Measuring the public-health impact of candidate HIV vaccines as part of the licensing process

Impact of a potential Chlamydia vaccine in the USA: mathematical modelling analyses

Introduction

Chlamydia trachomatis (CT) infection is a global health challenge. New approaches are needed to control CT disease burden.

Methods

An age-structured deterministic mathematical model calibrated to nationally representative population-based data was developed to investigate the impact of CT vaccination on the population of the USA if a vaccine becomes available. The model's parameters were chosen based on current knowledge from the literature on CT's natural history and epidemiology. The model's calibration used age-specific CT prevalence data sourced from the biannual rounds of the National Health and Nutrition Examination Surveys. The reported data are based on the outcomes generated by the model's simulations.

Results

Over a 10-year period, vaccinating 80% of individuals aged 15-49 with a vaccine that reduces by 50% susceptibility to infection (V E S = 50 % ), infectiousness (V E I = 50 % ) or duration of infection (V E P = 50 % ) resulted, respectively, in 36.3%, 26.5% and 42.1% reduction in CT prevalence, and 38.8%, 28.6% and 24.1% reduction in CT incidence rate. Number of averted infections was 11 346 000, 7 583 000 and 6 012 000, respectively. When efficacies acted together (V E S = V E I = V E P = 50 % ), CT prevalence and incidence rate were reduced by 66.3% and 61.0%, respectively. Number of vaccinations needed to avert one infection was 17.7 forV E S = 50 % , 26.5 forV E I = 50 % , 33.4 forV E P = 50 % and 12.0 forV E S = V E I = V E P = 50 % . Vaccinating individuals aged 15-19 and at highest risk of infection was most effective, requiring only 7.7 and 1.8 vaccinations to prevent one infection, respectively. Vaccination benefits were larger beyond 10 years.

Conclusion

A moderately efficacious CT vaccine can significantly reduce CT disease burden. Targeting specific populations can maximise cost-effectiveness. Additional potential 'breakthrough' effects of the vaccine on infectiousness and duration of infection could greatly increase its impact. CT vaccine development and implementation should be a public health priority.

Epidemiological Impact of SARS-CoV-2 Vaccination: Mathematical Modeling Analyses

This study aims to inform SARS-CoV-2 vaccine development/licensure/decision-making/implementation, using mathematical modeling, by determining key preferred vaccine product characteristics and associated population-level impacts of a vaccine eliciting long-term protection. A prophylactic vaccine with efficacy against acquisition (VE_S) ≥70% can eliminate the infection. A vaccine with VE_S <70% may still control the infection if it reduces infectiousness or infection duration among those vaccinated who acquire the infection, if it is supplemented with <20% reduction in contact rate, or if it is complemented with herd-immunity. At VE_S of 50%, the number of vaccinated persons needed to avert one infection is 2.4, and the number is 25.5 to avert one severe disease case, 33.2 to avert one critical disease case, and 65.1 to avert one death. The probability of a major outbreak is zero at VE_S ≥70% regardless of the number of virus introductions. However, an increase in social contact rate among those vaccinated (behavior compensation) can undermine vaccine impact. In addition to the reduction in infection acquisition, developers should assess the natural history and disease progression outcomes when evaluating vaccine impact.

Epidemiological Impact of SARS-CoV-2 Vaccination: Mathematical Modeling Analyses

This study aims to inform SARS-CoV-2 vaccine development/licensure/decision-making/implementation, using mathematical modeling, by determining key preferred vaccine product characteristics and associated population-level impacts of a vaccine eliciting long-term protection. A prophylactic vaccine with efficacy against acquisition (VES) ≥70% can eliminate the infection. A vaccine with VES <70% may still control the infection if it reduces infectiousness or infection duration among those vaccinated who acquire the infection, if it is supplemented with <20% reduction in contact rate, or if it is complemented with herd-immunity. At VES of 50%, the number of vaccinated persons needed to avert one infection is 2.4, and the number is 25.5 to avert one severe disease case, 33.2 to avert one critical disease case, and 65.1 to avert one death. The probability of a major outbreak is zero at VES ≥70% regardless of the number of virus introductions. However, an increase in social contact rate among those vaccinated (behavior compensation) can undermine vaccine impact. In addition to the reduction in infection acquisition, developers should assess the natural history and disease progression outcomes when evaluating vaccine impact.

Epidemiological Impact of Novel Preventive and Therapeutic HSV-2 Vaccination in the United States: Mathematical Modeling Analyses

This study aims to inform herpes simplex virus type 2 (HSV-2) vaccine development, licensure, and implementation by delineating the population-level impact of vaccination. Mathematical models were constructed to describe the transmission dynamics in presence of prophylactic or therapeutic vaccines assuming 50% efficacy, with application to the United States. Catch-up prophylactic vaccination will reduce, by 2050, annual number of new infections by 58%, incidence rate by 60%, seroprevalence by 21%, and avert yearly as much as 350,000 infections. Number of vaccinations needed to avert one infection was only 50 by 2050, 34 by prioritizing those aged 15-19 years, 4 by prioritizing the highest sexual risk group, 43 by prioritizing women, and 47 by prioritizing men. Therapeutic vaccination of infected adults with symptomatic disease will reduce, by 2050, annual number of new infections by 12%, incidence rate by 13%, seroprevalence by 4%, and avert yearly as much as 76,000 infections. Number of vaccinations needed to avert one infection was eight by 2050, two by prioritizing those aged 15-19 years, three by prioritizing the highest sexual risk group, seven by prioritizing men, and ten by prioritizing women. HSV-2 vaccination offers an impactful and cost-effective intervention to prevent genital herpes medical and psychosexual disease burden.

Mathematical Modeling of the Transmission Dynamics of Clostridium difficile Infection and Colonization in Healthcare Settings: A Systematic Review

BACKGROUND

We conducted a systematic review of mathematical models of transmission dynamic of Clostridium difficile infection (CDI) in healthcare settings, to provide an overview of existing models and their assessment of different CDI control strategies.

METHODS

We searched MEDLINE, EMBASE and Web of Science up to February 3, 2016 for transmission-dynamic models of Clostridium difficile in healthcare settings. The models were compared based on their natural history representation of Clostridium difficile, which could include health states (S-E-A-I-R-D: Susceptible-Exposed-Asymptomatic-Infectious-Resistant-Deceased) and the possibility to include healthcare workers and visitors (vectors of transmission). Effectiveness of interventions was compared using the relative reduction (compared to no intervention or current practice) in outcomes such as incidence of colonization, CDI, CDI recurrence, CDI mortality, and length of stay.

RESULTS

Nine studies describing six different models met the inclusion criteria. Over time, the models have generally increased in complexity in terms of natural history and transmission dynamics and number/complexity of interventions/bundles of interventions examined. The models were categorized into four groups with respect to their natural history representation: S-A-I-R, S-E-A-I, S-A-I, and S-E-A-I-R-D. Seven studies examined the impact of CDI control strategies. Interventions aimed at controlling the transmission, lowering CDI vulnerability and reducing the risk of recurrence/mortality were predicted to reduce CDI incidence by 3-49%, 5-43% and 5-29%, respectively. Bundles of interventions were predicted to reduce CDI incidence by 14-84%.

CONCLUSIONS

Although CDI is a major public health problem, there are very few published transmission-dynamic models of Clostridium difficile. Published models vary substantially in the interventions examined, the outcome measures used and the representation of the natural history of Clostridium difficile, which make it difficult to synthesize results and provide a clear picture of optimal intervention strategies. Future modeling efforts should pay specific attention to calibration, structural uncertainties, and transparent reporting practices.

A Bayesian approach to estimate changes in condom use from limited human immunodeficiency virus prevalence data

Evaluation of large‐scale intervention programmes against human immunodeficiency virus (HIV) is becoming increasingly important, but impact estimates frequently hinge on knowledge of changes in behaviour such as the frequency of condom use over time, or other self‐reported behaviour changes, for which we generally have limited or potentially biased data. We employ a Bayesian inference methodology that incorporates an HIV transmission dynamics model to estimate condom use time trends from HIV prevalence data. Estimation is implemented via particle Markov chain Monte Carlo methods, applied for the first time in this context. The preliminary choice of the formulation for the time varying parameter reflecting the proportion of condom use is critical in the context studied, because of the very limited amount of condom use and HIV data available. We consider various novel formulations to explore the trajectory of condom use over time, based on diffusion‐driven trajectories and smooth sigmoid curves. Numerical simulations indicate that informative results can be obtained regarding the amplitude of the increase in condom use during an intervention, with good levels of sensitivity and specificity performance in effectively detecting changes. The application of this method to a real life problem demonstrates how it can help in evaluating HIV interventions based on a small number of prevalence estimates, and it opens the way to similar applications in different contexts.

Potential future impact of a partially effective HIV vaccine in a southern African setting

Background

It is important for public health and within the HIV vaccine development field to understand the potential population level impact of an HIV vaccine of partial efficacy—both in preventing infection and in reducing viral load in vaccinated individuals who become infected—in the context of a realistic future implementation scenario in resource limited settings.

Methods

An individual level model of HIV transmission, progression and the effect of antiretroviral therapy was used to predict the outcome to 2060 of introduction in 2025 of a partially effective vaccine with various combinations of efficacy characteristics, in the context of continued ART roll-out in southern Africa.

Results

In the context of our base case epidemic (in 2015 HIV prevalence 28% and incidence 1.7 per 100 person years), a vaccine with only 30% preventative efficacy could make a substantial difference in the rate with which HIV incidence declines; the impact on incidence in relative terms is projected to increase over time, with a projected 67% lower HIV incidence in 2060 compared with no vaccine introduction. The projected mean decline in the general adult population death rate 2040–2060 is 11%. A vaccine with no prevention efficacy but which reduces viral load by 1 log is predicted to result in a modest (14%) reduction in HIV incidence and an 8% reduction in death rate in the general adult population (mean 2040–2060). These effects were broadly similar in multivariable uncertainty analysis.

Interpretation

Introduction of a partially effective preventive HIV vaccine would make a substantial long-term impact on HIV epidemics in southern Africa, in addition to the effects of ART. Development of an HIV vaccine, even of relatively low apparent efficacy at the individual level, remains a critical global public health goal.

Beating the placebo in HIV prevention efficacy trials: the role of the minimal efficacy bound

OBJECTIVES

To produce an effect on the HIV epidemic, preventive interventions need to achieve a minimum level of efficacy to offset potential indirect effects such as an increase in risky behavior. The current generation of HIV prevention trials on oral preexposure prophylaxis and on vaginal microbicides were designed using different set points for minimum individual-level efficacy (MIE). Some trials were designed not only to show superiority over placebo but also to rule out lower efficacies. The MIE has a substantial impact on the size and cost of a trial. Ideally, the MIE should be chosen to reduce uncertainty in the estimation of population-level effects. In this article, we investigate the effect of MIE on estimates of population-level impact to better inform trial design.

METHODS

We used mathematical model simulations assuming various rates of efficacy obtained from trials and different MIEs to study the impact of wide-scale interventions on 2 public health indicators.

RESULTS

Implementation factors were the main drivers of uncertainty in public health indicators for an intervention, although MIE also contributed. The level of uncertainty introduced by the MIE was substantially lower than that of the other factors.

CONCLUSIONS

Investigators in clinical trials have set the MIE solely on the basis of potential public health impact. However, the substantial increase in trial costs associated with a large MIE is unlikely to be justified. These additional funds would be better spent in evaluating more critical implementation factors that cannot be assessed in clinical trials.

Categorization of methods used in cost-effectiveness analyses of vaccination programs based on outcomes from dynamic transmission models

The aim of this study is to categorize methods used to estimate the cost-effectiveness of vaccination programs using dynamic transmission models, and assess value to decision-makers. A targeted literature search of PubMed has been carried out for this purpose. A review of 43 articles presenting cost-effectiveness analyses of vaccination programs based on dynamic transmission models identified four methods for the estimation of a cost-effectiveness ratio: cumulative population values over a fixed time horizon; population values for a steady-state year; cohort values from time of program initiation; and cohort values at steady state. Cost-effectiveness estimates are sensitive to the choice of time horizon or number of cohorts included. Estimates at steady state are the most comparable to estimates for other healthcare interventions but do not account for pre-steady-state periods. Population values provide estimates of budget impact. In conclusion, four different methods were identified for converting clinical outcomes from a dynamic transmission model to cost-effectiveness estimates. Sensitivity analyses for time horizon or number of cohorts considered should be routinely performed.

HIV treatment as prevention: considerations in the design, conduct, and analysis of cluster randomized controlled trials of combination HIV prevention

The rigorous evaluation of the impact of combination HIV prevention packages at the population level will be critical for the future of HIV prevention. In this review, we discuss important considerations for the design and interpretation of cluster randomized controlled trials (C-RCTs) of combination prevention interventions. We focus on three large C-RCTs that will start soon and are designed to test the hypothesis that combination prevention packages, including expanded access to antiretroviral therapy, can substantially reduce HIV incidence. Using a general framework to integrate mathematical modelling analysis into the design, conduct, and analysis of C-RCTs will complement traditional statistical analyses and strengthen the evaluation of the interventions. Importantly, even with combination interventions, it may be challenging to substantially reduce HIV incidence over the 2- to 3-y duration of a C-RCT, unless interventions are scaled up rapidly and key populations are reached. Thus, we propose the innovative use of mathematical modelling to conduct interim analyses, when interim HIV incidence data are not available, to allow the ongoing trials to be modified or adapted to reduce the likelihood of inconclusive outcomes. The preplanned, interactive use of mathematical models during C-RCTs will also provide a valuable opportunity to validate and refine model projections.

High Multiplicity Infection by HIV-1 in Men Who Have Sex with Men

Elucidating virus-host interactions responsible for HIV-1 transmission is important for advancing HIV-1 prevention strategies. To this end, single genome amplification (SGA) and sequencing of HIV-1 within the context of a model of random virus evolution has made possible for the first time an unambiguous identification of transmitted/founder viruses and a precise estimation of their numbers. Here, we applied this approach to HIV-1 env analyses in a cohort of acutely infected men who have sex with men (MSM) and found that a high proportion (10 of 28; 36%) had been productively infected by more than one virus. In subjects with multivariant transmission, the minimum number of transmitted viruses ranged from 2 to 10 with viral recombination leading to rapid and extensive genetic shuffling among virus lineages. A combined analysis of these results, together with recently published findings based on identical SGA methods in largely heterosexual (HSX) cohorts, revealed a significantly higher frequency of multivariant transmission in MSM than in HSX [19 of 50 subjects (38%) versus 34 of 175 subjects (19%); Fisher's exact p = 0.008]. To further evaluate the SGA strategy for identifying transmitted/founder viruses, we analyzed 239 overlapping 5′ and 3′ half genome or env-only sequences from plasma viral RNA (vRNA) and blood mononuclear cell DNA in an MSM subject who had a particularly well-documented virus exposure history 3–6 days before symptom onset and 14–17 days before peak plasma viremia (47,600,000 vRNA molecules/ml). All 239 sequences coalesced to a single transmitted/founder virus genome in a time frame consistent with the clinical history, and a molecular clone of this genome encoded replication competent virus in accord with model predictions. Higher multiplicity of HIV-1 infection in MSM compared with HSX is consistent with the demonstrably higher epidemiological risk of virus acquisition in MSM and could indicate a greater challenge for HIV-1 vaccines than previously recognized.

Understanding the biology of sexual transmission of HIV-1 could contribute importantly to the development of effective prevention measures. However, different routes of virus transmission (vaginal, rectal, penile or oral) and inaccessibility of tissues at or near the time of virus transmission make this goal elusive. Here, we apply single genome amplification and sequencing of plasma HIV-1 and a model of random virus evolution to a cohort of acutely infected men who have sex with men (MSM) and find that MSM are twice as likely as heterosexuals to become infected by multiple viruses as opposed to a single virus. Some MSM subjects were infected by as many as 7 to 10 or more genetically distinct viruses as a consequence of a single exposure event. We go on to molecularly clone the first full-length transmitted/founder subtype B HIV-1 virus and show that it is highly replicative in human CD4+ T-cells but not macrophages. Our study provides the first comparative, quantitative analysis of the multiplicity of HIV-1 infection in the two primary risk groups—MSM and heterosexuals—driving the global pandemic, and we discuss the implications of the findings to HIV-1 vaccine development and prevention research.

Social and behavioral science in HIV vaccine trials: a gap assessment of the literature

Social and behavioral science research is integral to the conduct of HIV vaccine trials, especially because the vaccine targets an infection laden with sensitive human issues. Although social and behavioral sciences have played a larger role in HIV vaccine clinical trials than other vaccine clinical trials to date, this role should be expanded. Fortunately, related publications, conference coverage and research proposals are on the rise; community engagement is receiving more attention during the earlier stages of product development; and collaboration between HIV vaccine scientists and social and behavioral scientists is being fostered. Greater attention to social and behavioral science issues could not only facilitate accrual, but also improve research efficiency and relevance. In this review, gaps in the literature on social and behavioral science issues in HIV vaccine clinical research, including barriers and facilitators to trial participation, enhancing feasibility of trial success, health systems, policy and monitoring social and behavioral issues, are identified and directions are suggested for filling those gaps. Development of a safe, efficacious and acceptable HIV vaccine will be nurtured by addressing the gaps through interdisciplinary collaborations.

Incremental role of male circumcision on a generalised HIV epidemic through its protective effect against other sexually transmitted infections: from efficacy to effectiveness to population-level impact

BACKGROUND

Male circumcision (MC) can reduce HIV acquisition. However, a better understanding of the indirect protective effect of MC on sexually transmitted infections (STIs) is required.

OBJECTIVE

To assess the incremental benefits conferred by MC on HIV infection at the individual level in circumcision trials (no herd immunity effect) and at the population level (with herd immunity effect) owing to its protective effect against other STIs.

METHODS

A dynamic stochastic model of HIV and STI infections in a Kenyan population was used to simulate the impact of MC offered to a few trial participants or to a large proportion of men in order to study the protective role of MC on HIV infection at the individual and population levels.

RESULTS

Fewer than 20% of the HIV infections prevented in the circumcised arm of the circumcision trials (individual level) could be attributable to the efficacy of MC against STIs rather than against HIV. At the population level, MC can significantly reduce the prevalence of HIV, especially among men and women in the longer term. However, even at the population level, the long-term incremental impact of MC on HIV due to the protection against STI is modest (even if MC efficacy against the STI and STI prevalence was high).

CONCLUSIONS

The protection of MC against STI contributes little to the overall effect of MC on HIV. Additional work is needed to determine whether, and under what conditions, the protective effect of MC efficacy against STIs can have a significant incremental benefit on the HIV epidemic.