Carbamazepine: a bioequivalence study and limited sampling modeling

OBJECTIVES

To assess the bioequivalence of 2 formulations of carbamazepine and to develop and validate limited sampling strategy (LSS) models for estimating the area under the plasma concentration-time curve (AUC0-infinity) and the peak plasma concentration (Cmax) of carbamazepine.

METHODS

Twenty-four (12 men, 12 women) healthy volunteers received single oral doses (400 mg) of carbamazepine, as reference and test conventional-release formulations, in a standard 2-sequence, 2-period crossover design. Bioequivalence assessment was based on the individual ratios of log-transformed values of AUC0-infinity and Cmax LSS modeling was developed in a training set of 12 randomly assigned volunteers and was validated on the other 12 subjects (validation set).

RESULTS

Carbamazepine AUC0-infinity and Cmax can be accurately predicted (R2 = 0.89 - 0.95, precision = 2.6 - 7.2%) by single-point (72 h) and 2-point LSS models (6, 32 h), respectively. Bioequivalence assessments based on LSS-derived AUC0-infinity and Cmax provided results similar to those obtained using all the concentration-in-plasma data points, and indicated that the 2 formulations are bioequivalent.

CONCLUSION

One-and 2-point LSS models provided accurate estimates of carbamazepine's AUC0-infinity and Cmax, and allowed correct assessment of bioequivalence between the formulations studied.

Malnutrition and susceptibility to enteroparasites: reinfection rates after mass chemotherapy

The evidence that relates malnutrition to enteroparasite infections arises from studies that demonstrate the improvement of nutritional indicators after antiparasitism treatment. However, the role of malnutrition as an aggravating factor to the susceptibility to enteroparasite infections is still not fully understood. We investigated the correlation between malnutrition and enteroparasite infection after mass chemotherapy, in a poor city of São Paulo State, Brazil. The sample comprised 759 children between 1 and 10 years of age of whom 585 were followed up for a period of 1 year and periodically assessed for reinfection with enteroparasites. One year of follow-up after mass chemotherapy demonstrated that 38 of the undernourished children were reinfected with enteroparasites, as compared with 25 of the eutrophic children (P = 0.033). The survival multivariate analysis demonstrated that, after controlling for the potential confounding variables, maternal literacy and per capita income rate, malnutrition was associated with susceptibility to reinfection (P = 0.13). We demonstrate that, although maternal literacy and per capita income rate are indeed confounding variables, malnutrition contributes to an increase in the risk of enteroparasite infections.

A method for estimating time dependent intervention benefits under arbitrarily varying age and exogenous components of hazard

A method for estimating the dependence of intrinsic intervention benefits on time elapsed since the intervention took place is proposed. The method is aimed at intervention programs against diseases where one or all of the following components of hazard intensity may undergo important and unknown variations: 1) the intervention benefits to a subject are a function of the time elapsed since the intervention took place, or since inception for a continuing treatment, 2) the subjects vulnerability is an unknown function of their age, 3) the exogenous or environmental baseline intensity, to which all are assumed subjected, fluctuates arbitrarily with calendar time. During the time span of a study, these variables interact in a complex way, possibly masking the real contribution of the intervention. However, with very general assumptions about how hazard components interact, the cumulative hazards of subpopulations treated at different times in the past are shown to be described mathematically by a convolution of the time elapsed dependent intervention benefit function with the age and calendar time dependent baseline intensity. Starting from the cumulative hazards of untreated and treated subpopulations that had the intervention at different times in the past, a method of deconvolution through regularization is proposed to reconstruct the time elapsed dependence of the intervention benefit function. The regularization technique used is of the 'penalized least square smoothing' type, it is applied to the solution of Volterra integral equations of the first kind under noisy inputs. Simulations, to test for the reconstruction of different modes of time elapsed variation of the intervention benefits, are carried out on realistically noisy 'data sets' taken to be available at a limited number of time points. The stability of the estimated reconstructions, to measurement errors, is examined through repeated simulations with random noise added to inputs. The method is applied to a Brazilian data set where BCG vaccination resulted in a small reduction in the cumulated risk of leprosy infection.

Limited-sampling strategy models for estimating the pharmacokinetic parameters of 4-methylaminoantipyrine, an active metabolite of dipyrone

Bioanalytical data from a bioequivalence study were used to develop limited-sampling strategy (LSS) models for estimating the area under the plasma concentration versus time curve (AUC) and the peak plasma concentration (Cmax) of 4-methylaminoantipyrine (MAA), an active metabolite of dipyrone. Twelve healthy adult male volunteers received single 600 mg oral doses of dipyrone in two formulations at a 7-day interval in a randomized, crossover protocol. Plasma concentrations of MAA (N = 336), measured by HPLC, were used to develop LSS models. Linear regression analysis and a "jack-knife" validation procedure revealed that the AUC(0-infinity) and the Cmax of MAA can be accurately predicted (R2>0.95, bias <1.5%, precision between 3.1 and 8.3%) by LSS models based on two sampling times. Validation tests indicate that the most informative 2-point LSS models developed for one formulation provide good estimates (R2>0.85) of the AUC(0-infinity) or Cmax for the other formulation. LSS models based on three sampling points (1.5, 4 and 24 h), but using different coefficients for AUC(0-infinity) and Cmax, predicted the individual values of both parameters for the enrolled volunteers (R2>0.88, bias = -0.65 and -0.37%, precision = 4.3 and 7.4%) as well as for plasma concentration data sets generated by simulation (R2>0.88, bias = -1.9 and 8.5%, precision = 5.2 and 8.7%). Bioequivalence assessment of the dipyrone formulations based on the 90% confidence interval of log-transformed AUC(0-infinity) and Cmax provided similar results when either the best-estimated or the LSS-derived metrics were used.

Development and validation of limited-sampling strategies for predicting amoxicillin pharmacokinetic and pharmacodynamic parameters

Amoxicillin plasma concentrations (n = 1,152) obtained from 48 healthy subjects in two bioequivalence studies were used to develop limited-sampling strategy (LSS) models for estimating the area under the concentration-time curve (AUC), the maximum concentration of drug in plasma (C(max)), and the time interval of concentration above MIC susceptibility breakpoints in plasma (T>MIC). Each subject received 500-mg amoxicillin, as reference and test capsules or suspensions, and plasma concentrations were measured by a validated microbiological assay. Linear regression analysis and a "jack-knife" procedure revealed that three-point LSS models accurately estimated (R(2), 0.92; precision, <5.8%) the AUC from 0 h to infinity (AUC(0-infinity)) of amoxicillin for the four formulations tested. Validation tests indicated that a three-point LSS model (1, 2, and 5 h) developed for the reference capsule formulation predicts the following accurately (R(2), 0.94 to 0.99): (i) the individual AUC(0-infinity) for the test capsule formulation in the same subjects, (ii) the individual AUC(0-infinity) for both reference and test suspensions in 24 other subjects, and (iii) the average AUC(0-infinity) following single oral doses (250 to 1,000 mg) of various amoxicillin formulations in 11 previously published studies. A linear regression equation was derived, using the same sampling time points of the LSS model for the AUC(0-infinity), but using different coefficients and intercept, for estimating C(max). Bioequivalence assessments based on LSS-derived AUC(0-infinity)'s and C(max)'s provided results similar to those obtained using the original values for these parameters. Finally, two-point LSS models (R(2) = 0.86 to 0.95) were developed for T>MICs of 0.25 or 2.0 microg/ml, which are representative of microorganisms susceptible and resistant to amoxicillin.

Design and interpretation of vaccine field studies

There are many different effects to consider when evaluating vaccines in the field. In this review, we have covered some of the various measures and issues related to study design and interpretation of the different measures. We emphasize that in designing and understanding vaccine studies, it is necessary to be specific about what the effect of interest is and about the assumptions underlying the interpretation of the results. Halloran et al. (81) present design, analysis, and interpretation of vaccine studies in more detail.

Acute retrovirus syndrome among prospectively identified homosexual men with incident HIV infection in Brazil. Projecto Praça Onze Study Group

BACKGROUND

Symptoms associated with HIV infection are common among HIV seroconverters, but the acute retroviral syndrome (ARS) is a diagnostic challenge because of the absence of a sensitive and specific case definition. We conducted an analysis of HIV seroconverters in Projeto Praça Onze, a HIVNET HIV seroincidence study among homosexual men in Rio de Janeiro.

METHODS

Information from study subjects enrolled in Projeto Praça Onze who were documented HIV seroconverters were compared with nonseroconverters. At each semiannual study visit, participants were asked about HIV seroconversion symptoms and sexually transmitted diseases (STDs) during the preceding 6 months. All information was collected before the laboratory evaluation. A classification tree analysis was used to identify an ARS case definition, first using clinical information and then after including risk factor data for seroconversion in our cohort.

RESULTS

As of July 1998, 674 volunteers were enrolled and 34 of these seroconverted; information was available for 33 of these. Among the seroconverters, 11 (34%) denied any symptoms, and 22 (66%) reported one or more symptoms, the most common of which were fever (25% of seroconverters versus 7% of nonseroconverters; p <.01), night sweats (9% versus 2%, respectively; p =.05), incapacitating disease (ID) for >/=3 days (27% versus 7%, respectively; p <.001), and weight loss of >/=2 kg (21% versus 9%, respectively; p =.05). STDs were more common in seroconverters (gonorrhea: 9% versus 1%, respectively; p <.01 and condyloma: 9% versus 3%, respectively; p =. 08). The first case definition was ID for >3 days, fever, pharyngitis, and myalgia (seroconverters, 3 of 32, versus nonseroconverters, 2 of 640). The second case definition was was ID for >3 days, anti-core hepatitis b-positive, and age <21 years (seroconverters: 6 of 32 versus nonseroconverters 4 of 640). The sensitivity and specificity for the first and second case definitions were: 9.4%, 99.4%, and 18.8%, 99.8%, respectively.

CONCLUSIONS

Among HIV seroconverters, symptoms consistent with ARS were common. We were unable to identify a sensitive case definition that could be used as a screening tool. Although the clinical case definition was not validated, the specificity of our case definitions was high, suggesting that subjects within this HIV risk group who fulfill the case definition should be tested for HIV.

[Dynamic models and social networks: a review and reflections on their contribution to understanding the HIV epidemic]

Due to certain specificities in the HIV epidemic, its spread has escaped the random transmission pattern of other infectious diseases. Observation of the epidemic has shown that individual behavior - relational patterns among individuals - plays a crucial role in HIV transmission and that strategies to prevent the epidemic's spread should take this factor into account in order to foster efficient allocation of existing resources. Mathematical and statistical models applying the behavioral approach to the epidemic have estimated interactions between groups whose characteristics and behaviors varied. However, such models have been more "post-dictive" than predictive, due to the inadequate representation of social structures in populations through which infectious agents spread. The social network methodology thus came to be applied to the approach to the HIV epidemic. This article discusses alternatives for the application of this methodology to the Brazilian epidemic, considering that sociometric risk networks structure the flow of infectious agents in communities, creating unique opportunities to interrupt their spread.

Vaccine-associated paralytic poliomyelitis: a retrospective cohort study of acute flaccid paralyses in Brazil

BACKGROUND

At the present time, in Brazil and other countries in the Americas, the only cases of paralytic poliomyelitis due to poliovirus are caused by vaccine strains. The recognition of possible determinants of vaccine-associated paralytic poliomyelitis (VAPP) by public health surveillance and immunization programmes is relevant to inform the debate on criteria for case definition and vaccination strategies.

METHODS

A retrospective cohort study based on the cases of acute flaccid paralysis (AFP) reported to the Ministry of Health (MoH) was designed, with the objective of studying cases of VAPP in Brazil between 1989 and 1995. Clinical, laboratory and epidemiological data from 3656 acute flaccid paralysis (AFP) cases, 30 of them diagnosed as VAPP, were analysed.

RESULTS

An 8.88 risk ratio of VAPP (95% CI : 4.37-18.03) was found when comparing individuals who received oral poliovirus vaccine (OPV) between 4 and 40 days before the onset of paralysis and individuals who did not receive the vaccine within this period. A risk of 1 case/2.39 million first doses and 1 case/13.03 million OPV doses administered was estimated for the general population.

CONCLUSIONS

Cases of AFP who received OPV between 4 and 40 days before the onset of paralysis and had fever, a prodrome of gastrointestinal symptoms, history of first dose of OPV, isolation of vaccine poliovirus type 2, and young age deserve careful investigation, since they are at increased risk for the condition studied.

Vaccine-associated paralytic poliomyelitis in Brazil, 1989-1995

At the present time, the only poliovirus-caused poliomyelitis cases reported in Brazil and other countries of the Americas are of vaccine etiology. It is important for epidemiological surveillance and immunization programs to evaluate the epidemiological profile of cases of vaccine-associated paralytic poliomyelitis (VAPP) in order to establish criteria for case definition and vaccination strategies. To research VAPP in Brazil, 30 cases diagnosed and classified as such by the Ministry of Health between 1989 and 1995 were submitted to a descriptive study of clinical, laboratory, and epidemiological data. In addition, the risk of occurrence of VAPP was estimated in relation to determinants based on a cohort of 3,656 persons with acute flaccid paralysis. Among individuals who had received oral polio vaccine (OPV) from 4 to 40 days before the onset of paralysis, we found a relative risk of 8.88 (95% CI: 4.37-18.03) for VAPP as compared with persons who had not been vaccinated during the same time interval. For individuals who developed VAPP in the period following national vaccination days, the estimated relative risk was 2.94 (95% CI: 1.44-6.00). For the first dose of OPV administered to the general population the estimated risk was 1 case of VAPP for every 2.39 million doses; for total doses of OPV the risk was 1 case in 13.03 million doses. A major share of VAPP cases were related to children affected by prodromes (fever and gastrointestinal signs and/or symptoms), isolation of vaccine poliovirus type 2, paralysis of the lower limbs, and a mean age of 1 year.

Modelos dinâmicos e redes sociais: revisão e reflexões a respeito de sua contribuição para o entendimento da epidemia do HIV

Há especificidades na epidemia do HIV que fazem com que sua transmissão fuja à aleatoriedade verificada na transmissão de outras doenças infecciosas. A observação da epidemia tem mostrado que os comportamentos individuais - padrões de relação que os indivíduos mantêm entre si - desempenham papel crucial na transmissão do HIV e que as estratégias de prevenção do crescimento da epidemia devem tomar em conta este fator para a alocação eficiente dos recursos existentes. Modelos matemáticos e estatísticos que utilizam a abordagem compartimental aplicada à epidemia estimavam as interações entre grupos cujas características e comportamentos variavam. Contudo, tais modelos eram mais "pós-ditivos" que preditivos, atribuindo-se isso à representação inadequada da estrutura social das populações pelas quais se disseminam os agentes infecciosos. Assim, passou-se a aplicar a metodologia de redes sociais à abordagem da epidemia do HIV. Este artigo discute alternativas à aplicação desta metodologia à epidemia brasileira, ponderando que as redes sociométricas de risco estruturam o fluxo de agentes infecciosos em comunidades, criando oportunidades ímpares a sua interrupção.

Limited-sampling strategy models for estimating the area under the plasma concentration-time curve for amlodipine

OBJECTIVE

Develop and validate limited-sampling strategy (LSS) models for estimating the area under the plasma concentration versus time curve (AUC) of amlodipine, using data from a bioequivalence study.

METHODS

Sixteen healthy volunteers received single 5-mg oral doses of amlodipine, as reference or test formulation, at a 14-day interval, in a randomized, crossover protocol. Plasma concentrations of amlodipine (n = 288), measured by mass spectrometry, were used to develop LSS models.

RESULTS

Linear regression analysis of the AUC0-72 and a "jack-knife" validation procedure revealed that LSS models based on two sampling times (12 h and 48 h) predict accurately (R2 = 0.99; bias < 0.01%; precision = 0.03%) the AUC0-72 of amlodipine for each formulation. Validation tests indicate that the 2-point LSS model developed for the reference formulation predicts accurately (R2 > 0.90): (a) the individual AUC0-72 for the test formulation in the same group of volunteers; (b) the individual AUC0-72 for the same reference formulation in another bioequivalence study in Brazilian volunteers; (c) the average AUC0-72 reported in seven additional international studies performed under protocols similar to the present investigation; (d) the individual AUC0-72 corresponding to concentration data points provided by a first-order compartmental pharmacokinetic model, when the relative values of either the absorption rate (Kabs) or the bioavailability (F) model parameters were set at 0.85 or 0.6, of their respective original values.

CONCLUSIONS

The 2-point LSS models developed in the current study predict accurately the AUC of amlodipine under a variety of experimental conditions and, thus, may be valuable for exploring the relationships between the pharmacokinetics and pharmacodynamics of this calcium antagonist, at reduced costs of sample acquisition and analysis, and avoiding sampling at "unsociable" hours.

Hepatitis A incidence rate estimates from a pilot seroprevalence survey in Rio de Janeiro, Brazil

BACKGROUND

To assess the impact of water sanitation and sewage disposal, part of a major environmental control programme in Rio de Janeiro, we carried out sero-prevalence studies for Hepatitis A virus (HAV) in three micro-regions in Rio de Janeiro. Each region varied with regard to level of sanitation. We are interested in assessing the discriminating power of age-specific prevalence curves for HAV as a proxy for improvement in sanitation. These curves will serve as baseline information to future planned surveys as the sanitation programme progresses.

METHODS

Incidence rate curves from prevalence data are estimated parametrically via a Weibull-like survival function, and non-parametrically via maximum likelihood and monotonic splines. Sera collected from children and adults in the three areas are used to detect antibodies against HAV through ELISA.

RESULTS

We compare baseline incidence curves at the three sites estimated by the three methods. We observe a strong negative correlation between level of sanitation and incidence rates for HAV infection. Incidence estimates yielded by the parametric and non-parametric approaches tend to agree at early ages in the microregion showing the best level of sanitation and to increasingly disagree in the other two.

CONCLUSION

Our results support the choice of HAV as a sentinel disease that is associated with level of sanitation. We also introduce monotonic splines as a novel non-parametric approach to estimate incidence from prevalence data. This approach outperforms current estimating procedures.

A non-parametric method for the reconstruction of age- and time-dependent incidence from the prevalence data of irreversible diseases with differential mortality

A method is proposed for reconstructing the time and age dependence of incidence rates from successive age-prevalence cross sections taken from the sentinel surveys of irreversible diseases when there is an important difference in mortality between the infected and susceptible subpopulations. The prevalence information at different time-age points is used to generate a surface; the time-age variations along the life line profiles of this surface and the difference in mortality rates are used to reconstruct the time and age dependence of the incidence rate. Past attempts were based on specified parametric forms for the incidence or on the hypothesis of time-invariant forms for the age-prevalence cross sections. The proposed method makes no such assumptions and is thus capable of coping with rapidly evolving prevalence situations. In the simulations carried out, it is found to be resilient to important random noise components added to a prescribed incidence rate input. The method is also tested on a real data set of successive HIV age-prevalence cross sections from Burundi coupled to differential mortality data on HIV(+) and HIV(-) individuals. The often-made assumption that the incidence rate can be written as the product of a calendar time component and an age component is also examined. In this case, a pooling procedure is proposed to estimate the time and the age profiles of the incidence rate using the reconstructed incidence rates at all time-age points.

Detection of antibodies against hepatitis A virus in eluates of blood spotted on filter-paper: a pilot study in Rio de Janeiro, Brazil

The validity of blood spotted on to filter-paper (BSOFP) eluates for the detection of antibodies against hepatitis A virus (HAV) was investigated in 718 individuals (children and adults) during a field study in a small area in Rio de Janeiro State, Brazil. Serum samples were considered the 'gold standard'. BSOFP eluates were analyzed by 2 different techniques: microplate competitive enzyme-linked immunosorbent assay (ELISA) of the whole study group and microparticle enzyme immune assay (MEIA) of a subsample of 59 individuals. For BSOFP eluates by ELISA, sensitivity and specificity were 89.6% (95% CI: 84.7-93.1) and 97.5% (95% CI: 95.6-98.7), respectively. For a seroprevalence of anti-HAV antibodies of 32%, the positive predictive value was 94.5% (95% CI: 90.3-97.0) and the negative predictive value was 95.2% (95% CI: 92.8-96.8). The test efficiency was 95.0% (95% CI: 93.1-96.4). Similar results were found for BSOFP eluates by MEIA. Agreement between the 2 techniques used for BSOFP (ELISA and MEIA) was also high (kappa = 0.93). These results encourage the more widespread application of BSOFP as a means of surveillance for large-scale epidemiological studies for hepatitis A.

[Leprosy epidemiology in a cohort of household contacts in Rio de Janeiro (1987-1991)]

This study aimed to identify factors influencing the development of leprosy (Hansen's disease) in household contacts. A dynamic cohort was analyzed from 1987 to 1991 at the Hansen's Disease Department of the Oswaldo Cruz Foundation in Rio de Janeiro. The incidence rate was 0.01694 person-years of follow-up. Nevertheless, for subjects at the end of the first year of follow-up the incidence rate was 0.06385 (end of second year, 0.03299; end of third year, 0.02370; end of fourth year, 0.018622; and end of observation period, 0.01694). A stepwise multivariate logistic regression model was proposed to study the risk of developing leprosy, including co-prevalent cases, totaling 758 contacts. In the final model, the risk was associated with a negative Mitsuda skin test (OR = 3.093; CI 95% = 1.735-5.514), prior BCG vaccination (OR = 0.3802; CI 95% = 0.2151-0.66719), and multibacillary primary clinical form (OR = 2.547; CI 95% = 1.249-5.192). The results showed that both multibacillary leprosy and specific immune status are significant indicators for developing the disease in a cohort of household contacts.

Protease inhibitors as initial therapy for individuals with an intermediate risk of HIV disease progression: is more necessarily better?

OBJECTIVE

To compare three possible therapeutic strategies for the treatment of patients with an intermediate risk of HIV disease progression.

DESIGN

Mathematical modeling based on assumptions derived from published data.

METHODS

A parametric survival model was fitted to empirical data to describe the survival trajectory of untreated individuals. It was assumed that successful treatment decreases the risk of disease progression curing the first year after its introduction by a constant that is dependent on the magnitude of the initial drop in HIV viral load. Thereafter, individual members of the treatment cohort follow different pathways, depending on the duration of the initial response or, in case of virologic failure, the response to a new drug regimen.

RESULTS

Sub-groups of patients starting therapy with two nucleoside reverse transcriptase inhibitors (NRTI) or two NRTI and a protease inhibitor had the highest instantaneous risk of disease progression at the end of the 5-year follow-up period. Patients who started therapy with two NRTI and a non-NRTI had the lowest likelihood of progression to AIDS or death at 5 years of follow-up. This is because, in the case of the subgroup whose initial treatment included a protease inhibitor, failure rates due to non-adherence to therapy are high and response to salvage therapy is limited by past protease inhibitor experience.

CONCLUSIONS

Despite the superior virologic potency of the protease inhibitor-containing regimens, in this analysis other strategies performed equally well or even better. In the absence of solid empirical data and until the advent of antiretroviral regimens that are shown to be safe, simple to take, and maximally suppress viral load, caution may be required in selecting the long-term therapy for patients with less advanced HIV disease.

Limited-sampling strategy models for itraconazole and hydroxy-itraconazole based on data from a bioequivalence study

The extensive interindividual variability in oral bioavailability of itraconazole prompted an assessment of the bioequivalence of two formulations marketed in Brazil, namely, Sporanox (reference) and Traconal (test). Eighteen healthy volunteers received single 200-mg oral doses of each formulation at 2-week intervals in a randomized, crossover protocol. The concentrations of itraconazole and hydroxy-itraconazole in plasma were measured by high-performance liquid chromatography, and the datum points (n = 396) were subsequently used to develop limited-sampling strategy models for estimation of the areas under the curve (AUCs) for both compounds. The 90% confidence intervals for individual percent ratios (test/reference formulations) of the maximum concentration of drug in serum, the AUC from 0 to 48 h and the AUC from time zero to infinity (AUC0-infinity) for itraconazole and hydoxy-itraconazole were below the range of 80 to 125%, suggesting that these formulations are not bioequivalent. Linear regression analysis of the AUC0-infinity against time and a "jackknife" validation procedure revealed that models based on three sampling times accurately predict (R2, >0.98; bias, <3%; precision, 3 to 7%) the AUC0-infinity for each of the four formulation-compound pairs tested. Increasing the number of sampling points to more than three adds little to the accuracy of the estimates of AUC0-infinity. The three-point models developed for the reference formulation were validated retrospectively and were found to predict within 2% the AUC0-infinity reported in previous studies performed under similar protocols. In conclusion, the data in this study indicate (i) that the tested formulations are not bioequivalent when single doses are compared and (ii) that limited-sampling strategy models based on three points predict accurately the AUC0-infinitys for itraconazole and hydroxy-itraconazole and could be a valuable tool in pharmacokinetic and bioequivalence studies of single oral doses of itraconazole.

Evaluation of SPf66 malaria vaccine efficacy in Brazil

This paper reports the efficacy results of the randomized, placebo-controlled, field trial of SPf66 malaria vaccine in Costa Marques, Rondonia, Brazil. This region is characterized by the seasonal distribution of Plasmodium falciparum and P. vivax infections, and the recent occupation by migrants from nonendemic areas. A total of 800 individuals of both sexes, ranging in age from seven to 60 years, were included in the study. Of the initial cohort, 572 participants completed the vaccination schedule. Clinical and parasitologic evaluations were obtained by active and passive searches on a periodic basis. The overall protective efficacy against P. falciparum infections was -1.6% (-32.9% to 22.4%), and 14.1% (-17.0% to 36.9%) for the first episode. The overall protective efficacy for P. vivax infections was -19.7% (-44.8% to 1.03%), and -10.8% (-41.1% to 12.8%) for the first episode. No statistical evidence of an overall significant protective effect of SPf66 malaria vaccine against P. falciparum and P. vivax malaria was obtained in this trial.

Study designs for evaluating different efficacy and effectiveness aspects of vaccines

Vaccine efficacy and effectiveness (VE) are generally measured as 1 minus some measure of relative risk (RR) in the vaccinated group compared with the unvaccinated group (VE = 1 - RR). In designing a study to evaluate vaccination, the type of effect and the question of interest determine the appropriate choice of comparison population and parameter. Possible questions of interest include that of the biologic effect of vaccination on susceptibility, on infectiousness, or on progression to disease in individuals. The indirect effects, total effects, and overall public health benefits of widespread vaccination of individuals within the context of a vaccination program might also be of primary concern. The change in behavior induced by belief in the protective effects of vaccination might influence the estimates of these effects or might itself be of interest. In this paper, the authors present a framework of study designs that relates the scientific question of interest to the choice of comparison groups, the unit of observation, the level of information available for analysis, and the parameter of effect.

Rate estimation from prevalence information on a simple epidemiologic model for health interventions

Health intervention control programs, such as vaccination, can be evaluated by comparing incidence rates of infection between unprotected and protected individuals in a population. The ratio of incidence rates is usually estimated by following up control and treated groups in order to collect information on person-time and cases in each group. This approach can be expensive and time consuming. An alternative approach is to use prevalence data to reconstitute incidence. Current-status are readily available or easily gathered and can be used to estimate incidence rates. Under certain assumptions of irreversibility for the outcome of interest, we discuss a simple transmission model appropriate to evaluate health interventions that confer long term protection. Rates and populations are parameter-free functions of age and calendar time. We develop general mathematical relationships that link incidence and intervention rates to prevalence which could be estimated from sampling without requiring knowledge of subpopulation demographics.

Safety evaluation of SPf66 malaria vaccine in Brazil

The frequency and description of side effects secondary to the subcutaneous application of SPf66 malaria vaccine and placebo are reported for each dose of application in the participants of the vaccine efficacy trial in Brazil. Side effects evaluated two hours after each application were detected in 8.0%, 30.2% and 8.8%, for the 1st, 2nd and 3rd dose, respectively, in the SPf66 group, and in 7.0%, 8.5% and 2.9% in the placebo group. Local reactions such as mild inflammation, nodule and pain or erythema frequently accompanied by pruritus were the most common reactions detected in both groups (3.8%, 29.1% and 8.5% in the SPf66 group and 4.0%, 7.6% and 2.5% in the placebo group). Among vaccinees, local side effects after the 2nd dose were more frequent in females. Systemic side effects were expressed mainly through general symptoms referred by the participants and were most frequent after the 1st dose in both groups (4.3% in the SPf66 group and 3.0% in the placebo group). Muscle aches and fever were referred by few participants. No severe adverse reactions were detected for either dose of application or group.

Estimability and interpretation of vaccine efficacy using frailty mixing models

The authors consider estimability and interpretation of vaccine efficacy based on time to event data, allowing that some of the population might have a very low probability of acquiring disease, and the rest have partial, possibly continuously distributed, susceptibility. The efficacy parameters of interest in the frailty mixing model include the fraction highly unlikely to acquire the infection or disease due to the vaccine, the degree of partial protection in those still susceptible, and the average protection or summary measure of efficacy under heterogeneity. The efficacy estimates can still be usefully interpreted when the heterogeneity results from heterogeneity in contact patterns, contact rates, or infectiousness of the contacts, as long as these are equal in the vaccinated and unvaccinated groups. A likelihood-based method allows estimation of the efficacy parameters of interest from grouped time to event data. Simulated vaccine studies assuming different levels and distributions of efficacy demonstrate that ignoring heterogeneity in susceptibility or exposure to infection generally results in underestimation of vaccine efficacy as well as incorrect interpretation of the estimates. The approach is also applicable to other covariates affecting susceptibility or exposure to infection in infectious diseases. Exploitation of the dependent happening structure of infectious diseases to obtain a shape for the baseline hazard may help identifiability. The authors recommend fitting several models to time to event data in vaccine studies.

SPf66 vaccine trial in Brazil: conceptual framework study design and analytical approach

This paper describes the study population and the study design of the phase III field trail of the SPf66 vaccine in Brazil. Assessment of validity and precision principles necessary for the appropriate evaluation of the protective effect of the vaccine are discussed, as well as the results of the preliminary analyses of the gathered data. The analytical approach for the estimation of the protective effect of the vaccine is presented. This paper provides the conceptual framework for future publications.