was a good approximation to the combined male and female prevalence for the same age group. CONCLUSIONS: This study suggests that HSV-2 was already widespread before the HIV epidemic and has not been greatly influenced by it. It also demonstrates that ANC surveillance may be useful for estimating community HSV-2 prevalence.

Assessment and evaluation of contact as a risk factor for tuberculosis in rural Africa.

SETTING: A rural district in Malawi. OBJECTIVE: To determine the effect of inaccurate recall on estimates of the proportion of tuberculosis (TB) cases attributable to contact with identifiable prior cases. DESIGN: Case-control study of laboratory-confirmed TB cases and community controls, comparing family, household and area contacts identified from a database of TB cases with those named at interview. Estimation of prior contact as a risk factor for TB and identified factors associated with being a named contact. RESULTS: Ninety-five per cent of named contacts were known TB cases. The proportion of total identified contacts who were named at interview was 75%, and was similar for cases and controls. Cases were twice as likely as controls to identify prior contacts. Adding database information did not affect odds ratios, but increased the proportion of TB cases attributable to prior contact. Smear-positive, male and human immunodeficiency virus (HIV) negative TB patients were more likely to be named by subsequent cases. Identifiable recent contact with known smear-positive cases accounted for 12.5% of the TB burden. CONCLUSIONS: Reporting of putative source contacts showed little evidence of recall bias and gave estimates of the relative risk of TB associated with identifiable contact. The lower likelihood of HIV-positive cases being named as contacts may reflect reduced infectiousness.

Ascertainment of childhood vaccination histories in northern Malawi.

OBJECTIVE: To assess factors related to recorded vaccine uptake, which may confound the evaluation of vaccine impact. METHODS: Analysis of documented vaccination histories of children under 5 years and demographic and socio-economic characteristics collected by a demographic surveillance system in Karonga District, Malawi. Associations between deviations from the standard vaccination schedule and characteristics that are likely to be associated with increased mortality were determined by multivariate logistic regression. RESULTS: Approximately 78% of children aged 6-23 months had a vaccination document, declining to <50% by 5 years of age. Living closer to an under-5 clinic, having a better educated father, and both parents being alive were associated with having a vaccination document. For a small percentage of children, vaccination records were incomplete and/or faulty. Vaccination uptake was high overall, but delayed among children living further from the nearest under-5 clinic or from poorer socio-economic backgrounds. Approximately 9% of children had received their last dose of DPT with or after measles vaccine. These children were from relatively less educated parents, and were more likely to have been born outside the health services. CONCLUSIONS: Though overall coverage in this community was high and variation in coverage according to child or parental characteristics small, there was strong evidence of more timely coverage among children from better socio-economic conditions and among those who lived closer to health facilities. These factors are likely to be strong confounders in the association of vaccinations with mortality, and may offer an alternative explanation for the non-specific mortality impact of vaccines described by other studies.

The social and economic impact of parental HIV on children in northern Malawi: retrospective population-based cohort study.

From population-based surveys in the 1980s in Karonga district, northern Malawi, 197 'index individuals' were identified as HIV-positive. 396 HIV-negative 'index individuals' were selected as a comparison group. These individuals, and their spouses and children, were followed up in 1998-2000. 582 of 593 index individuals were traced. 487 children of HIV-positive, and 1493 children of HIV-negative, parents were included in analyses. Rates of paternal, maternal, and double orphanhood among children with one or both parents HIV-positive were respectively 6, 8, and 17 times higher than for children with HIV-negative parents. Around 50% of children living apart from both parents had a grandparent as their guardian; for most of the rest the guardian was an aunt, uncle, or sibling. There were no child-headed households. Almost all children aged 6-14 were attending primary school. There was no evidence that parental HIV affected primary school attainment among children <15 years old. Children of HIV-positive parents were less likely to have attended secondary school than those of HIV-negative parents. The extended family has mitigated the impact of orphanhood on children, but interventions to reduce the incidence of orphanhood, and/or which strengthen society's ability to support orphans, are essential, especially as the HIV epidemic matures and its full impact is felt.

HIV epidemic trend and antiretroviral treatment need in Karonga District, Malawi.

We describe the development of the HIV epidemic in Karonga District, Malawi over 22 years using data from population surveys and community samples. These data are used to estimate the trend in HIV prevalence, incidence and need for antiretroviral treatment (ART) using a simple mathematical model. HIV prevalence rose quickly in the late 1980s and early 1990s, stabilizing at around 12% in the mid-1990s. Estimated annual HIV incidence rose quickly, peaking in the early 1990s at 2.2% among males and 3.1% among females, and then levelled off at 1.3% among males and 1.1% among females by the late 1990s. Assuming a 2-year eligibility period, both our model and the UNAIDS models predicted 2.1% of adults were in need of ART in 2005. This prediction was sensitive to the assumed eligibility period, ranging from 1.6% to 2.6% if the eligibility period was instead assumed to be 1.5 or 2.5 years, respectively.

Time delays between patient and laboratory selectively affect accuracy of helminth diagnosis.

Studies of intestinal helminth infections are influenced by the constraints of sample collection, as identification of helminth ova in stools is affected by the time since evacuation from the host. Different methods may be required to optimise diagnostic sensitivity under different study conditions. In the context of studies in rural Malawi, we collected stool samples with different time delays from production by subjects to sample collection by field staff, to examination in the laboratory. Stools were processed by Kato-Katz (KK) or formol-ether concentration (FEC) methods. Hookworm and Schistosoma mansoni were the most common helminths identified. The prevalence of hookworm was higher with KK (270/988, 27%) than with FEC (191/988, 19%). Comparison was made between the results from the two methods according to the timing of the processing steps. Delays in processing did not affect retrieval of S. mansoni. A decrease in sensitivity of almost 50% for detection of hookworm was observed with either method when preservation/refrigeration was delayed by more than 3h. A delay of 1 day from refrigeration or preservation to laboratory processing also reduced the sensitivity for hookworm by 50% for both methods. Care must be taken in studies of multiple helminth infections owing to the selective reduction of hookworm ova during transport. This is particularly critical when samples are not preserved, even over short periods of time, and even with formalin preservation.

The influence of previous exposure to environmental mycobacteria on the interferon-gamma response to bacille Calmette-Guérin vaccination in southern England and northern Malawi.

We report a large study of the effect of BCG vaccination on the in vitro 6-day whole blood interferon-gamma (IFN-gamma) response to antigens from eight species of mycobacteria among schoolchildren in south-eastern England, where bacille Calmette-Guérin (BCG) vaccination is highly protective against pulmonary tuberculosis, and among young adults in northern Malawi, where BCG vaccination is not protective. In the UK children, BCG induced an appreciable increase in IFN-gamma response to antigens from most species of mycobacteria. The degree of change was linked to the relatedness of the species to Mycobacterium bovis BCG, and provides further evidence of the cross-reactivity of mycobacterial species in priming of the immune system. IFN-gamma responses to purified protein derivatives (PPDs) from M. tuberculosis and environmental mycobacteria were more prevalent in the Malawian than the UK group prior to vaccination; BCG vaccination increased the prevalence of responses to these PPDs in the UK group to a level similar to that in Malawi. There was no evidence that the vaccine-induced change in IFN-gamma response was dependent upon the magnitude of the initial response of the individual to environmental mycobacteria in the United Kingdom or in Malawi. These observations should assist the development and interpretation of human clinical trials of new vaccines against M. tuberculosis in areas of both low and high exposure to environmental mycobacteria.