Background Pneumococcal conjugate vaccine (PCV) and rotavirus vaccine (RV) are key tools for reducing common causes of infant mortality. However, measurement of population-level mortality impact is lacking from sub-Saharan Africa. We evaluated mortality impact and vaccine effectiveness (VE) of PCV13 introduced in November 2011, with subsequent RV1 roll-out in October 2012, in Malawi. Methods We conducted two independent community-based birth cohort studies. Study 1, in northern Malawi (40000population), evaluated population impact using change-point analysis and negative-binomial regression of non-traumatic 14–51-week infant mortality preintroduction (1 January 2004 to 31 September 2011) and postintroduction (1 October 2011 to 1 July 2019), and against three-dose coverage. Study 2, in central Malawi (465 000 population), was recruited from 24 November 2011 to 1 June 2015. In the absence of preintroduction data, individual three-dose versus zero-dose VE was estimated using individual-level Cox survival models. In both cohorts, infants were followed with household visits to ascertain vaccination, socioeconomic and survival status. Verbal autopsies were conducted for deaths. Results Study 1 included 20 291 live births and 216 infant deaths. Mortality decreased by 28.6% (95% CI: 15.3 to 39.8) post-PCV13 introduction. A change point was identified in November 2012. Study 2 registered 50 731 live births, with 454 deaths. Infant mortality decreased from 17 to 10/1000 live births during the study period. Adjusted VE was 44.6% overall (95% CI: 23.0 to 59.1) and 48.3% (95% CI: −5.9 to 74.1) against combined acute respiratory infection, meningitis and sepsis-associated mortality. Conclusion These data provide population-level evidence of infant mortality reduction following sequential PCV13 and RV1 introduction into an established immunisation programme in Malawi. These data support increasing coverage of vaccine programmes in high-burden settings.

Background and Purpose— The incidence of stroke in Malawi is unknown but major risk factors, including hypertension, obesity, and diabetes mellitus, are highly prevalent. We sought to understand community-level knowledge about stroke. Methods— A population-based cross-sectional study was conducted in rural Malawi (2016–2017). Adults aged ≥15 years were randomly selected and interviewed about their knowledge and perceptions of stroke symptoms, risk factors, and prevention. Logistic regression was used to investigate sociodemographic factors associated with stroke knowledge. Results— Of 812 selected, 739 (91% response rate) were seen and consented; 57% were female, and the median age was 52.0 years. Knowledge of stroke was poor: 71% knew no (correct) risk factors. Witchcraft (20.6%) was mentioned as frequently as hypertension (19.8%) as a cause. Knowledge of stroke was greatest in the most educated and wealthy and lowest in men, the never married, and the youngest age group. HIV-positive individuals had higher knowledge of prevention (odds ratio, 2.91; 95% CI, 1.21–7.03) than HIV negative individuals. Conclusions— Knowledge about stroke is very low in this community, particularly among the least educated and poor. Programs to support prevention, early recognition, and timely hospital presentation after a stroke are needed.

There remains a pressing need for biomarkers that can predict who will progress to active tuberculosis (TB) after exposure to Mycobacterium tuberculosis (MTB) bacterium. By analyzing cohorts of household contacts of TB index cases (HHCs) and a stringent non-human primate (NHP) challenge model, we evaluated whether integration of blood transcriptional profiling with serum metabolomic profiling can provide new understanding of disease processes and enable improved prediction of TB progression. Compared to either alone, the combined application of pre-existing transcriptome- and metabolome-based signatures more accurately predicted TB progression in the HHC cohorts and more accurately predicted disease severity in the NHPs. Pathway and data-driven correlation analyses of the integrated transcriptional and metabolomic datasets further identified novel immunometabolomic signatures significantly associated with TB progression in HHCs and NHPs, implicating cortisol, tryptophan, glutathione, and tRNA acylation networks. These results demonstrate the power of multi-omics analysis to provide new insights into complex disease processes.

Objectives This prospective cohort study sought to estimate health system and household costs for episodes of diarrhoeal illness in Malawi.Setting Data were collected in two Malawian settings: a rural health centre in Chilumba and an urban tertiary care hospital in Blantyre.Participants Children under 5 years of age presenting with diarrhoeal disease between 1 January 2013 and 21 November 2014 were eligible for inclusion. Illnesses attributed to other underlying causes were excluded, as were illnesses commencing more than 2 weeks prior to presentation. Complete data were collected on 514 cases at both the time of the initial visit to the participating healthcare facility and 6 weeks after discharge.Primary and secondary outcome measures The primary outcome measure was the total cost of an episode of illness. Costs to the health system were gathered from chart review (drugs and diagnostics) and actual hospital expenditure (staff and facility costs). Household costs, including lost income, were obtained by interview with the parents/guardians of patients.Results Total costs in 2014 US$ for rural inpatient, rural outpatient, urban inpatient and urban outpatient were $65.33, $8.89, $60.23 and $14.51, respectively (excluding lost income). Mean household contributions to these costs were 15.8%, 9.8%, 21.3% and 50.6%.Conclusion This study found significant financial burden from childhood diarrhoeal disease to the healthcare system and to households. The latter face the risk of consequent impoverishment, as the study demonstrates how the costs of seeking treatment bring the income of the majority of families in all income strata below the national poverty line in the month of illness.

Background Malawi introduced pneumococcal conjugate vaccine (PCV13) and monovalent rotavirus vaccine (RV1) in 2011 and 2012 respectively, and is planning the introduction of a second-dose measles vaccine (MV). We assessed predictors of availability, uptake and timeliness of these vaccines in a rural Malawian setting. Methods Commencing on the first date of PCV13 eligibility we conducted a prospective population-based birth cohort study of 2,616 children under demographic surveillance in Karonga District, northern Malawi who were eligible for PCV13, or from the date of RV1 introduction both PCV13 and RV1. Potential predictors of vaccine uptake and timeliness for PCV13, RV1 and MV were analysed respectively using robust Poisson and Cox regression. Results Vaccine coverage was high for all vaccines, ranging from 86.9% for RV1 dose 2 to 95.4% for PCV13 dose 1. Median time delay for PCV13 dose 1 was 17 days (IQR 7–36), 19 days (IQR 8–36) for RV1 dose 1 and 20 days (IQR 3–46) for MV. Infants born to lower educated or farming mothers and those living further away from the road or clinic were at greater risk of being not fully vaccinated and being vaccinated late. Delays in vaccination were also associated with non-facility birth. Vaccine stock-outs resulted in both a delay in vaccine timeliness and in a decrease in completion of schedule. Conclusion Despite high vaccination coverage in this setting, delays in vaccination were common. We identified programmatic and socio-demographic risk factors for uptake and timeliness of vaccination. Understanding who remains most vulnerable to be unvaccinated allows for focussed delivery thereby increasing population coverage and maximising the equitable benefits of universal vaccination programmes.

Background. Rotavirus vaccination reduces childhood hospitalization in Africa, but cost-effectiveness has not been determined using real-world effectiveness and costing data. We sought to determine monovalent rotavirus vaccine cost-effectiveness in Malawi, one of Africa's poorest countries and the first Gavi-eligible country to report disease reduction following introduction in 2012.Methods.aEuro integral This was a prospective cohort study of children with acute gastroenteritis at a rural primary health center, a rural first referral-level hospital and an urban regional referral hospital in Malawi. For each participant we itemized household costs of illness and direct medical expenditures incurred. We also collected Ministry of Health vaccine implementation costs. Using a standard tool (TRIVAC), we derived cost-effectiveness.Results.aEuro integral Between 1 January 2013 and 21 November 2014, we recruited 530 children aged < 5 years with gastroenteritis. Costs did not differ by rotavirus test result, but were significantly higher for admitted children and those with increased severity on Vesikari scale. Adding rotavirus vaccine to the national schedule costs Malawi $0.42 per dose in system costs. Vaccine copayment is an additional $0.20. Over 20 years, the vaccine program will avert 1 026 000 cases of rotavirus gastroenteritis, 78 000 inpatient admissions, 4300 deaths, and 136 000 disability-adjusted-life-years (DALYs). For this year's birth cohort, it will avert 54 000 cases of rotavirus and 281 deaths in children aged < 5 years. The program will cost $10.5 million and save $8.0 million in averted healthcare costs. Societal cost per DALY averted was $10, and the cost per rotavirus case averted was $1.Conclusions.aEuro integral Gastroenteritis causes substantial economic burden to Malawi. The rotavirus vaccine program is highly cost-effective. Together with the demonstrated impact of rotavirus vaccine in reducing population hospitalization burden, its cost-effectiveness makes a strong argument for widespread utilization in other low-income, high-burden settings.

Background: Pneumonia and gastroenteritis are leading causes of vaccine-preventable childhood morbidity and mortality. Malawi introduced pneumococcal conjugate and rotavirus vaccines to the immunisation programme in 2011 and 2012, respectively. Evaluating their effectiveness is vital to ensure optimal implementation and justify Sustained investment.Methods/Design: A national evaluation platform was established to determine vaccine effectiveness and impact in Malawi. Impact and effectiveness against vaccine-type invasive pneumococcal disease, radiological pneumonia and rotavirus gastroenteritis are investigated using before-after incidence comparisons and case-control designs, respectively. Mortality is assessed using a prospective population cohort. Cost-effectiveness evaluation is nested within the case-control studies. We describe platform characteristics including strengths and weaknesses for conducting vaccine evaluations.Discussion: Integrating data from individual level and ecological methods across multiple sites provides comprehensive information for policymakers on programme impact and vaccine effectiveness including changes in serotype/genotype distribution over time. Challenges to robust vaccine evaluation in real-world conditions include: vaccination ascertainment; pre-existing rapid decline in mortality and pneumococcal disease in the context of non-vaccine interventions; and the maintenance of completeness and quality of reporting at scale and over time. In observational non-randomised designs ascertainment of vaccine status may be biased particularly in infants with fatal outcomes. In the context of multiple population level interventions targeting study endpoints attribution of reduced incidence to vaccine impact may be flawed. Providing evidence from several independent but complementary studies will provide the greatest confidence in assigning impact. Welcome declines in disease incidence and in child mortality make accrual of required sample sizes difficult, necessitating large studies to detect the relatively small but potentially significant contribution of vaccines to mortality prevention. Careful evaluation of vaccine effectiveness and impact in such settings is critical to sustaining support for vaccine programmes. Our evaluation platform covers a large population with a high prevalence of HIV and malnutrition and its findings will be relevant to other settings in sub-Saharan Africa. (c) 2015 The Authors. Published by Elsevier Ltd.

Background: Under-five mortality is decreasing but with little change in neonatal mortality rates. We examined the effect of maternal HIV status on under-five mortality and cause of death since widespread availability of antiretroviral therapy in rural Malawi. Methods: Children born in 2006–2011 in the Karonga demographic surveillance area were included. Maternal HIV status was available from HIV serosurveys. Age-specific mortality rate ratios for children born to HIV-positive and HIV-negative mothers were obtained by fitting a Poisson model accounting for child clustering by mother and adjusting for potential confounders. Cause of death was ascertained by verbal autopsy. Findings: There were 352 deaths among 6913 under-five singleton children followed for 20,754 person-years (py), giving a mortality rate of 17.0/1000 py overall, 218/1000 py (16.5/1000 live births) in neonates, 20/1000 py (17.4/1000 live births) in postneonatal infants, and 8/1000 py in 1–4 years old. Comparing those born to HIV-positive and HIV-negative mothers, the rate ratio adjusted for child age, sex, maternal age, parity, and drinking water source was 1.5 (95% confidence interval [CI]: 0.6 to 3.7) in neonates, 11.5 (95% CI: 7.2 to 18.5) in postneonatal infants, and 4.6 (95% CI: 2.7 to 7.9) in 1–4 years old. Birth injury/asphyxia, neonatal sepsis, and prematurity contributed >70% of neonatal deaths, whereas acute infections, malaria, diarrhea, and pneumonia accounted for most deaths in older children. Conclusions: Maternal HIV status had little effect on neonatal mortality but was associated with much higher mortality in the postneonatal period and among older children. Greater attention to HIV care in pregnant women and mothers should help improve child survival, but broader interventions are needed to reduce neonatal mortality.

Little is known about the pattern or risk factors for deaths from external causes in sub-Saharan Africa: there is a lack of reliable data, and public health priorities have been focussed on other causes. This study assessed the prevalence and risk factor for deaths from external causes in rural Malawi.

Background: BCG immunogenicity in infants differs between populations and these differences have been attributed to various factors. In this study, the influence of geographical location, season of birth, timing of vaccination, micronutrient status (zinc) and inflammatory status (C-reactive protein, CRP) were assessed.Methods: Immunogenicity was assessed by cytokine signature in culture supernatants from diluted whole blood samples stimulated with M. tuberculosis PPD, using a multiplex bead assay.Results were correlated with the plasma zinc and CRP concentrations at the time of sampling, and with interview and household data. BCG vaccinated infants were recruited in Malawi, The Gambia and the UK. Results: In Malawi, infants vaccinated within the first week after birth showed lower production of most cytokines measured than those vaccinated later. The number of cytokines showing significant differences between Malawian and Gambian infants decreased after adjusting for season of birth. In Malawi, a proportion of infants had zinc deficiency and elevated plasma CRP (>10 mg/L), but neither zinc deficiency nor high CRP was associated with production of any of the cytokines measured.Conclusions: The cytokine/chemokine signatures observed in response to M. tuberculosis PPD in infants at 3 months post BCG vaccination were affected by geographical location, season of birth, and timing of vaccination but not associated with the concentration of plasma zinc or inflammatory status. These factors should be considered in future trials of new TB vaccines.

BACKGROUND:Under-five mortality is decreasing but with little change in neonatal mortality rates. We examined the effect of maternal HIV status on under-five mortality and cause of death since widespread availability of antiretroviral therapy in rural Malawi.METHODS:Children born in 2006-2011 in the Karonga demographic surveillance area were included. Maternal HIV status was available from HIV serosurveys. Age-specific mortality rate ratios for children born to HIV-positive and HIV-negative mothers were obtained by fitting a Poisson model accounting for child clustering by mother and adjusting for potential confounders. Cause of death was ascertained by verbal autopsy.FINDINGS:There were 352 deaths among 6913 under-five singleton children followed for 20,754 person-years (py), giving a mortality rate of 17.0/1000 py overall, 218/1000 py (16.5/1000 live births) in neonates, 20/1000 py (17.4/1000 live births) in postneonatal infants, and 8/1000 py in 1-4 years old. Comparing those born to HIV-positive and HIV-negative mothers, the rate ratio adjusted for child age, sex, maternal age, parity, and drinking water source was 1.5 (95% confidence interval [CI]: 0.6 to 3.7) in neonates, 11.5 (95% CI: 7.2 to 18.5) in postneonatal infants, and 4.6 (95% CI: 2.7 to 7.9) in 1-4 years old. Birth injury/asphyxia, neonatal sepsis, and prematurity contributed >70% of neonatal deaths, whereas acute infections, malaria, diarrhea, and pneumonia accounted for most deaths in older children.CONCLUSIONS:Maternal HIV status had little effect on neonatal mortality but was associated with much higher mortality in the postneonatal period and among older children. Greater attention to HIV care in pregnant women and mothers should help improve child survival, but broader interventions are needed to reduce neonatal mortality.

BACKGROUND:Antiretroviral (ART) scale-up in Malawi has been achieved on a large scale based mainly on clinical criteria. Simple markers of prognosis are useful, and we investigated the value of very early anthropometric changes in predicting mortality.METHODS:PRINCIPAL FINDINGS:Adult patients who initiated ART in Karonga District, northern Malawi, between September 2005 and August 2006 were included in a prospective cohort study, and followed for up to one year. We used Cox regression to examine the association between anthropometric changes at 2 and 6 weeks and deaths within the first year. 573 patients were included, of whom 59% were women; the median age at initiation was 37 and 64% were in WHO stage 4. Both body mass index (BMI) and mid-upper arm circumference (MUAC) increased linearly with increased time on ART, and were closely correlated with each other. There were 118 deaths. After 2 weeks on ART, a BMI increase of <0.5 kg/m(2) (HR 2.47, 95%CI 1.24-4.94, p=0.005) or a MUAC increase of <0.5cm (HR 2.79, 95%CI 1.19-6.55, p=0.008) were strong predictors of death, and these associations were stronger after adjusting for baseline charactertistics. Similar results were found after 6 weeks on ART.CONCLUSIONS:Very early anthropometric changes, after 2 and 6 weeks on ART, are strong predictors of survival, independent of baseline characteristics. This should help identify patients requiring more detailed assessment where facilities are limited. MUAC is particularly valuable, requiring the simplest equipment and being appropriate for patients who have problems standing.

OBJECTIVES:Developing countries are undergoing demographic transition with a shift from high mortality caused by communicable diseases (CD) to lower mortality rates caused by non-communicable diseases (NCD). HIV/AIDS has disrupted this trend in sub-Saharan Africa. However, in recent years, HIV-associated mortality has been reduced with the introduction of widely available antiretroviral therapy (ART). Side effects of ART may lead to increased risk of cardiovascular diseases, raising the prospects of an accelerated transition towards NCD as the primary cause of death. We report population-based data to investigate changes in cause of death owing to NCD during the first 4 years after introduction of HIV treatment.METHODS:We analysed data from a demographic surveillance system in Karonga district, Malawi, from September 2004 to August 2009. ART was introduced in mid-2005. Clinician review of verbal autopsies conducted 2-6 weeks after a death was used to establish a single principal cause of death.RESULTS:Over the entire period, there were 905 deaths, AIDS death rate fell from 505 to 160/100,000 person-years, and there was no evidence of an increase in NCD rates. The proportion of total deaths attributable to AIDS fell from 42% to 17% and from NCD increased from 37% to 49%.DISCUSSION:Our findings show that 4 years after the introduction of ART into HIV care in Karonga district, all-cause mortality has fallen dramatically, with no evidence of an increase in deaths owing to NCD.

Background. BCG vaccination of infants is thought to provide good protection in all settings. This study investigated whether Malawian infants made weaker responses across a cytokine panel after BCG vaccination, compared with UK infants.Methods. Diluted whole-blood samples were cultured with Mycobacterium tuberculosis purified protein derivative for 6 days from BCG-vaccinated infants 3 months (n = 40 Malawi, 28 UK) and 12 months (n = 34 Malawi, 26 UK) after vaccination, and also from UK unvaccinated infants (n = 9 at 3 months, n = 10 at 12 months). Forty-two cytokines were measured in supernatants using a multiplex bead array assay. Principal component analysis was used to summarize the overall patterns in cytokine responses.Results. We found differences in median responses in 27 of the 42 cytokines: 7 higher in the UK and 20 higher in Malawi. The cytokines with higher responses in the UK were all T helper 1 related. The cytokines with higher responses in Malawi included innate proinflammatory cytokines, regulatory cytokines, interleukin 17, T helper 2 cytokines, chemokines, and growth factors. Principal component analysis separated the BCG-vaccinated infants from Malawi from the UK vaccinated infants and from the unvaccinated infants.Conclusions. Malawian infants make cytokine responses following BCG vaccination, but the cytokine profile is different from that in the UK. The different biosignatures following BCG vaccination in the 2 settings may indicate variability in the protective efficacy of infant BCG vaccination.

Introduction Global child mortality rates declined by 2.5% annually between 1960 and 1990, but the decline was slowest in sub-Saharan Africa. (1) The fourth United Nations Millennium Development Goal (MDG4) is to reduce child mortality by two-thirds by 2015. (2) Although some countries have made dramatic gains in child survival, more improvement is needed, especially in the sub-Saharan region. Malawi had the second highest under-5 mortality rate in southern and eastern Africa in 1990. In contrast to Kenya, Zambia and Zimbabwe, where rates have stabilized or increased, Malawi saw a substantial, steady decline, (3) from 221 per 1000 live births in 1990 to 120 per 1000 by 2006. (4-6) However, to achieve MDG4 an increase in the annual rate of decline from 3.5% to 5.4% is needed. (7) Estimating child mortality levels and trends is difficult. (8) In the absence of vital registration systems in most developing countries, mortality is estimated indirectly, from censuses and surveys, (6,9) particularly Demographic and Health Surveys (DHS) and World Fertility Surveys. These estimates are subject to several errors, including omission of births and deaths, misreporting of age and omission of maternal orphans (since mothers are the source of information). (1) In Malawi under-5 mortality has decreased, but estimates have shown considerable variation, (4-6) so the actual rate, and therefore the likely future trajectory, are not clear. (10) Many of the problems in estimation occur because data are collected retrospectively. These problems can be overcome by using demographic surveillance data. In generalized epidemics of human immunodeficiency virus (HIV) infection, paediatric acquired immunodeficiency syndrome (AIDS) and orphanhood have started to erode recent gains in child survival. (1,11,12) The relative impact of HIV infection on child mortality depends on the background level of mortality from other causes. (13) A model based on national HIV infection prevalence in pregnant women, average risk of mother-to-child transmission and survival probabilities in infected children, with adjustment for competing causes of death, predicted a mean rate of under-5 mortality attributable to H IV infection for Malawi in 1999 of 20.9 per 1000 births (plausible range 12.8-31.0). (14) To help validate national estimates based on routine surveys, we used data from Malawi's only demographic surveillance system to determine child mortality rates between 2002 and 2006 and to estimate the effect of HIV infection on these rates. Methods Demographic surveillance We set up a continuous-registration demographic surveillance system (DSS), covering a population of 32 000, in Karonga district (northern Malawi) in 2002. (15) An initial house-to-house census (baseline census) recorded personal identifiers and sociodemographic data for all individuals, and economic data and physical location for every household. Demographic surveillance was started in 230 geographically-defined clusters, each containing 15-60 households, immediately after completion of the baseline census in a given cluster. Within each cluster, one village informant was trained to record births, deaths and migrations. Project field staff followed up and recorded births and deaths monthly and migrations annually. To check completeness of infant death registration, all babies whose birth was recorded in the DSS and who were not known to have died or migrated out of the area were visited at 12 months of age. This follow-up visit detected only one extra infant death among 662 eligible babies. The total DSS population was re-censused after 2 years. This showed that the monthly and annual reporting system had registered 99% of deaths, 97% of births and 92% of migrations. (15) This report includes data from the beginning of the DSS in August 2002 until February 2006. Although the initial protocol was limited to live births, fetal deaths were also reported. …

BACKGROUND:Routine ART programme statistics generally only provide information about individuals who start treatment. We aimed to investigate the outcome of those who are eligible but do not start ART in the Malawi programme, factors associated with this dropout, and reasons for not starting treatment, in a prospective cohort study.METHODS:Individuals having a first screening visit at the ART clinic at Karonga District Hospital, northern Malawi, between September 2005 and July 2006 were interviewed. Study follow-up to identify treatment outcomes was conducted at the clinic and in the community. Logistic regression models were used to identify factors associated with dropout before ART initiation among participants identified as clinically eligible for ART.RESULTS:88 participants eligible for ART at their first screening visit (out of 633, 13.9%) defaulted before starting ART. Participants with less education, difficulties in dressing, a more delayed ART initiation appointment, and mid-upper arm circumference (MUAC) < 22 cm were significantly less likely to have visited the clinic subsequently. Thirty-five (58%) of the 60 participants who defaulted and were tracked at home had died, 21 before their ART initiation appointment.CONCLUSIONS:MUAC and reported difficulties in dressing may provide useful screening indicators to identify sicker ART-eligible individuals at high risk of dropping out of the programme who might benefit from being brought back quickly or admitted to hospital for observation. Individuals with less education may need adapted health information at screening. Deaths of ART-eligible individuals occurring prior to ART initiation are not included in routine programme statistics. Considering all those who are eligible for ART as a denominator for programme indicators would help to highlight this vulnerable group, in order to identify new opportunities for further improving ART programmes.

Bacille Calmette-Guérin (BCG) vaccination induces a marked increase in the interferon (IFN)-gamma response to Mycobacterium tuberculosis purified protein derivative (Mtb PPD) in UK adolescents, but not in Malawian adolescents. We hypothesized that Mtb PPD-induced IFN-gamma after BCG vaccination would be similar in infants from these 2 countries. Infants were vaccinated with BCG during the first 3-13 weeks of life. Three months after BCG vaccination, 51 (100%) of 51 UK infants had an IFN-gamma response to Mtb PPD, compared to 41 (53%) of 78 of Malawian infants, in whom responses varied according to their season of birth. We conclude that population differences in immune responses after BCG vaccination are observed among infants, as well as among young adults.

Background Differences in degree of environmental exposure to antigens in early life have been hypothesized to lead to differences in immune status in individuals from different populations, which may have implications for immune responses in later years. Methods Venous blood from HIV-negative adolescents and blood from the umbilical cords of babies, born to HIV-negative women, post-delivery was collected and analysed using flow cytometry. T cell phenotype was determined from peripheral blood lymphocytes and cytomegalovirus (CMV) seropositivity was assessed by ELISA in adolescents. Results HIV-negative Malawian adolescents were shown to have a lower percentage of naïve T cells (CD45RO - CD62L hi CD11a lo ), a higher proportion of memory T cells and a higher percentage of CD28 - memory (CD28 - CD45RO + ) T cells compared to age-matched UK adolescents. Malawian adolescents also had a lower percentage of central memory (CD45RA - CCR7 + ) T cells and a higher percentage of stable memory (CD45RA + CCR7 - ) T cells than UK adolescents. All of the adolescents tested in Malawi were seropositive for CMV (59/59), compared to 21/58 (36%) of UK adolescents. CMV seropositivity in the UK was associated with a reduced percentage of naïve T cells and an increased percentage of CD28 - memory T cells in the periphery. No differences in the proportions of naïve and memory T cell populations were observed in cord blood samples from the two sites. Conclusion It is likely that these differences between Malawian and UK adolescents reflect a greater natural exposure to various infections, including CMV, in the African environment and may imply differences in the ability of these populations to induce and maintain immunological memory to vaccines and natural infections.