Stanford
Prevention Policy Modeling Lab

Sexually Transmitted Diseases

Research Area

Sexually Transmitted Diseases

Click on a project title to learn more about PPML's work on Sexually Transmitted Diseases.

Persons who are justice system-involved are more likely to experience risk factors for hepatitis C and sexually transmitted infections (STI), and prevalence of these infections is higher than in the general population. For many incarcerated individuals, correctional health services may be the first or only touchpoint for testing and treatment. CDC recommends comprehensive opt-out testing at intake, treatment for those infected, and linkage to care upon release. Significant barriers exist for facilities to fully implement these recommendations. However, with dedicated resources and leadership support, some state department of corrections, jails, local health departments, and other community services have come together to overcome these barriers and implement successful programs tailored to their local situation.

Successful implementation of the CDC Recommendations for Correctional Settings across the nation could have important impacts on the community prevalence of these diseases and subsequent transmission. Decreasing the prevalence of these diseases among the incarcerated population and their communities could lead to improved population health and economic outcomes as well as safer work environment for correctional staff. Averted costs through reduced transmission could translate to increased funding to support the development and implementation of interventions and strategies to further reduce disparities in health outcomes for incarcerated populations.

This project will estimate the change in hepatitis C and syphilis national prevalence if all U.S. adult jails fully, successfully implemented the CDC Recommendations for Correctional Settings. It also aims to better understand the factors that could increase or decrease the health and economic impact of the CDC Recommendations and their collaboration with health departments.

Doxycycline post-exposure prophylaxis (Doxy-PEP) has been shown to reduce risk of infection with bacterial sexually transmitted infections (STI) among men who have sex with men (MSM). Following recommendations by several public health departments, Doxy-PEP is being implemented across the U.S. There is concern that increased use will lead to emergence of resistance in N.gonorrhoeae, and without alternative interventions, to increased gonorrhea incidence. A clear strategy is needed to monitor the impact of Doxy-PEP on antimicrobial resistance and inform treatment recommendations.

The primary objectives in this project are to estimate the reduction in gonorrhea incidence and changes in the doxycycline resistance proportion with rollout of Doxy-PEP and compare surveillance strategies in their ability to identify increases in doxycycline resistance. This project will utilize a previously developed stochastic compartmental model to estimate the undetected disease burden of a new strain of ceftriaxone-resistant gonorrhea in Massachusetts. We will expand this model to include the rollout of both Doxy-PEP and vaccination in a population of MSM.

Analyses of the impact and cost-effectiveness of congenital syphilis (CS) prevention interventions require data on the lifetime health impact and cost per case of congenital syphilis. Current estimates of the cost and number of quality-adjusted life years (QALY) lost per case of CS account for stillbirths and neonatal death and short-term outcomes, but ignore potential lifetime effects of CS. Further, the lifetime effects of CS can vary depending on whether CS is diagnosed and treated at birth.

This project will estimate the average lifetime direct medical cost, lifetime productivity cost, and number of QALYs lost per case of CS in the U.S., considering all possible combinations of birth outcomes and treatment status of infant.

The number of congenital syphilis (CS) cases has continued to grow, surpassing 3,800 reported cases in 2023, 8 times the number reported in 2014. CDC recommends that all pregnant women undergo syphilis testing at their initial prenatal visit, and that those at high risk be screened again at 28 weeks and at delivery. Prevention of congenital syphilis requires innovative strategies, including reaching pregnant women who are not reached by traditional prenatal care.

Alternate strategies can provide avenues for reducing the burden of CS, but their population-level health impact remains unclear. Leveraging a previously developed CS model, we will evaluate health impacts of strategies to prevent CS both at the population level, and in disproportionately affected populations.

Past Projects

The U.S. Preventive Services Task Force recommends annual screening for chlamydia in sexually active women under the age of 25. Evidence suggests that chlamydia screening may be protective against pelvic inflammatory disease, which can lead to more severe health outcomes, like tubal factor infertility, ectopic pregnancy, and chronic pelvic pain. In men, chlamydia can cause urethritis and epididymitis. Previous studies suggest that chlamydia screening can be cost-effective; however, no transmission model has assessed the cost-effectiveness of chlamydia screening in the United States at the levels that have been achieved up to the present. 

This project used a calibrated chlamydia transmission model to estimate the number of sequelae averted and quality-adjusted life years (QALY) gained by chlamydia screening between 2000 and 2015 and assesses the cost-effectiveness of chlamydia screening.

Associated Publication

Estimates of the quality-of-life impacts of sexually transmitted infections (STI) are needed to quantify the health burden of STIs and to inform cost-effectiveness analyses of STI prevention interventions. The use of QALYs enables comparison across a wide range of conditions and outcomes. 

The purpose of this project is to develop estimates of the expected lifetime number of QALYs lost per infection for major STIs: chlamydia, gonorrhea, syphilis, herpes simplex virus type 2 (HSV-2), and trichomoniasis. In addition to informing cost-effectiveness analyses of STI prevention interventions, this project’s results, when combined with estimates of STI incidence, provide estimates of the overall population health burden of each STI. 

This project built on previous PPML work to estimate and value downstream consequences of infection across different racial/ethnic groups in the United States, as well as updates a prior analysis to compute QALYs lost for chlamydia and trichomoniasis. A Markov modeling approach and evidence synthesis computes QALYs lost for syphilis and HSV-2.

Associated Publications

The COVID-19 pandemic disrupted sentinel surveillance for antimicrobial resistant gonorrhea (ARGC). Fewer isolates were available at ARGC sentinel surveillance sites due to the reduction of STD clinic services. More broadly, healthcare seeking was impacted in most parts of the country, limiting the opportunity for patients to be screened or receive diagnostic testing for sexually transmitted infections. In particular, the reduction in screening and treatment (and the shift from injectable to oral treatment for gonorrhea in response to clinic closures) may have increased the incidence and prevalence of ARGC.

This project adapted an existing model for the spread of antimicrobial-resistant gonococcal infection to project the effective lifespan of five antibiotics (ciprofloxacin, ceftriaxone, azithromycin, penicillin, and tetracycline) if rapid point-of-care diagnosis were to become widely available.

Associated Publication

Estimating the return on investments in programs for sexually transmitted infections (STI) is an important priority, and previous work has analyzed program impact for syphilis, chlamydia, and gonorrhea. Results suggest that funding STI control activities is associated with reduced STI diagnoses in subsequent years at the state-level, with the strongest association observed for gonorrhea and syphilis. The regression modeling framework is not amenable to identifying the mechanisms through which the impact of funding is achieved, and as a non-dynamic model using ecological data, there is uncertainty around the estimated impact. 

This project triangulated results across different modeling frameworks to compare and validate earlier findings estimating the impact of STD Prevention and Control for Health Departments-funded STD prevention activities.

Associated Publication

Despite advances in prevention and treatment, significant disparities exist in rates of sexually transmitted infections (STI), tuberculosis (TB), and viral hepatitis in the United States. Multiple causes, many rooted in social determinants of health, increase risks of exposure, reduce access to preventive services such as testing, and delay linkages to care and treatment for underserved populations. Disparities are expressed in worse health outcomes, and reduced prevention effectiveness also may result in higher care and treatment expenditures. This project evaluated and quantified the potential health and economic impacts of addressing the strategies, goals, and indicators, as defined by the STI, Viral Hepatitis, and TB National Strategic Plans to reduce racial and ethnic disparities.

Early detection and treatment are core components of congenital syphilis (CS) prevention. The risk of adverse outcomes is lower if a pregnant person with syphilis receives adequate treatment during early pregnancy. Improving antenatal care access and quality has benefits that go vastly beyond the prevention of CS, including: 

  • Reducing disparities in a range of adverse outcomes for pregnant people by race/ethnicity
  • Strengthening healthcare systems
  • Improving integration of antenatal care with broader reproductive healthcare 

This project modeled the status quo of antenatal care access and syphilis screening cascade, compared to increased antenatal care access, to understand how to reduce the burden and disparities in CS through improved quality and access to care for pregnant people.

Antimicrobial resistant gonorrhea (ARGC) has been identified as a serious public health threat. Only one antibiotic—ceftriaxone—remains recommended by CDC guidelines for empirical treatment of gonorrhea in the United States, underscoring the need to develop new antibiotics to treat this highly prevalent sexually transmitted infection.

With the prospect of new anti-gonococcal antibiotics being approved, this project evaluates to deploy a new drug to maximize its clinically useful lifespan. This study focuses specifically on comparing introduction strategies for a novel, first-in-class antibiotic for gonorrhea treatment within the existing ceftriaxone monotherapy landscape. It provides evidence that is pertinent to treatment protocol development once an antibiotic candidate for gonorrhea treatment gains regulatory approval.

Associated Publication

Optimizing the use of sentinel surveillance can improve effectiveness at detecting emerging antimicrobial resistant gonorrhea (ARGC) and extending the life of effective treatments. Prior PPML work showed that increasing the scale of sentinel surveillance could increase the effective lifespan of antibiotics used for the treatment of gonorrhea. Evidence is lacking about the cost-effectiveness of this and other strategies that change the number of isolates tested dynamically in response to prior periods’ estimates of resistance prevalence.

This project aimed to identify cost-effective strategies to inform the annual number of isolates tested for drug susceptibility in sentinel surveillance.

Associated Publication

Identifying geographic areas of populations at risk for higher and worsening rates of STIs can help to prioritize resource allocations and prevention efforts. This project used multilevel regression modeling to identify indicators predictive of: 

  • Gonorrhea cases that are most likely to be antibiotic-resistant
  • Jurisdictions that have increased vulnerability to STI increases and outbreaks

With the appearance of penA 60.001-carrying N. gonorrhoeae strains in the United States, a critical question is the prevalence of these strains. This project answers how to estimate this prevalence, how these estimates and the associated uncertainty change as time passes and new cases are or are not detected, and the key parameters needed to make and improve these estimates.

This project evaluates resistance prevalence from an N. gonorrhoeae resistance surveillance system once a novel resistant strain is detected.

Associated Publication

The Gonococcal Isolate Surveillance Project (GISP) was established in 1986 to monitor trends in antimicrobial susceptibilities of N. gonorrhoeae strains in the United States to establish an evidence-based rationale for selection of gonococcal therapies. Although providing evidence to inform gonorrhea treatment guidelines is of substantial public health value, this value has never been explicitly quantified.  This project develops a simulation model of antimicrobial resistant gonorrhea among men who have sex with men in the United States to project the cost and quality adjusted life years associated with different scenarios of surveillance specifications for AMR gonorrhea, as well as conducts a cost effectiveness analysis to estimate the value of GISP and compare its cost-effectiveness with other scenarios of surveillance.

This project estimated the value – in terms of averted gonococcal infections and costs saved – of the Gonococcal Isolate Surveillance Project.

This project explored interlocking relationships between HIV prevention methods, such as PrEP, and STD transmission and treatment, to understand the future course of the HIV and STD epidemics for MSM overall and by race. Because of the complex feedback and high dimensionality of the behaviors and subgroups of interest, and the infeasibility of conducting randomized trials to probe the questions of interest, agent-based modeling is well-suited.

Agent-based models of MSM have been developed to represent the epidemics in Atlanta, GA, and Baltimore, MD, and used to assess the impact of PrEP guidelines, the role of STI clinics, HIV/STI coinfection, and expedited partner therapy (EPT) options for STI prevention.

This project developed an agent-based transmission model of gonorrhea in MSM to the unique epidemiological characteristics, as well as diagnostic and treatment challenges. The model was used to investigate the importance of different sites of infection for gonorrhea transmission in MSM and the potential impact of site-specific screening strategies on the ability to control gonorrhea transmission.

Associated Publications

The clinical and public health community are facing the challenge of increasing antibiotic resistance in gonorrhea. In the search for strategies to control resistance, one proposed approach is the use of rapid diagnostics that would allow clinicians to tailor treatment to the antibiotic susceptibilities of individual infections, rather than treating empirically based on data from population-wide surveillance. In fact, efforts to develop such diagnostics are underway.

The hypothesis motivating this strategy – that resistance emerges under antibiotic selection, so tailored therapy should prolong the utility of anti-gonococcal agents and control resistance better that the current strategy of empiric treatment – has not been evaluated. The lack of studies on this topic has been due to lack of data on several key parameters, including the current extent of resistance in the population, likelihood of emergence of resistance on treatment, and fitness cost of resistance mutations.

This project assessed the impact of tailored therapy on slowing the emergence and spread of antibiotic resistance in gonorrhea, with consideration of resistance to the extended spectrum cephalosporins, macrolides, and fluoroquinolones.

Associated Publication

Syphilis is resurgent in the United States. The most striking epidemiological feature of the current epidemic is the disproportionate representation of MSM among cases, with incidence also varying by geography and ethnicity. Cases of CS have been increasing in recent years, as have rates in women, indicative of a changing epidemic. These trends suggest that the previously primarily MSM-focused epidemic has expanded into heterosexual populations. Frequent screening of at-risk individuals remains the best available tool for syphilis control, but current public health efforts have not had the desired effect in reducing the disease burden.

A risk-structured transmission model was developed to characterize the epidemiology of syphilis in the United States and evaluate the potential impact of different approaches to syphilis screening on epidemic dynamics and the health of affected individuals. The model incorporated both MSM and heterosexual populations. The model was fit for outbreaks in different jurisdictions, recognizing the importance that core group composition and geography will have on epidemic characteristics and the implications for sustaining syphilis control in different contexts.

Associated Publication

Chlamydia remains one of the most prevalent STIs. Untreated infection can result in pelvic inflammatory disease (PID) and infertility among women, presenting a challenge to improve on prevention strategies. This project explored a range of novel intervention approaches for testing and treatment, using a pair formation model of chlamydia to investigate the potential strengths and limitations of different prevention strategies, namely:

  • Partner services
  • More frequent screening of those with a higher rate of chlamydia
  • Point-of-care testing

This model was also used to examine the potential impact of more frequent screening among those who have previously been tested and treated for chlamydia, a group that has an elevated risk for re-infection. This project highlighted the strengths and limitations of the different strategies and their potential impact on controlling the transmission dynamics of chlamydia. Such analyses of novel prevention strategies can inform policy decisions and be used to allocate resources.

Associated Publication

The overall burden of gonorrhea in the United States is not equally distributed across the population, with marked disparities in terms of the geographical distribution of cases and the reported race/ethnicity and sexual orientation of cases. There is significant geographic heterogeneity in affected population groups across the United States that a national-level model is not able to capture. Some regions in the country are experiencing primarily Black heterosexual outbreaks, others predominantly MSM outbreaks, and others a mixture of the two.

Given this regional variation, the types of interventions used and the success of screening programs might be expected to differ by region, depending on the affected populations and epidemic profile. To better understand how demographic differences influence the effectiveness of screening interventions, this project used local-level models that reproduce regional trends to evaluate the effect of current and novel approaches to screening and treatment in the face of different outbreak characteristics. This approach can help local public health practitioners better tailor approaches to screening based on local gonorrhea epidemiology.

Associated Publication

The clinical and public health community are facing the challenge of increasing antibiotic resistance in gonorrhea. PPML modified an existing gonorrhea point-of-care testing model to investigate how different properties associated with a newly introduced antibiotic influence decision-making for gonorrhea treatment recommendations.

The model described a range of properties associated with a novel antimicrobial, such as the probability of resistance emergence on treatment and an estimate of fitness cost associated with resistance. Using near real-time observations, PPML’s adaptive decision-making framework defined treatment policies that are expected to prevent the most gonorrhea cases while requiring the fewest number of switches between available treatment recommendations.

Associated Publications

Chlamydia remains one of the most prevalent STIs in the United States due to a significant asymptomatic reservoir. Strategies to prevent STIs include a wide range of interventions, including those that:

  • Focus on individual behavior change, such as behavioral counseling, condoms, and sexual health messaging
  • Improve detection and treatment of cases and partners
  • Target societal/structural change through stigma reduction, legalization of gay marriage, or alcohol taxes

However, how best to allocate limited resources across these interventions to achieve maximal impact on population rates of infection, reduction of disparities impacting marginalized sub-populations, and reduction of sequelae, is unknown. This project developed a model of heterosexual transmission of C. trachomatis infection to enable examination of the health impact, costs, and cost-effectiveness of different intervention strategies in specific settings defined by epidemiologic, economic, demographic, and healthcare profiles.

Associated Publications

Gonorrhea is the second most prevalent bacterial STI in the United States, with the highest prevalence observed regionally in the Southeast, and among MSM, African American, and young adult populations. Gonorrhea infection can be asymptomatic, resulting in many undetected cases in the absence of targeted screening policies. Undetected cases have an increased risk of progressing to adverse health outcomes, such as pelvic inflammatory disease.

Another significant element in the current epidemiology of gonorrhea is the development of antimicrobial resistance in many Neisseria gonorrhoea strands. This model captured the essential features of gonorrhea epidemiology in the U.S. population, including antimicrobial resistance and genotyping, to assess interventions for reducing the burden of disease. PPML developed a novel mathematical model for evaluating the transmission of gonorrhea strains in heterosexual and homosexual populations in the United States.

 Associated Publication

Estimating the incidence and prevalence of STIs in the United States is necessary for policy and planning purposes, but direct evidence for these data is lacking and challenges in obtaining reliable estimates exist. Most commonly, cross-sectional data have been used to estimate prevalence, which is then used to back-calculate incidence estimates.

This project developed a statistical model to estimate the prevalence and incidence of chlamydia, gonorrhea, trichomoniasis, and syphilis in the United States from 2000-2014 using laboratory examination results from national surveys, surveillance case reports, and self-reported diagnoses, among others.

Gonorrhea infection has a low prevalence in the general population of the United States, but disproportionate effects are seen in subgroups of the population. Increased risk of gonorrhea infection has been found in African American and MSM communities and is associated with low socioeconomic status.

This project explored the consequences of the unequal burden of gonorrhea in the United States. It utilized a Markov cohort model to translate estimated disparities in incidence and prevalence of gonorrhea into estimates of the cost and burden of disease by racial and ethnic groups, with the latter quantified in QALYs lost. It also examined the potential health and economic consequences of different intervention strategies on racial and ethnic disparities.

Associated Publication

Chlamydia diagnosis rates are highest among people under the age of 25. A significant percentage of this population is school-aged children currently enrolled in educational facilities. Schools offer a structured setting for targeted interventions intended to reduce chlamydia prevalence, such as screening. Prior studies on school-based screening programs found variation in their potential impact, with several pilots failing to demonstrate a stable reduction in chlamydia prevalence in the intervention schools. Certain factors, such as the mixing of students between intervention and non-intervention schools and variation in school participation and parental consent, are thought to limit potential impact.

This project evaluated the impact and cost-effectiveness of school-based screening for chlamydia within a variety of community settings. The analysis explored the impact of screening programs by modeling a range of intervention strategies in urban settings, characterized by varying profiles and pre-existing levels of chlamydia screening in public high schools. Three urban school-based chlamydia screening programs provided a strong empirical basis for modeling specific programs and associated outcomes. The model incorporated the data and profile of each participating site, accommodating replication of existing programs as well as generalization to enable exploration of potential outcomes under a wide range of different circumstances reflecting variation in epidemiology and existing health services coverage.

Associated Publication