Plasmodium knowlesi transmission: integrating quantitative approaches from epidemiology and ecology to understand malaria as a zoonosis.

The public health threat posed by zoonotic Plasmodium knowlesi appears to be growing: it is increasingly reported across South East Asia, and is the leading cause of malaria in Malaysian Borneo. Plasmodium knowlesi threatens progress towards malaria elimination as aspects of its transmission, such as spillover from wildlife reservoirs and reliance on outdoor-biting vectors, may limit the effectiveness of conventional methods of malaria control. The development of new quantitative approaches that address the ecological complexity of P. knowlesi, particularly through a focus on its primary reservoir hosts, will be required to control it. Here, we review what is known about P. knowlesi transmission, identify key knowledge gaps in the context of current approaches to transmission modelling, and discuss the integration of these approaches with clinical parasitology and geostatistical analysis. We highlight the need to incorporate the influences of fine-scale spatial variation, rapid changes to the landscape, and reservoir population and transmission dynamics. The proposed integrated approach would address the unique challenges posed by malaria as a zoonosis, aid the identification of transmission hotspots, provide insight into the mechanistic links between incidence and land use change and support the design of appropriate interventions.

Sexual-stage antibody responses to P. falciparum in endemic populations.

Gametocytes and sporogonic stages are responsible for the spread of disease and drug resistance in the population. Sexual stage immunity affects the infectiousness of gametocytes to mosquitoes. Specific antibodies including anti-Pfs48/45 and anti-Pfs230 antibodies are found in individuals with limited prior exposure to malaria. Sexual stage antibodies are rapidly acquired after infection and are relatively prevalent in gametocytaemic individuals. Functional transmission reducing activity (TRA) is found after primary infections and in young children and appears to depend on recent rather than cumulative exposure to gametocytes. Exposure to gametocytes decreases with age most likely as a consequence of the acquisition of asexual-stage immunity that controls asexual parasite density and consequently gametocytaemia. This results in lower exposure to the antigenic load of gametocytes in semi-immune individuals. Since sexual stage immunity is probably short-lived in the absence of gametocytes, we hypothesize that sexual stage immunity will wane, resulting in low antibody and TRA prevalences in clinically semi-immune carriers.

Spatial and temporal variation in malaria transmission in a low endemicity area in northern Tanzania.

BACKGROUND: Spatial and longitudinal monitoring of transmission intensity will allow better targeting of malaria interventions. In this study, data on meteorological, demographic, entomological and parasitological data over the course of a year was collected to describe malaria epidemiology in a single village of low transmission intensity. METHODS: Entomological monitoring of malaria vectors was performed by weekly light trap catches in 10 houses. Each house in the village of Msitu wa Tembo, Lower Moshi, was mapped and censused. Malaria cases identified through passive case detection at the local health centre were mapped by residence using GIS software and the incidence of cases by season and distance to the main breeding site was calculated. RESULTS: The principle vector was Anopheles arabiensis and peak mosquito numbers followed peaks in recent rainfall. The entomological inoculation rate estimated was 3.4 (95% CI 0.7-9.9) infectious bites per person per year. The majority of malaria cases (85/130) occurred during the rainy season (chi2 = 62,3, p < 0.001). Living further away from the river (OR 0.96, CI 0.92-0.998, p = 0.04 every 50 m) and use of anti-insect window screens (OR 0.65, CI 0.44-0.94, p = 0.023) were independent protective factors for the risk of malaria infection. Children aged 1-5 years and 5-15 years were at greater risk of clinical episodes (OR 2.36, CI 1.41-3.97, p = 0.001 and OR 3.68, CI 2.42-5.61, p < 0.001 respectively). CONCLUSION: These data show that local malaria transmission is restricted to the rainy season and strongly associated with proximity to the river. Transmission reducing interventions should, therefore, be timed before the rain-associated increase in mosquito numbers and target households located near the river.

Polymorphism of the Pfmdr1 gene and chloroquine resistance in Plasmodium falciparum in The Gambia.

To assess the level of resistance to chloroquine (CQ) of Plasmodium falciparum in The Gambia in 1995-1996 we measured susceptibility in vivo by quantifying parasitaemia of children with mild malaria on days 4 and 8 after treatment. Pretreatment blood samples were used for susceptibility testing in vitro by the World Health Organization microtest and the prevalence of the tyrosine (tyr)86 allele of the Pfmdr1 gene was assessed by the polymerase chain reaction and restriction fragment length polymorphism analysis. Seven of 42 children (17%) treated with CQ remained parasitaemic on day 4 and required a change of antimalarial treatment. Susceptibility assays in vitro were performed on 50 P. falciparum isolates obtained from eligible children before treatment; 36 (72%) were resistant to CQ (> or = 1.6 mumol/L). The median minimum inhibitory concentration (MIC) of artemether was 3.38 nmol/L (range 0.42-13.51 nmol/L) and the median MIC of dihydroartemisinin was 0.88 nmol/L (range 0.22-14.04 nmol/L). Susceptibility in vitro to CQ and the Pfmdr1 genotype were determined for 31 fresh isolates. The allele was present in 12 of 22 isolates found to be resistant to CQ in vitro, but in none of the 9 isolates which were susceptible (Fisher's exact test, P = 0.005). All P. falciparum isolates with the tyr86 allele were CQ resistant in vitro, but since only half of all resistant isolates contained the allele, additional explanations for CQ resistance are required.

Estimates of the infectious reservoir of Plasmodium falciparum malaria in The Gambia and in Tanzania.

Separate studies carried out in Farafenni, The Gambia and Ifakara, Tanzania in 1990-94 provided comparative data on population age structure, population gametocyte prevalences and gametocyte carrier infectivity. The percentage of the population estimated to be infective to mosquitoes was 5.5% and 3.8% in The Gambia and Tanzania, respectively. The age groups 1-4 years, 5-9 years, 10-19 years and 20 years or more comprised 17.5%, 21.7%, 22.2% and 37.9%, respectively, of the infectious population in The Gambia; the corresponding figures for Tanzania were 30.9%, 25.2%, 15.7% and 28.1%. These figures are in broad agreement with those from other published studies which estimated the infectious reservoir directly and suggest that adults contribute significantly to the infectious reservoir of malaria, particularly in areas of intense seasonal transmission. Control measures aimed at reduction of transmission may have only a limited effect in areas of moderate seasonal transmission if directed only at children.

Factors that are associated with the risk of acquiring Plasmodium knowlesi malaria in Sabah, Malaysia: a case-control study protocol.

INTRODUCTION: Plasmodium knowlesi has long been present in Malaysia, and is now an emerging cause of zoonotic human malaria. Cases have been confirmed throughout South-East Asia where the ranges of its natural macaque hosts and Anopheles leucosphyrus group vectors overlap. The majority of cases are from Eastern Malaysia, with increasing total public health notifications despite a concurrent reduction in Plasmodium falciparum and P. vivax malaria. The public health implications are concerning given P. knowlesi has the highest risk of severe and fatal disease of all Plasmodium spp in Malaysia. Current patterns of risk and disease vary based on vector type and competence, with individual exposure risks related to forest and forest-edge activities still poorly defined. Clustering of cases has not yet been systematically evaluated despite reports of peri-domestic transmission and known vector competence for human-to-human transmission. METHODS AND ANALYSIS: A population-based case-control study will be conducted over a 2-year period at two adjacent districts in north-west Sabah, Malaysia. Confirmed malaria cases presenting to the district hospital sites meeting relevant inclusion criteria will be requested to enrol. Three community controls matched to the same village as the case will be selected randomly. Study procedures will include blood sampling and administration of household and individual questionnaires to evaluate potential exposure risks associated with acquisition of P. knowlesi malaria. Secondary outcomes will include differences in exposure variables between P. knowlesi and other Plasmodium spp, risk of severe P. knowlesi malaria, and evaluation of P. knowlesi case clustering. Primary analysis will be per protocol, with adjusted ORs for exposure risks between cases and controls calculated using conditional multiple logistic regression models. ETHICS: This study has been approved by the human research ethics committees of Malaysia, the Menzies School of Health Research, Australia, and the London School of Hygiene and Tropical Medicine, UK.

A longitudinal study of immune responses to Plasmodium falciparum sexual stage antigens in Tanzanian adults.

Next to children, adults form a substantial part of the infectious reservoir that is responsible for the spread of malaria. In this longitudinal study, we determined sexual stage immune responses to Plasmodium falciparum and infectiousness to mosquitoes in adults from an area with intense malaria transmission. A cohort of 43 Tanzanian adults was followed for 18 months. Parasitological data were collected monthly; serum once every three months. Antibody prevalences were determined for sexual stage antigens Pfs230 and Pfs48/45 and circumsporozoite protein (NANP5)(n = 199). Functional transmission reducing activity (TRA) was assessed by standard membrane feeding assay (SMFA; n = 85). Cumulative parasite prevalence was 67.4% (29/43) for asexual stages and 34.9% (15/43) for gametocytes. Enrolment antibody prevalence was 95.3% (41/43) for NANP5, 18.9% (7/37) for Pfs230 and 7% (3/43) for Pfs48/45 epitope 3. TRA > 50% reduction was seen in 48.2% (41/85) and TRA > 90% reduction in 4.7% (4/85) of the samples. Our findings of low and inconsistent sexual stage immune responses are likely to be the result of a low exposure to gametocytes in this older age group. This may in turn be caused by effective asexual stage immunity. We conclude that the infectivity of older individuals is less likely to be affected by sexual stage immunity.

Transmission-reducing immunity is inversely related to age in Plasmodium falciparum gametocyte carriers.

Immunity to the sexual stages of Plasmodium falciparum is induced during natural infections and can significantly reduce the transmission of parasites to mosquitoes (transmission reducing activity; TRA) but little is known about how these responses develop with increasing age/exposure to malaria. Routinely TRA is measured in the standard membrane feeding assay (SMFA). Sera were collected from a total of 199 gametocyte carriers (median age 4 years, quartiles 2 and 9 years) near Ifakara, Tanzania; 128 samples were tested in the SMFA and generated TRA data classified as a reduction of > 50% and > 90% of transmission. TRA of > 50% was highest in young children (aged 1-2) with a significant decline with age (chi(2) trend = 5.79, P = 0.016) and in logistic regression was associated with prevalence of antibodies to both Pfs230 and Pfs48/45 (OR 4.03, P = 0.011 and OR 2.43 P = 0.059, respectively). A TRA of > 90% reduction in transmission was not age related but was associated with antibodies to Pfs48/45 (OR 2.36, P = 0.055). Our data confirm that antibodies are an important component of naturally induced TRA. However, whilst a similar but small proportion of individuals at all ages have TRA > 90%, the gradual deterioration of TRA > 50% with age suggests decreased antibody concentration or affinity. This may be due to decreased exposure to gametocytes, probably as a result of increased asexual and/or gametocyte specific immunity.

Estimating medium- and long-term trends in malaria transmission by using serological markers of malaria exposure.

The implementation and evaluation of malaria control programs would be greatly facilitated by new tools for the rapid assessment of malaria transmission intensity. Because acquisition and maintenance of antimalarial antibodies depend on exposure to malaria infection, such antibodies might be used as proxy measures of transmission intensity. We have compared the prevalence of IgG antibodies with three Plasmodium falciparum asexual stage antigens in individuals of all ages living at varying altitudes encompassing a range of transmission intensities from hyper- to hypoendemic in northeastern Tanzania, with alternative measures of transmission intensity. The prevalence of antibodies to merozoite surface protein-1(19) was significantly more closely correlated with altitude than either point-prevalence malaria parasitemia or single measures of hemoglobin concentration. Analysis of age-specific seroprevalence rates enabled differentiation of recent (seasonal) changes in transmission intensity from longer-term transmission trends and, using a mathematical model of the annual rate of seroconversion, estimation of the longevity of the antibody response. Thus, serological tools allow us to detect variations in malaria transmission over time. Such tools will be invaluable for monitoring trends in malaria endemicity and the effectiveness of malaria control programs.

Transmission-blocking effects of sera from malaria-exposed individuals on Plasmodium falciparum isolates from gametocyte carriers.

Sera from donors exposed to malaria were tested for their ability to block the transmission of isolates from Cameroonian Plasmodium falciparum gametocyte carriers. Sera were selected from amongst Cameroonian and Gambian donors who had positive antibody reactivity against the surface of activated gametes and against epitopes of Pfs 48/45 (a potential transmission-blocking vaccine candidate antigen). Aliquots of washed blood from gametocyte carriers were resuspended in test and control sera and fed to An. gambiae mosquitoes via a membrane feeder. Comparisons of the prevalence and intensity of infections is dissected mosquitoes showed variations in the ability of sera to block the transmission of the different isolates. Sera were identified that had little or no blocking effect on the transmission of isolates unless the isolate was poorly infectious. Some sera completely blocked the transmission of some isolates whilst having little or no effect on others. The observed variation in transmission-modulating activity may have implications for the development of a transmission-blocking vaccine.

Host haematological factors influencing the transmission of Plasmodium falciparum gametocytes to Anopheles gambiae s.s. mosquitoes.

We investigated the relationship between selected host haematological and parasitological parameters and the density and infectivity of Plasmodium falciparum gametocytes. 143 individuals (age range 1-62 years) attending an outpatient clinic in Farafenni, The Gambia, who had peripheral blood gametocytaemia were recruited (mean gametocyte density 123.7/microl, range 5-17,000/microl). Of the parameters measured, packed cell volume (PCV), reticulocyte count (RetC) and lymphocyte count (LyC) were significantly associated with gametocyte density (r = - 0.17, P < 0.05; r = 0.21, P < 0.01; r = 0.18, P < 0.05, respectively). Data from membrane feeding experiments in which 15 or more mosquitoes were dissected showed that 60.7% (53/87) of gametocyte carriers infected one or more mosquitoes. Gametocyte density was strongly correlated with transmission success (TS) (r = 0.3, P < 0.005) and, in successful infections, with both mosquito prevalence (MP) (r = 0.36, P < 0.005) and mean oocyst burden (MOB) (r = 0.65, P < 0.0001). None of the other factors measured were significantly associated with any of these indices in bivariate analysis. Regression modelling showed that both gametocyte density and PCV were positively associated with gametocyte carrier infectivity to mosquitoes (LRchi2 = 100.7 and 47.2, respectively) and, in successful infections, with MOB (beta = 0.16, t = 4.9, P < 0.001; beta = 0.02, t = 2.3, P < 0.05, respectively). The positive association with PCV suggests that blood meal quality influences infection probably as a nutritional requirement, however, as this effect was most apparent at high gametocyte densities, its epidemiological significance is questionable. Though the haematological parameters associated with gametocyte density are a direct consequence of asexual infection, they may also represent an adaptive mechanism for optimization of sexual stage development.

Diversion of Anopheles gambiae from children to other hosts following exposure to permethrin-treated bednets.

Permethrin-treated bednets reduce mortality and morbidity from malaria in Gambian children. However, it is not certain how this effect is achieved, as neither mosquito numbers nor the human blood index of indoor-resting female Anopheles gambiae Giles sensu lato (Diptera: Culicidae) mosquitoes have been reduced when treated bednets were introduced into a community. One possibility is that insecticide-treated bednets divert mosquitoes from children to adults. To investigate this hypothesis, a cross-over trial with insecticide-treated bednets was undertaken in two small Gambian villages. To differentiate mosquitoes that had fed on children from those that had fed on adults, all children in the study villages were immunized with rabies vaccine before the trial. Using the detection of rabies antibody in a bloodmeal as an indicator that a mosquito had bitten a child, it was found that the percentage of blood-fed mosquitoes caught indoors that had bitten a child fell significantly from 30.8% to 9.2% and from 28.0% to 6.9% in each village after insecticide-treated bednets were introduced. To investigate the possibility that some diversion to animals had occurred, a PCR analysis for human beta-globin DNA was undertaken on selected samples. The results of this investigation were confusing, as some rabies-antibody positive bloodmeals were negative for human DNA. This may have been due to cross-reacting antibodies in animal sera and/or DNA degradation by digestion in the mosquito. Although good evidence for diversion of mosquitoes away from children was obtained, it remains uncertain whether diversion was mainly to adult humans, to animals or to both.

Humoral immune responses in Gambians to Pfs16, an immunodominant, Plasmodium falciparum integral membrane protein.

Naturally acquired humoral immune responses to Pfs16, an integral membrane protein expressed in Plasmodium falciparum gametocytes and sporozoites, were investigated in The Gambia. A high prevalence of antibodies to this molecule was detected by peptide ELISA. Ninety-three per cent of the people taking part in a survey at the end of the rainy season (November) had serum antibodies to one or more synthetic peptides spanning the sequence: 88% reacted with one particular peptide sequence (IMLIILSGIVGFKVK) whereas only one out of ten non-Gambians (taking anti-malarial prophylaxis with no history of infection) reacted with the peptide. Epitope mapping with mouse MoAbs has shown that this peptide is located on a part of the molecule differing from immunodominant regions of the molecule identified in a previous study in Papua New Guinea.

Serological responses of Gambian children to immunization with the malaria vaccine SPf66.

Antibody responses to the malaria vaccine SPf66 and to its constituent peptides were measured over a period of 2 years in Gambian children who had been immunized with SPf66 or with a control vaccine (inactivated polio vaccine). Three hundred and six of 308 children (99%) who had received three doses of SPf66 vaccine had antibodies to SPf66 at a level above that found in European controls who had not been exposed to malaria. Responses to the constituent peptides derived from 35.1, 55.1 and 83.1-kDa proteins were found in 88%, 97% and 97% of children, respectively; 26% had an antibody response to the NANP repeat peptide of circumsporozoite protein which is also included in the SPf66 vaccine. A response to SPf66 was found in 22% of children who had received the control vaccine. Antibody responses to NANP, 35.1, 55.1 and 83.1-kDa peptide were found in 3%, 33%, 49% and 33% of these children. Overall, no significant correlation was found between the level of anti-SPf66 antibody at the beginning of the malaria transmission season following vaccination and the subsequent risk of malaria. However, further analysis showed that among the control children who had acquired antibodies to SPf66 as a result of natural exposure to malaria, those with high levels of anti-SPf66 were less at risk of malaria, perhaps reflecting their greater previous exposure and thus immunity. In contrast, among children who had received three doses of SPf66, those with high antibody levels were at greater risk of have malaria during the subsequent malaria transmission season.

Effects of azithromycin on malariometric indices in The Gambia.

Azithromycin (a macrolide-like antibiotic) has antimalarial effects in vitro and in animal models. In the course of a randomised trial of trachoma control we examined the effects of azithromycin on parasite and spleen rates in the population aged 5-14 years from eight villages in the Farafenni study area in The Gambia, West Africa. The entire population of four treatment villages received three doses of azithromycin 20 mg/kg weekly (days 1, 8, and 15) and four control villages received daily tetracycline eye ointment topically (days 1-42). Among 226 children studied before treatment and at day 28, azithromycin reduced the proportions with Plasmodium falciparum parasites (rate ratio 0.56, 95% confidence interval 0.44-0.71; p < 0.0001), with palpable spleens (RR 0.50, 95% CI 0.36-0.70; p < 0.0001), with febrile parasitaemia (RR 0.45, 95% CI 0.27-0.75; p < 0.01), and with P malariae infection (p < 0.001). This effect was related more to resolution of parasitaemia than to prevention of new infections.

A pilot safety and immunogenicity study of the malaria vaccine SPf66 in Gambian infants.

A pilot safety and immunogenicity trial of the malaria vaccine SPf66 has been undertaken in 150 Gambian infants. No significant systemic side effects were recorded but modest local reactions were seen after the administration of a third 1.0 mg dose. SPf66 produced in Colombia was more immunogenic than SPf66 produced in the USA and a 1.0 mg dose of each vaccine gave higher antibody levels than a 0.5 mg dose. However, antibody levels fell rapidly after administration of the third dose of vaccine and showed little change over the following malaria transmission season. The incidence of clinical malaria was higher among children who received SPf66 than among children who received inactivated polio vaccine, the effect being most marked among children who received 1.0 mg Colombian SPf66. As the trial was not designed to measure the effect of SPf66 on morbidity from malaria, the significance of this finding is uncertain.

Parasite infectivity and immunity to Plasmodium falciparum gametocytes in Gambian children.

Immunity to the sexual stages of Plasmodium falciparum can be induced during natural infections. Characterization of this immunity may facilitate the design of a transmission-blocking vaccine (TBV). This study aimed to assess the prevalence and serological correlates of functional transmission-blocking immunity in Gambian children (aged 1-4 years old) who were P. falciparum gametocyte carriers. Serological assays showed 100% response to fixed, whole parasites but only 42% to live gametes. Responses to the antigens Pfs230 and Pfs48/45 were 54.1% and 37.3%, respectively, in an IgG1 ELISA. 14/55 sera were capable of reducing the infectivity of laboratory isolate NF54 in a standard membrane-feeding assay (SMFA). This activity was strongly correlated with IgG1 responses to Pfs48/45 (r = 0.49, P < 0.001) and to a serological reaction with epitopes of the same molecule (r = 0.38, P = 0.003). A weaker correlation was observed with IgG1 to Pfs230 (r = 0.29, P = 0.03). In direct membrane feeding assays (DMFA) with autologous isolates, sera from 4/29 children showed transmission-blocking activity. There was no correlation with serological assays and the DMFA or between the SMFA and DMFA. This may be caused by variation in sexual stage antigens and/or alternative modes of transmission-blocking immunity, both of which have implications for vaccine implementation.

Evidence for selection for the tyrosine-86 allele of the pfmdr 1 gene of Plasmodium falciparum by chloroquine and amodiaquine.

The 4-aminoquinolines chloroquine (CQ) and amodiaquine (AM) were used to treat Gambian children with uncomplicated falciparum malaria in a randomized drug trial. Blood samples were taken immediately before treatment (day 0), and at day 7 and day 28 after treatment. Samples from those parasitologically positive at day 7 following treatment ('early positives') and those positive at day 28 but negative at day 7 ('late positives') have been studied by PCR followed by restriction enzyme digestion to determine the allelic status of the pfmdr 1 locus at the codon-86 position (asparagine or tyrosine), previously associated with resistance to CQ. A significantly higher prevalence of the tyr-86 allele was observed in samples taken immediately before treatment (day 0) in the early positives group when compared with the late positives group. This suggests the tyr-86 allele contributes to drug resistance in the early positives group. This association remained significant for both CQ and AM groups, implying a common genetic basis of resistance. Predominance of the allele at day 7 is consistent with a strong selection in the first week following treatment. In the late positives group, a significantly higher prevalence of the tyr-86 allele was observed in the samples at day 28 when compared with those at day 0, suggestive of selection during the period day 7 to day 28. Differences were observed in the extent of this selection in the CQ and AM groups. The samples were genotyped at 3 unlinked polymorphic loci. These analyses suggested that most parasites observed at day 7 were probably recrudescences whereas most of those at day 28 were reinfections.

Efficacy trial of malaria vaccine SPf66 in Gambian infants.

SPf66 malaria vaccine is a synthetic protein with aminoacid sequences derived from pre-erythrocytic and asexual blood-stage proteins of Plasmodium falciparum. SPf66 was found to have a 31% protective efficacy in an area of intensive malaria transmission in Tanzanian children, 1-5 years old. We report a randomised, double-blind, placebo-controlled trial of SPf66 against clinical P falciparum malaria in Gambian infants. 630 children, aged 6-11 months at time of the first dose, received three doses of SPf66 or injected polio vaccine (IPV). Morbidity was monitored during the following rainy season by means of active and passive case detection. Cross-sectional surveys were carried out at the beginning and at the end of the rainy season. An episode of clinical malaria was defined as fever (> or = 37.5 degrees C) and a parasite density of 6000/microL or more. Analysis of efficacy was done on 547 children (316 SPf66/231 IPV). No differences in mortality or in health centre admissions were found between the two groups of children. 347 clinical episodes of malaria were detected during the three and a half months of surveillance. SPf66 vaccine was associated with a protective efficacy against the first or only clinical episode of 8% (95% CI -18 to 29, p = 0.50) and against the overall incidence of clinical episodes of malaria of 3% (95% CI -24 to 24, p = 0.81). No significant differences in parasite rates or in any other index of malaria were found between the two groups of children. The findings of this study differ from previous reports on SPf66 efficacy from South America and from Tanzania. In The Gambia, protection against clinical attacks of malaria during the rainy season after immunisation in children 6-11 months old at time of the first dose was not achieved.

PIP: During December 1993 to November 1994, in the Upper River Division of The Gambia, 630 infants aged 6-11 months at time of first dose received three doses of malaria vaccine SPf66 or injected polio vaccine (IPV) and were followed up at home during the rainy season using active and passive case detection methods to determine the protective efficacy of SPf66 against clinical episodes of malaria due to Plasmodium falciparum. The researchers were able to use data on only 547 children (316 SPf66/231 IPV) to determine efficacy. The definition of clinical malaria was fever (37.5 degrees Celsius or higher) and a parasite density of at least 6000/mcl. The two groups were essentially the same in terms of mortality, health center admissions, parasite rates, or any other index of malaria. Health workers identified 347 clinical episodes of malaria during the three months of surveillance. SPf66 vaccine had a protective efficacy against first or only clinical episode of malaria of 8% (p = 0.5). Its protective efficacy was 3% against all clinical episodes of malaria. The results of this trial were different than those from earlier reports on SPf66 efficacy from South America and Tanzania. Possible reasons accounting for the different findings were a mistake in coding syringes for the third dose, substantially less intensity of malaria infection in The Gambia than South America, younger children in The Gambia than in Tanzania, genetic differences in the populations, and difference in length of follow-up. In conclusion, protection against malaria during the rainy season after immunization with SPf66 vaccine in infants aged 6-11 months did not occur.