The macroeconomic consequences of renouncing to universal access to antiretroviral treatment for HIV in Africa: a micro-simulation model

Aim

Previous economic literature on the cost-effectiveness of antiretroviral treatment (ART) programs has been mainly focused on the microeconomic consequences of alternative use of resources devoted to the fight against the HIV pandemic. We rather aim at forecasting the consequences of alternative scenarios for the macroeconomic performance of countries.

Methods

We used a micro-simulation model based on individuals aged 15–49 selected from nationally representative surveys (DHS for Cameroon, Tanzania and Swaziland) to compare alternative scenarios : 1-freezing of ART programs to current levels of access, 2- universal access (scaling up to 100% coverage by 2015, with two variants defining ART eligibility according to previous or current WHO guidelines). We introduced an “artificial” ageing process by programming methods. Individuals could evolve through different health states: HIV negative, HIV positive (with different stages of the syndrome). Scenarios of ART procurement determine this dynamics. The macroeconomic impact is obtained using sample weights that take into account the resulting age-structure of the population in each scenario and modeling of the consequences on total growth of the economy.

Results

Increased levels of ART coverage result in decreasing HIV incidence and related mortality. Universal access to ART has a positive impact on workers'' productivity; the evaluations performed for Swaziland and Cameroon show that universal access would imply net cost-savings at the scale of the society, when the full macroeconomic consequences are introduced in the calculations. In Tanzania, ART access programs imply a net cost for the economy, but 70% of costs are covered by GDP gains at the 2034 horizon, even in the extended coverage option promoted by WHO guidelines initiating ART at levels of 350 cc/mm³ CD4 cell counts.

Conclusion

Universal Access ART scaling-up strategies, which are more costly in the short term, remain the best economic choice in the long term. Renouncing or significantly delaying the achievement of this goal, due to “legitimate” short term budgetary constraints would be a misguided choice.     

Stunting,double orphanhood and unequal access to public services in democratic South Africa

Orphans who lack household or community support face significant socio-economic disadvantages. In particular, they are at greater risk of malnutrition and stunting in developing countries. Children who have no living parents, also called double orphans, are most likely to require support from extended families or public institutions. This paper explores how WASH infrastructure, and public health and social services relate to stunting. It is one of the first studies to analyse these factors with a specific focus on double orphans, who tend to live in under-serviced areas with high stunting rates and poor access to public resources. We collate a cross sectional spatial dataset with local child stunting rates from 2013, rates of double orphanhood, private household resources, and public services from 2011 for South Africa, a country where the HIV/AIDS pandemic has led to high rates of double orphanhood. We estimate spatial econometric models that account for unobserved regional shocks and measurement bias, but which do not address other biases. Our results show that high stunting rates, particularly in areas with high proportions of double orphans, overlap strongly with poor provision of WASH and the availability of household resources. By contrast, other softer services accessed outside the home, such as access to health, social welfare and early childhood development facilities are not correlated with stunting in the same way. WASH is more strongly related to reduced stunting when infrastructure covers larger geographic areas and with the combined use of services from adjacent areas. This occurs because of economies of scale in provision and preventing transmission of disease across regions. Policy makers can explore the option to reduce stunting by expanding geographic networks of WASH service delivery into under-serviced areas where double orphans tend to locate.     

Evolution of antiretroviral drug costs in Brazil in the context of free and universal access to AIDS treatment

Background

Little is known about the long-term drug costs associated with treating AIDS in developing countries. Brazil''s AIDS treatment program has been cited widely as the developing world''s largest and most successful AIDS treatment program. The program guarantees free access to highly active antiretroviral therapy (HAART) for all people living with HIV/AIDS in need of treatment. Brazil produces non-patented generic antiretroviral drugs (ARVs), procures many patented ARVs with negotiated price reductions, and recently issued a compulsory license to import one patented ARV. In this study, we investigate the drivers of recent ARV cost trends in Brazil through analysis of drug-specific prices and expenditures between 2001 and 2005.

Methods and Findings

We compared Brazil''s ARV prices to those in other low- and middle-income countries. We analyzed trends in drug expenditures for HAART in Brazil from 2001 to 2005 on the basis of cost data disaggregated by each ARV purchased by the Brazilian program. We decomposed the overall changes in expenditures to compare the relative impacts of changes in drug prices and drug purchase quantities. We also estimated the excess costs attributable to the difference between prices for generics in Brazil and the lowest global prices for these drugs. Finally, we estimated the savings attributable to Brazil''s reduced prices for patented drugs. Negotiated drug prices in Brazil are lowest for patented ARVs for which generic competition is emerging. In recent years, the prices for efavirenz and lopinavir–ritonavir (lopinavir/r) have been lower in Brazil than in other middle-income countries. In contrast, the price of tenofovir is US$200 higher per patient per year than that reported in other middle-income countries. Despite precipitous price declines for four patented ARVs, total Brazilian drug expenditures doubled, to reach US$414 million in 2005. We find that the major driver of cost increases was increased purchase quantities of six specific drugs: patented lopinavir/r, efavirenz, tenofovir, atazanavir, enfuvirtide, and a locally produced generic, fixed-dose combination of zidovudine and lamivudine (AZT/3TC). Because prices declined for many of the patented drugs that constitute the largest share of drug costs, nearly the entire increase in overall drug expenditures between 2001 and 2005 is attributable to increases in drug quantities. Had all drug quantities been held constant from 2001 until 2005 (or for those drugs entering treatment guidelines after 2001, held constant between the year of introduction and 2005), total costs would have increased by only an estimated US$7 million. We estimate that in the absence of price declines for patented drugs, Brazil would have spent a cumulative total of US$2 billion on drugs for HAART between 2001 and 2005, implying a savings of US$1.2 billion from price declines. Finally, in comparing Brazilian prices for locally produced generic ARVs to the lowest international prices meeting global pharmaceutical quality standards, we find that current prices for Brazil''s locally produced generics are generally much higher than corresponding global prices, and note that these prices have risen in Brazil while declining globally. We estimate the excess costs of Brazil''s locally produced generics totaled US$110 million from 2001 to 2005.

Conclusions

Despite Brazil''s more costly generic ARVs, the net result of ARV price changes has been a cost savings of approximately US$1 billion since 2001. HAART costs have nevertheless risen steeply as Brazil has scaled up treatment. These trends may foreshadow future AIDS treatment cost trends in other developing countries as more people start treatment, AIDS patients live longer and move from first-line to second and third-line treatment, AIDS treatment becomes more complex, generic competition emerges, and newer patented drugs become available. The specific application of the Brazilian model to other countries will depend, however, on the strength of their health systems, intellectual property regulations, epidemiological profiles, AIDS treatment guidelines, and differing capacities to produce drugs locally.     

Pregnancy and virologic response to antiretroviral therapy in South Africa

Background

Although women of reproductive age are the largest group of HIV-infected individuals in sub-Saharan Africa, little is known about the impact of pregnancy on response to highly active antiretroviral therapy (HAART) in that setting. We examined the effect of incident pregnancy after HAART initiation on virologic response to HAART.

Methods and Findings

We evaluated a prospective clinical cohort of adult women who initiated HAART in Johannesburg, South Africa between 1 April 2004 and 30 September 2009, and followed up until an event, death, transfer, drop-out, or administrative end of follow-up on 31 March 2010. Women over age 45 and women who were pregnant at HAART initiation were excluded from the study; final sample size for analysis was 5,494 women. Main exposure was incident pregnancy, experienced by 541 women; main outcome was virologic failure, defined as a failure to suppress virus to ≤400 copies/ml by six months or virologic rebound >400 copies/ml thereafter. We calculated adjusted hazard ratios using marginal structural Cox proportional hazards models and weighted lifetable analysis to calculate adjusted five-year risk differences. The weighted hazard ratio for the effect of pregnancy on time to virologic failure was 1.34 (95% confidence limit [CL] 1.02, 1.78). Sensitivity analyses generally confirmed these main results.

Conclusions

Incident pregnancy after HAART initiation was associated with modest increases in both relative and absolute risks of virologic failure, although uncontrolled confounding cannot be ruled out. Nonetheless, these results reinforce that family planning is an essential part of care for HIV-positive women in sub-Saharan Africa. More work is needed to confirm these findings and to explore specific etiologic pathways by which such effects may operate.     

Affordable ART services in Africa: synthesis and adaptation of laboratory services

¹ Correspondence address. E-mail: carin.huyser{at}up.ac.za The aim of this paper is to provide information, opinions andsuggestions on affordable laboratory-orientated fertility screeningand treatment. Resource management to provide such servicesin developing countries, basic and advanced assisted reproductiveservices and assisted reproduction treatment (ART) of patientswith sexually transmitted infections are addressed. Alternativeviewpoints and parallel thinking should be encouraged to synthesizeand adapt first-world ART guidelines and recommendations intosafe and workable directives for developing regions. AffordableAfrican ART programmes, devoid of commercialism, can provideessential sexual health screening services en route to safefertility services for human immunodeficiency virus type-1 (HIV-1)serodiscordant couples (male HIV-positive), who wish to havetheir own biological child.     

Contraceptive use and method preference among women in Soweto, South Africa: the influence of expanding access to HIV care and treatment services

Objective

Preventing unintended pregnancy among HIV-positive women constitutes a critical and cost-effective approach to primary prevention of mother-to-child transmission of HIV and is a global public health priority for addressing the desperate state of maternal and child health in HIV hyper-endemic settings. We sought to investigate whether the prevalence of contraceptive use and method preferences varied by HIV status and receipt of highly active antiretroviral therapy (HAART) among women in Soweto, South Africa.

Methods

We used survey data from 563 sexually active, non-pregnant women (18–44 years) recruited from the Perinatal HIV Research Unit in Soweto (May–December, 2007); 171 women were HIV-positive and receiving HAART (median duration of use = 31 months; IQR = 28, 33), 178 were HIV-positive and HAART-naïve, and 214 were HIV-negative. Medical record review was conducted to confirm HIV status and clinical variables. Logistic regression models estimated adjusted associations between HIV status, receipt of HAART, and contraceptive use.

Results

Overall, 78% of women reported using contraception, with significant variation by HIV status: 86% of HAART users, 82% of HAART-naïve women, and 69% of HIV-negative women (p<0.0001). In adjusted models, compared with HIV-negative women, women receiving HAART were significantly more likely to use contraception while HAART-naïve women were non-significantly more likely (AOR: 2.40; 95% CI: 1.25, 4.62 and AOR: 1.59; 95% CI: 0.88, 2.85; respectively). Among HIV-positive women, HAART users were non-significantly more likely to use contraception compared with HAART-naïve women (AOR: 1.55; 95% CI: 0.84, 2.88). Similar patterns held for specific use of barrier (primarily male condoms), permanent, and dual protection contraceptive methods.

Conclusion

Among HIV-positive women receiving HAART, the observed higher prevalence of contraceptive use overall and condoms in particular promises to yield fewer unintended pregnancies and reduced risks of vertical and sexual HIV transmission. These findings highlight the potential of integrated HIV and reproductive health services to positively impact maternal, partner, and child health.     

Risk factors associated with increased mortality among HIV infected children initiating antiretroviral therapy (ART) in South Africa

Objective

To identify demographic and clinical risk factors associated with mortality after initiation of antiretroviral therapy (ART) in a cohort of human immunodeficiency (HIV) infected children in KwaZulu-Natal, South Africa.

Methods

We performed a retrospective cohort study of 537 children initiating antiretroviral therapy at McCord Hospital in KwaZulu-Natal, South Africa. Data were extracted from electronic medical records and risk factors associated with mortality were assessed using Cox regression analysis.

Results

Overall there were 47 deaths from the cohort of 537 children initiating ART with over 991 child-years of follow-up (median 22 months on ART), yielding a mortality rate of 4.7 deaths per 100 child years on ART. Univariate analysis indicated that mortality was significantly associated with lower weight-for-age Z-score (p<0.0001), chronic diarrhea (p = 0.0002), lower hemoglobin (p = 0.002), age <3 years (p = 0.003), and CD4% <10% (p = 0.005). The final multivariable Cox proportional hazards mortality model found age less than 3 years (p = 0.004), CD4 <10% (p = 0.01), chronic diarrhea (p = 0.03), weight-for-age Z-score (<0.0001) and female gender as a covariate varying with time (p = 0.03) all significantly associated with mortality.

Conclusion

In addition to recognized risk factors such as young age and advanced immunosuppression, we found female gender to be significantly associated with mortality in this pediatric ART cohort. Future studies are needed to determine whether intrinsic biologic differences or socio-cultural factors place female children with HIV at increased risk of death following initiation of ART.     

Linkage to HIV care and antiretroviral therapy in Cape Town, South Africa

Background

Antiretroviral therapy (ART) has been scaled-up rapidly in Africa. Programme reports typically focus on loss to follow-up and mortality among patients receiving ART. However, little is known about linkage and retention in care of individuals prior to starting ART.

Methodology

Data on adult residents from a periurban community in Cape Town were collected at a primary care clinic and hospital. HIV testing registers, CD4 count results provided by the National Health Laboratory System and ART registers were linked. A random sample (n = 885) was drawn from adults testing HIV positive through antenatal care, sexual transmitted disease and voluntary testing and counseling services between January 2004 and March 2009. All adults (n = 103) testing HIV positive through TB services during the same time period were also included in the study. Linkage to HIV care was defined as attending for a CD4 count measurement within 6 months of HIV diagnosis. Linkage to ART care was defined as initiating ART within 6 months of HIV diagnosis in individuals with a CD4 count ≤200 cells/µl taken within 6 months of HIV diagnosis.

Findings

Only 62.6% of individuals attended for a CD4 count measurement within 6 months of testing HIV positive. Individuals testing through sexually transmitted infection services had the best (84.1%) and individuals testing on their own initiative (53.5%) the worst linkage to HIV care. One third of individuals with timely CD4 counts were eligible for ART and 66.7% of those were successfully linked to ART care. Linkage to ART care was highest among antenatal care clients. Among individuals not yet eligible for ART only 46.3% had a repeat CD4 count. Linkage to HIV care improved in patients tested in more recent calendar period.

Conclusion

Linkage to HIV and ART care was low in this poor peri-urban community despite free services available within close proximity. More efforts are needed to link VCT scale-up to subsequent care.     

Expanding ART for treatment and prevention of HIV in South Africa: estimated cost and cost-effectiveness 2011-2050

Background

Antiretroviral Treatment (ART) significantly reduces HIV transmission. We conducted a cost-effectiveness analysis of the impact of expanded ART in South Africa.

Methods

We model a best case scenario of 90% annual HIV testing coverage in adults 15–49 years old and four ART eligibility scenarios: CD4 count <200 cells/mm³ (current practice), CD4 count <350, CD4 count <500, all CD4 levels. 2011–2050 outcomes include deaths, disability adjusted life years (DALYs), HIV infections, cost, and cost per DALY averted. Service and ART costs reflect South African data and international generic prices. ART reduces transmission by 92%. We conducted sensitivity analyses.

Results

Expanding ART to CD4 count <350 cells/mm³ prevents an estimated 265,000 (17%) and 1.3 million (15%) new HIV infections over 5 and 40 years, respectively. Cumulative deaths decline 15%, from 12.5 to 10.6 million; DALYs by 14% from 109 to 93 million over 40 years. Costs drop $504 million over 5 years and $3.9 billion over 40 years with breakeven by 2013. Compared with the current scenario, expanding to <500 prevents an additional 585,000 and 3 million new HIV infections over 5 and 40 years, respectively. Expanding to all CD4 levels decreases HIV infections by 3.3 million (45%) and costs by $10 billion over 40 years, with breakeven by 2023. By 2050, using higher ART and monitoring costs, all CD4 levels saves $0.6 billion versus current; other ART scenarios cost $9–194 per DALY averted. If ART reduces transmission by 99%, savings from all CD4 levels reach $17.5 billion. Sensitivity analyses suggest that poor retention and predominant acute phase transmission reduce DALYs averted by 26% and savings by 7%.

Conclusion

Increasing the provision of ART to <350 cells/mm3 may significantly reduce costs while reducing the HIV burden. Feasibility including HIV testing and ART uptake, retention, and adherence should be evaluated.     

Public-health and individual approaches to antiretroviral therapy: township South Africa and Switzerland compared

Background

The provision of highly active antiretroviral therapy (HAART) in resource-limited settings follows a public health approach, which is characterised by a limited number of regimens and the standardisation of clinical and laboratory monitoring. In industrialized countries doctors prescribe from the full range of available antiretroviral drugs, supported by resistance testing and frequent laboratory monitoring. We compared virologic response, changes to first-line regimens, and mortality in HIV-infected patients starting HAART in South Africa and Switzerland.

Methods and Findings

We analysed data from the Swiss HIV Cohort Study and two HAART programmes in townships of Cape Town, South Africa. We included treatment-naïve patients aged 16 y or older who had started treatment with at least three drugs since 2001, and excluded intravenous drug users. Data from a total of 2,348 patients from South Africa and 1,016 patients from the Swiss HIV Cohort Study were analysed. Median baseline CD4⁺ T cell counts were 80 cells/μl in South Africa and 204 cells/μl in Switzerland. In South Africa, patients started with one of four first-line regimens, which was subsequently changed in 514 patients (22%). In Switzerland, 36 first-line regimens were used initially, and these were changed in 539 patients (53%). In most patients HIV-1 RNA was suppressed to 500 copies/ml or less within one year: 96% (95% confidence interval [CI] 95%–97%) in South Africa and 96% (94%–97%) in Switzerland, and 26% (22%–29%) and 27% (24%–31%), respectively, developed viral rebound within two years. Mortality was higher in South Africa than in Switzerland during the first months of HAART: adjusted hazard ratios were 5.90 (95% CI 1.81–19.2) during months 1–3 and 1.77 (0.90–3.50) during months 4–24.

Conclusions

Compared to the highly individualised approach in Switzerland, programmatic HAART in South Africa resulted in similar virologic outcomes, with relatively few changes to initial regimens. Further innovation and resources are required in South Africa to both achieve more timely access to HAART and improve the prognosis of patients who start HAART with advanced disease.     

Attrition through Multiple Stages of Pre-Treatment and ART HIV Care in South Africa

Introduction

While momentum for increasing treatment thresholds is growing, if patients cannot be retained in HIV care from the time of testing positive through long-term adherence to antiretroviral therapy (ART), such strategies may fall short of expected gains. While estimates of retention on ART exist, few cohorts have data on retention from testing positive through long-term ART care.

Methods

We explored attrition (loss or death) at the Themba Lethu HIV clinic, Johannesburg, South Africa in 3 distinct cohorts enrolled at HIV testing, pre-ART initiation, and ART initiation.

Results

Between March 2010 and August 2012 we enrolled 380 patients testing HIV+, 206 initiating pre-ART care, and 185 initiating ART. Of the 380 patients enrolled at testing HIV-positive, 38.7% (95%CI: 33.9–43.7%) returned for eligibility staging within ≤3 months of testing. Of the 206 enrolled at pre-ART care, 84.5% (95%CI: 79.0–88.9%) were ART eligible at their first CD4 count. Of those, 87.9% (95%CI: 82.4–92.2%) initiated ART within 6 months. Among patients not ART eligible at their first CD4 count, 50.0% (95%CI: 33.1–66.9%) repeated their CD4 count within one year of the first ineligible CD4. Among the 185 patients in the ART cohort, 22 transferred out and were excluded from further analysis. Of the remaining 163, 81.0% (95%CI: 74.4–86.5%) were retained in care through two years on treatment.

Conclusions

Our findings from a well-resourced clinic demonstrate continual loss from all stages of HIV care and strategies to reduce attrition from all stages of care are urgently needed.     

The impact of the new WHO antiretroviral treatment guidelines on HIV epidemic dynamics and cost in South Africa

BACKGROUND: Since November 2009, WHO recommends that adults infected with HIV should initiate antiretroviral therapy (ART) at CD4+ cell counts of ≤350 cells/μl rather than ≤200 cells/μl. South Africa decided to adopt this strategy for pregnant and TB co-infected patients only. We estimated the impact of fully adopting the new WHO guidelines on HIV epidemic dynamics and associated costs. METHODS AND FINDING: We used an established model of the transmission and control of HIV in specified sexual networks and healthcare settings. We quantified the model to represent Hlabisa subdistrict, KwaZulu-Natal, South Africa. We predicted the HIV epidemic dynamics, number on ART and program costs under the new guidelines relative to treating patients at ≤200 cells/μl for the next 30 years. During the first five years, the new WHO treatment guidelines require about 7% extra annual investments, whereas 28% more patients receive treatment. Furthermore, there will be a more profound impact on HIV incidence, leading to relatively less annual costs after seven years. The resulting cumulative net costs reach a break-even point after on average 16 years. CONCLUSIONS: Our study strengthens the WHO recommendation of starting ART at ≤350 cells/μl for all HIV-infected patients. Apart from the benefits associated with many life-years saved, a modest frontloading appears to lead to net savings within a limited time-horizon. This finding is robust to alternative assumptions and foreseeable changes in ART prices and effectiveness. Therefore, South Africa should aim at rapidly expanding its healthcare infrastructure to fully embrace the new WHO guidelines.     

Land management implications for ecosystem services in a South African rangeland

In South Africa, restoration and sustainable management of historically overgrazed and degraded rangelands are promoted to increase biodiversity and ecosystem service provision. This study evaluates different land management scenarios in terms of ecosystem services in a South African rangeland, the Baviaanskloof catchment. As measured data were limited, we used simple models to quantify and map the effect of the different combination of agricultural, nature conservation and restoration practices on multiple ecosystem services. The land management scenarios were evaluated against management targets set for individual ecosystem services. Results highlight how the provision of ecosystem services is related to land management as unmanaged, pristine ecosystems provide a different mix of ecosystem services than ecosystems recently restored or managed as grazing lands. Results also indicate that historically overgrazed lands provide no forage, may retain 40% less sediment and have 38% lower biodiversity, while providing 60% more fuel wood and supplying two and half times more water (i.e. retaining less water), than pristine or restored lands. We conclude that a combination of light grazing, low input agriculture, nature conservation and restoration is the best for the sufficient provision of multiple ecosystem services. Applying such mixed management would improve biodiversity, ecotourism and maintain forage production and regulating services on farmers’ land. This management option also fits into and further optimizes local decision-makers’ vision regarding the future management of the area.     

Correcting mortality for loss to follow-up: a nomogram applied to antiretroviral treatment programmes in sub-Saharan Africa

Background

The World Health Organization estimates that in sub-Saharan Africa about 4 million HIV-infected patients had started antiretroviral therapy (ART) by the end of 2008. Loss of patients to follow-up and care is an important problem for treatment programmes in this region. As mortality is high in these patients compared to patients remaining in care, ART programmes with high rates of loss to follow-up may substantially underestimate mortality of all patients starting ART.

Methods and Findings

We developed a nomogram to correct mortality estimates for loss to follow-up, based on the fact that mortality of all patients starting ART in a treatment programme is a weighted average of mortality among patients lost to follow-up and patients remaining in care. The nomogram gives a correction factor based on the percentage of patients lost to follow-up at a given point in time, and the estimated ratio of mortality between patients lost and not lost to follow-up. The mortality observed among patients retained in care is then multiplied by the correction factor to obtain an estimate of programme-level mortality that takes all deaths into account. A web calculator directly calculates the corrected, programme-level mortality with 95% confidence intervals (CIs). We applied the method to 11 ART programmes in sub-Saharan Africa. Patients retained in care had a mortality at 1 year of 1.4% to 12.0%; loss to follow-up ranged from 2.8% to 28.7%; and the correction factor from 1.2 to 8.0. The absolute difference between uncorrected and corrected mortality at 1 year ranged from 1.6% to 9.8%, and was above 5% in four programmes. The largest difference in mortality was in a programme with 28.7% of patients lost to follow-up at 1 year.

Conclusions

The amount of bias in mortality estimates can be large in ART programmes with substantial loss to follow-up. Programmes should routinely report mortality among patients retained in care and the proportion of patients lost. A simple nomogram can then be used to estimate mortality among all patients who started ART, for a range of plausible mortality rates among patients lost to follow-up. Please see later in the article for the Editors'' Summary     

Spatial ecology of freshwater eels in South Africa: implications for conservation

Hydrobiologia - Four freshwater eel species (Anguilla spp.) occur in southern African rivers that are increasingly anthropogenically modified. Information on movements, habitat use and overlap by...     

Malaria mortality rates in South Asia and in Africa: implications for malaria control

Malaria mortality in human populations varies greatly under different circumstances. The intense malaria transmission conditions found in many parts of tropical Africa, the much lower malaria inoculation rates currently sustained in areas of southern Asia, and the epidemic outbreaks of malaria occasionally seen on both continents, present highly contrasting patterns of malaria-related mortality. Here Harsha Alles, Kamini Mendis and Richard Carter examine malaria-related mortality under different circumstances and discuss implications for the management of malaria in these settings. They emphasize the power of rapid case treatment to save lives at risk under virtually all circumstances of malaria transmission.