Modelling the influence of environmental parameters over marine planktonic microbial communities using artificial neural networks.

Guanabara Bay is a tropical estuarine ecosystem that receives massive anthropogenic impacts from the metropolitan region of Rio de Janeiro. This ecosystem suffers from an ongoing eutrophication process that has been shown to promote the emergence of potentially pathogenic bacteria, giving rise to public health concerns. Although previous studies have investigated how environmental parameters influence the microbial community of Guanabara Bay, they often have been limited to small spatial and temporal gradients and have not been integrated into predictive mathematical models. Our objective was to fill this knowledge gap by building models that could predict how temperature, salinity, phosphorus, nitrogen and transparency work together to regulate the abundance of bacteria, chlorophyll and Vibrio (a potential human pathogen) in Guanabara Bay. To that end, we built artificial neural networks to model the associations between these variables. These networks were carefully validated to ensure that they could provide accurate predictions without biases or overfitting. The estimated models displayed high predictive capacity (Pearson correlation coefficients ≥0.67 and root mean square error ≤ 0.55). Our findings showed that temperature and salinity were often the most important factors regulating the abundance of bacteria, chlorophyll and Vibrio (absolute importance ≥5) and that each of these has a unique level of dependence on nitrogen and phosphorus for their growth. These models allowed us to estimate the Guanabara Bay microbiome's response to changes in environmental conditions, which allowed us to propose strategies for the management and remediation of Guanabara Bay.

On the origin and timing of Zika virus introduction in Brazil.

The timing and origin of Zika virus (ZIKV) introduction in Brazil has been the subject of controversy. Initially, it was assumed that the virus was introduced during the FIFA World Cup in June-July 2014. Then, it was speculated that ZIKV may have been introduced by athletes from French Polynesia (FP) who competed in a canoe race in Rio de Janeiro in August 2014. We attempted to apply mathematical models to determine the most likely time window of ZIKV introduction in Brazil. Given that the timing and origin of ZIKV introduction in Brazil may be a politically sensitive issue, its determination (or the provision of a plausible hypothesis) may help to prevent undeserved blame. We used a simple mathematical model to estimate the force of infection and the corresponding individual probability of being infected with ZIKV in FP. Taking into account the air travel volume from FP to Brazil between October 2013 and March 2014, we estimated the expected number of infected travellers arriving at Brazilian airports during that period. During the period between December 2013 and February 2014, 51 individuals travelled from FP airports to 11 Brazilian cities. Basing on the calculated force of ZIKV infection (the per capita rate of new infections per time unit) and risk of infection (probability of at least one new infection), we estimated that 18 (95% CI 12-22) individuals who arrived in seven of the evaluated cities were infected. When basic ZIKV reproduction numbers greater than one were assumed in the seven evaluated cities, ZIKV could have been introduced in any one of the cities. Based on the force of infection in FP, basic reproduction ZIKV number in selected Brazilian cities, and estimated travel volume, we concluded that ZIKV was most likely introduced and established in Brazil by infected travellers arriving from FP in the period between October 2013 and March 2014, which was prior to the two aforementioned sporting events.

Magnitude and frequency variations of vector-borne infection outbreaks using the Ross-Macdonald model: explaining and predicting outbreaks of dengue fever.

The classical Ross-Macdonald model is often utilized to model vector-borne infections; however, this model fails on several fronts. First, using measured (or estimated) parameters, which values are accepted from the literature, the model predicts a much greater number of cases than what is usually observed. Second, the model predicts a single large outbreak that is followed by decades of much smaller outbreaks, which is not consistent with what is observed. Usually towns or cities report a number of recurrences for many years, even when environmental changes cannot explain the disappearance of the infection between the peaks. In this paper, we continue to examine the pitfalls in modelling this class of infections, and explain that, if properly used, the Ross-Macdonald model works and can be used to understand the patterns of epidemics and even, to some extent, be used to make predictions. We model several outbreaks of dengue fever and show that the variable pattern of yearly recurrence (or its absence) can be understood and explained by a simple Ross-Macdonald model modified to take into account human movement across a range of neighbourhoods within a city. In addition, we analyse the effect of seasonal variations in the parameters that determine the number, longevity and biting behaviour of mosquitoes. Based on the size of the first outbreak, we show that it is possible to estimate the proportion of the remaining susceptible individuals and to predict the likelihood and magnitude of the eventual subsequent outbreaks. This approach is described based on actual dengue outbreaks with different recurrence patterns from some Brazilian regions.

Proposal of fifteen new species of Parasynechococcus based on genomic, physiological and ecological features.

Members of the recently proposed genus Parasynechococcus (Cyanobacteria) are extremely abundant throughout the global ocean and contribute significantly to global primary productivity. However, the taxonomy of these organisms remains poorly characterized. The aim of this study was to propose a new taxonomic framework for Parasynechococcus based on a genomic taxonomy approach that incorporates genomic, physiological and ecological data. Through in silico DNA-DNA hybridization, average amino acid identity, dinucleotide signatures and phylogenetic reconstruction, a total of 15 species of Parasynechococcus could be delineated. Each species was then described on the basis of their gene content, light and nutrient utilization strategies, geographical distribution patterns throughout the oceans and response to environmental parameters.

Diversity and antibiotic resistance profiles of Pseudomonads from a hospital wastewater treatment plant.

AIMS: To evaluate the diversity of Pseudomonads and antibiotic resistance profiles of Pseudomonas aeruginosa in a hospital wastewater treatment plant (HWTP) located in Rio de Janeiro city, Brazil. Due its intrinsic multidrug resistance and its ability to colonize several environments, we selected Ps. aeruginosa isolates as indicator of antimicrobial resistance frequency. METHODS AND RESULTS: Twenty-seven Ps. aeruginosa strains isolated from five stages of HWTP identified by rrs 16S rDNA sequencing were submitted against 12 antimicrobials through disc diffusion method. Among these isolates, 62·9% showed aztreonam resistance, followed by ticarcillin/clavulanic acid (33·3%) and cefepime (22·2%). Of these isolates, 22·2% were classified as multidrug-resistant (MDR ≥ 3 classes). Five 16S rRNA gene libraries of Pseudomonas genus were constructed, one for each stages of the plant, yielding 93 sequences clustered in 41 Operational Taxonomic Units (OTUs). Each treatment step showed unique OTU's composition, suggesting changes in Pseudomonas spp. communities during the process. Several Pseudomonas species involved in biodegradation and bioremediation of xenobiotics were detected suggesting a positive impact in the wastewater treatment. CONCLUSIONS: Our strategy using metagenomics associated with the isolation of Ps. aeruginosa strains as bio-indicator allowed us to assess their antimicrobial susceptibility, the viability and diversity of Pseudomonas species in the hospital wastewater. SIGNIFICANCE AND IMPACT OF THE STUDY: The presence of MDR bacteria from treated effluents alerts for the need to improve these systems to avoid the spreading of resistance genes in aquatic ecosystems. This has special relevance in Brazil, where a significant portion of the population has no access to treated water.

Interpretations and pitfalls in modelling vector-transmitted infections.

In this paper we propose a debate on the role of mathematical models in evaluating control strategies for vector-borne infections. Mathematical models must have their complexity adjusted to their goals, and we have basically two classes of models. At one extreme we have models that are intended to check if our intuition about why a certain phenomenon occurs is correct. At the other extreme, we have models whose goals are to predict future outcomes. These models are necessarily very complex. There are models in between these classes. Here we examine two models, one of each class and study the possible pitfalls that may be incurred. We begin by showing how to simplify the description of a complicated model for a vector-borne infection. Next, we examine one example found in a recent paper that illustrates the dangers of basing control strategies on models without considering their limitations. The model in this paper is of the second class. Following this, we review an interesting paper (a model of the first class) that contains some biological assumptions that are inappropriate for dengue but may apply to other vector-borne infections. In conclusion, we list some misgivings about modelling presented in this paper for debate.

Will people change their vector-control practices in the presence of an imperfect dengue vaccine?

Human behaviours, which are influenced by social, cultural, economic and political factors, can increase or decrease the risk of dengue infection, depending on the relationship with the insect vector. Because no vaccine is currently available, the spread of dengue can only be curtailed by controlling vector populations (Aedes aegypti and others) and by protecting individuals. This study tested the hypothesis that dengue-affected populations are likely to relax their vector-control habits if a potentially protective vaccine becomes available. The hypothesis was tested using two approaches: a mathematical model designed to describe dengue transmission and an empirical field test in which the local population of an endemic area was interviewed about their vector-control habits given the presence of a theoretical vaccine. The model demonstrated that depending on the level of vector-control reduction, there is a threshold in vaccine efficacy below which it is better not to introduce the vaccine. The interview showed that people who were informed that a very effective vaccine is available would reduce their vector-control habits significantly compared to a group that was informed that the vaccine is not very effective.

First Report of Gray Mold (Amphobotrys ricini) on Copperleaf (Acalypha wilkesiana) in Brazil.

Acalypha wilkesiana (Euphorbiaceae), common names copperleaf or Jacob's coat (in Brazil, crista-de-peru), is a popular ornamental native from the Pacific islands. It is widely used in gardens in Brazil (4). In January 2012, a group of diseased A. wilkesiana was found in a nursery at the municipality of Itaboraí (state of Rio de Janeiro, Brazil). Later, another group of individuals of the same plant species bearing identical disease symptoms were found in a botanic garden in the city of Rio de Janeiro (Jardim Botânico do Rio de Janeiro). Diseased plants had intense leaf blight. Such leaves dropped over healthy leaves of the same or other plants and necrosis was hence initiated on such leaves. Inflorescences were also affected by blight and after becoming necrotic a dieback of supporting stems also resulted. Abundant grayish sporulation was easily observed over necrotic tissues. Samples were collected, dried in a plant press, and representative specimens were deposited in the herbarium at the Universidade Federal de Viçosa. These were from Itaboraí (VIC 31822) and from Rio de Janeiro (VIC 31931). Structures were mounted in lactophenol for observation under a microscope and isolated in pure culture on PCA plates. Isolates were deposited in the culture collection of the Universidade Federal de Viçosa with accession numbers of COAD 1112 and COAD 1108, respectively. The fungus had the following morphology: conidiophores cylindrical, up to 1,200 µm branching dicotomously at mid-length in broad angles and then branching secondarily, light brown; conidiogenous cells ampulliform, terminal, denticulate; conidia globose, 6 to 11 µm diam, subhyaline to pale brown, smooth. This combination of features is typical of Amphobotrys ricini (2), a common pathogen of castor bean (1) and several other members of the Euphorbiaceae. DNA was extracted from each isolate growing in pure culture and ITS sequences were generated and deposited in GenBank under the accession numbers JX961613 (COAD 1108) and JX961614 (COAD 1112). These were compared by BLASTn with other entries in GenBank, and the closest match for both isolates was A. ricini (JF433374) with 97% nucleotide homology (over 97% query coverage) for COAD 1112 and 98% nucleotide homology (over 98% query coverage) for COAD 1112. Pathogenicity of the isolate from A. wilkesiana was demonstrated through brush inoculation of a conidial suspension (3 × 106 conidia. mL-1) onto healthy leaves of a A. wilkesiana individual followed by its transfer to a humid chamber for 48 h. Symptoms appeared after 3 days of inoculation and sporulation appeared over necrotic tissues after 10 days. Despite the importance of A. ricini as a plant pathogen, little has been investigated on its taxonomy with molecular tools. Although morphology and host-association are the basis for the delimitation of A. ricini, our preliminary results for ITS sequences suggest that this species may include cryptic taxa that are not properly discriminated on a morphological and pathological basis. This report follows other novel reports of A. ricini on ornamental Euphorbiaceae in Brazil (3) and, to our knowledge, represents the first report of A. ricini on A. wilkesiana worldwide. References: (1) G. H. Godfrey. J. Agric. Res. 23:679, 1923. (2) G. L. Hennebert. Persoonia 7:183, 1973. (3) B. V. Lima et al. Australas. Plant Dis. Notes 3:5, 2008. (4) H. Lorenzi and H. M. Souza. Plantas Ornamentais no Brasil - Arbustivas, Herbáceas e Trepadeiras. Nova Odessa: Instituto Plantarum, 1999.

Atividade antimicrobiana dos extratos metanólicos da raiz, caule e folhas de Croton pulegioides Baill. (Zabelê) / Antimicrobial activity of methanol extracts of root, stem and leaves of Croton pulegioides Baill

O uso popular de plantas medicinais continua sendo de grande interesse para a população, principalmente em países em desenvolvimento, nos quais o acesso à medicina alopática ainda é muito restrito. Assim, além de sua importância na medicina tradicional, o conhecimento das plantas pode fornecer para a indústria farmacêutica, direta ou indiretamente, princípios ativos de interesse terapêutico. No presente trabalho foi avaliada a atividade antimicrobiana dos extratos metanólicos secos de Croton pulegioides Baill. obtidos a partir da raiz, do caule e das folhas nas concentrações de 25, 50 e 100mg/mL utilizando o método de difusão em Agar (técnica de poços) frente a cinco cepas Gram-positivas, três cepas de fungos e três cepas de bactérias Gram-negativas. Na execução do estudo foi utilizado o antibiótico Gentamicina (100µg/mL) no teste com bactérias, e Cetoconazol (300µg/mL) para as Candidas verificando-se, ao mesmo tempo, a ausência de atividade para o diluente DMSO 20% em água. Na observação dos resultados evidenciaram-se halos de inibição indicando atividade dos extratos metanólicos da raiz, caule e folhas, nas concentrações de 50 e 100mg/ml, frente Bacillus subtilis AM 04; dos extratos metanólicos da raiz e do caule, nas concentrações de 50 e 100mg/ml, frente Staphylococcus aureus AM 103 (ATCC 6538); e do extrato metanólico do caule, nas concentrações de 50 e 100mg/ml, frente S. epidermidis AM 235. Por outro lado, todos os extratos foram inativos nas concentrações avaliadas frente às bactérias Gram-negativas utilizadas no estudo (Pseudomonas aeruginosa AM 206, Escherichia coli AM 1050 e Klebsiella pneumoniae AM 410). Referente à atividade sobre fungos, os extratos metanólicos da raiz e do caule inibiram, nas três concentrações avaliadas, o crescimento de Candida tropicalis AM 1181 e C. albicans AM 1140, enquanto o extrato das folhas, inibiu o crescimento de C. tropicalis AM 1181 nas concentrações de 50 e 100mg/ml e de C. albicans AM 1140 apenas na concentração de 100mg/mL. Finalmente o extrato metanólico das folhas, na concentração de 100mg/mL, foi o único a inibir o crescimento de Candida krusei. Os resultados preliminares apresentados pelo presente trabalho sugeriram que a espécie Croton pulegioides Baill. apresentou resultados promissores quanto ao potencial antimicrobiano.

The employment of medicinal plants as a folk custom remains important to the present time, mainly in developing countries, where access to allopathic medicine is restricted. Besides their importance in traditional medicine, the knowledge on these plants can, directly or indirectly, provide information about the active ingredients involved, which may be of interest to the pharmaceutical industry. The present study evaluated the antimicrobial activity of dried methanolic extracts of Croton pulegioides Baill, extracted from its roots, stalk, and leaves, at the concentrations of 25, 50 and 100mg/ml. These were diffused in wells with an Agar medium, to which five strains of Gram-positive, three strains of fungi and three strains of Gram-negative bacteria had been added. For this study, the antibiotic Gentamicin (10µg/100µL) was used in tests with bacteria, and Ketoconazole (30µg/100µL), with Candidas. When DMSO diluted in 20% water was used, no activity was observed. The results produced evidence of inhibition halos, which indicates the activity of the methanolic extracts from the roots, stalk and leaves at the concentrations of 50 and 100 mg/ml for Bacillus subtilis AM 04; activity of the methanolic extracts from the roots and stalk at the concentrations of 50 and 100mg/ml for Staphylococcus aureus AM 103 (ATCC 6538); and activity of the methanolic extract from the stalk at the concentrations of 50 and 100mg/ml for Staphylococcus epidermidis AM 235. There was no activity in the concentrations studied for the Gram-negative bacteria (Pseudomonas aeruginosa AM 206, Escherichia coli AM 1050 and Klebsiella pneumoniae AM 410). With respect to fungal activity, the methanolic extracts from the roots and stalk inhibited Candida tropicalis AM 1181 and C. albicans AM 1140 at the three concentrations studied. The extract from the leaves inhibited the growth of C. tropicalis AM 1181 at the concentrations of 50 e 100mg/ml and C. albicans AM 1140 only at the concentration of 100mg/ml. Finally, the methanolic extract from the leaves, at the concentration of 100mg/ml, was the only one to inhibit the growth of Candida krusei. Preliminary results from this study suggest that the Croton pulegioides Baill showed promising results as a potential antimicrobial agent.

Estimation of R0 from the initial phase of an outbreak of a vector-borne infection.

The magnitude of the basic reproduction ratio R(0) of an epidemic can be estimated in several ways, namely, from the final size of the epidemic, from the average age at first infection, or from the initial growth phase of the outbreak. In this paper, we discuss this last method for estimating R(0) for vector-borne infections. Implicit in these models is the assumption that there is an exponential phase of the outbreaks, which implies that in all cases R(0)>1. We demonstrate that an outbreak is possible, even in cases where R(0) is less than one, provided that the vector-to-human component of R(0) is greater than one and that a certain number of infected vectors are introduced into the affected population. This theory is applied to two real epidemiological dengue situations in the southeastern part of Brazil, one where R(0) is less than one, and other one where R(0) is greater than one. In both cases, the model mirrors the real situations with reasonable accuracy.

A hypothesis for the 2007 dengue outbreak in Singapore.

A previous mathematical model explaining dengue in Singapore predicted a reasonable outbreak of about 6500 cases for 2006 and a very mild outbreak with about 2000 cases for 2007. However, only 3051 cases were reported in 2006 while more than 7800 were reported in the first 44 weeks of 2007. We hypothesized that the combination of haze with other local sources of particulate matter had a significant impact on mosquito life expectancy, significantly increasing their mortality rate. To test the hypothesis a mathematical model based on the reproduction number of dengue fever and aimed at comparing the impact of several possible alternative control strategies was proposed. This model also aimed at contributing to the understanding of the causes of dengue resurgence in Singapore in the last decade. The model's simulation demonstrated that an increase in mosquito mortality in 2006 and either a reduction in mortality or an increase in the carrying capacity of mosquitoes in 2007 explained the patterned observed in Singapore. Based on the model's simulation we concluded that the fewer than expected number of dengue cases in Singapore in 2006 was caused by an increase in mosquito mortality due to the disproportionate haze affecting the country that year and that particularly favourable environmental conditions in 2007 propitiated mosquitoes with a lower mortality rate, which explains the greater than expected number of dengue cases in 2007. Whether our hypothesis is plausible or not should be debated further.

A hypothesis for explaining single outbreaks (like the Black Death in European cities) of vector-borne infections.

We propose a mechanism by which single outbreaks of vector-borne infections can happen even when the value of the basic reproduction number, R(0), of the infection is below one. With this hypothesis we have shown that dynamical models simulations demonstrate that the arrival of a relatively small (with respect to the host population) number of infected vectors can trigger a short-lived epidemic but with a huge number of cases. These episodes are characterized by a sudden outbreak in a previously virgin area that last from weeks to a few months, and then disappear without leaving vestiges. The hypothesis proposed in this paper to explain those single outbreaks of vector-borne infections, even when total basic reproduction number, R(0), is less than one (which explain the fact that those infections fail to establish themselves at endemic levels), is that the vector-to-host component of R(0) is greater than one and that a sufficient amount of infected vectors are imported to the vulnerable area, triggering the outbreak. We tested the hypothesis by performing numerical simulations that reproduce the observed outbreaks of chikungunya in Italy in 2007 and the plague in Florence in 1348. The theory proposed provides an explanation for isolated outbreaks of vector-borne infections, ways to calculate the size of those outbreaks from the number of infected vectors arriving in the affected areas. Given the ever-increasing worldwide transportation network, providing a high degree of mobility from endemic to virgin areas, the proposed mechanism may have important implications for public health planning.

Cost-effectiveness analysis of a hypothetical hepatitis C vaccine compared to antiviral therapy.

We propose a mathematical model to simulate the dynamics of hepatitis C virus (HCV) infection in the state of São Paulo, Brazil. We assumed that a hypothetical vaccine, which cost was taken to be the initial cost of the vaccine against hepatitis B exists and it is introduced in the model. We computed its cost-effectiveness compared with the anti-HCV therapy. The calculated basic reproduction number was 1.20. The model predicts that without intervention a steady state exists with an HCV prevalence of 3%, in agreement with the current epidemiological data. Starting from this steady state three interventions were simulated: indiscriminate vaccination, selective vaccination and anti-HCV therapy. Selective vaccination proved to be the strategy with the best cost-effectiveness ratio, followed by indiscriminate vaccination and anti-HCV therapy.

Multiple endocrine neoplasia type 1 in Brazil: MEN1 founding mutation, clinical features, and bone mineral density profile.

OBJECTIVE: Only few large families with multiple endocrine neoplasia type 1 (MEN1) have been documented. Here, we aimed to investigate the clinical features of a seven-generation Brazilian pedigree, which included 715 at-risk family members. DESIGN: Genealogical and geographic analysis was used to identify the MEN1 pedigree. Clinical and genetic approach was applied to characterize the phenotypic and genotypic features of the family members. RESULTS: Our genetic data indicated that a founding mutation in the MEN1 gene has occurred in this extended Brazilian family. Fifty family members were diagnosed with MEN1. Very high frequencies of functioning and non-functioning MEN1-related tumors were documented and the prevalence of prolactinoma (29.6%) was similar to that previously described in prolactinoma-variant Burin (32%). In addition, bone mineral density analysis revealed severe osteoporosis (T, -2.87+/-0.32) of compact bone (distal radius) in hyperparathyroidism (HPT)/MEN1 patients, while marked bone mineral loss in the lumbar spine (T, -1.95+/-0.39), with most cancellous bone, and femoral neck (mixed composition; T, -1.48+/-0.27) were also present. CONCLUSIONS: In this study, we described clinically and genetically the fifth largest MEN1 family in the literature. Our data confirm previous findings suggesting that prevalence of MEN1-related tumors in large families may differ from reports combining cumulative data of small families. Furthermore, we were able to evaluate the bone status in HPT/MEN1 cases, a subject that has been incompletely approached in the literature. We discussed the bone loss pattern found in our MEN1 patients comparing with that of patients with sporadic primary HPT.

Modelling the control strategies against dengue in Singapore.

Notified cases of dengue infections in Singapore reached historical highs in 2004 (9459 cases) and 2005 (13,817 cases) and the reason for such an increase is still to be established. We apply a mathematical model for dengue infection that takes into account the seasonal variation in incidence, characteristic of dengue fever, and which mimics the 2004-2005 epidemics in Singapore. We simulated a set of possible control strategies and confirmed the intuitive belief that killing adult mosquitoes is the most effective strategy to control an ongoing epidemic. On the other hand, the control of immature forms was very efficient in preventing the resurgence of dengue epidemics. Since the control of immature forms allows the reduction of adulticide, it seems that the best strategy is to combine both adulticide and larvicide control measures during an outbreak, followed by the maintenance of larvicide methods after the epidemic has subsided. In addition, the model showed that the mixed strategy of adulticide and larvicide methods introduced by the government seems to be very effective in reducing the number of cases in the first weeks after the start of control.

Threshold conditions for a non-autonomous epidemic system describing the population dynamics of dengue.

A non-autonomous dynamical system, in which the seasonal variation of a mosquito vector population is modeled, is proposed to investigate dengue overwintering. A time-dependent threshold, R(t), is deduced such that when its yearly average, denoted by R, is less than 1, the disease does not invade the populations and when R is greater than 1 it does. By not invading the population we mean that the number of infected individuals always decrease in subsequent seasons of transmission. Using the same threshold, all the qualitative features of the resulting epidemic can be understood. Our model suggests that trans-ovarial infection in the mosquitoes facilitates dengue overwintering. We also explain the delay between the peak in the mosquitoes population and the peak in dengue cases.

The impact of imperfect vaccines on the evolution of HIV virulence.

A theoretical framework is proposed on which some hypotheses related to the impact of imperfect vaccines on the evolution of HIV virulence can be tested. For this, a linear increase of risk behaviour with vaccine efficacy is assumed. This is based on the hypothesis that people are prone to relax preventive measures by knowing that they and their partners are vaccinated and that this effect is more intense the more effective the vaccine is known to be. An additional, and perhaps more important hypothesis is related to the theoretical possibility that increased risk behaviour of some vaccinated individuals in partially protected populations could act as a selective pressure toward more virulent HIV strains. Those hypotheses were tested by a mathematical model that considers three different HIV strains competing against each other in a population partially protected by imperfect vaccines of distinct efficacies. Simulations of the model demonstrated that, under the above hypotheses, there is a shift in HIV virulence towards more aggressive strains with increase in vaccine efficacy, associated with a marked reduction in the total amount of transmission and, consequently, in the prevalence of HIV. Potential ways for further testing the theory/model and the implications of the results are discussed.