A risk-based model for predicting the impact of using condoms on the spread of sexually transmitted infections

We create and analyze a mathematical model to understand the impact of condom-use and sexual behavior on the prevalence and spread of Sexually Transmitted Infections(STIs).STIs remain significant public health challenges globally with a high burden of some Sexually Transmitted Diseases(STDs)in both developed and undeveloped countries.Although condom-use is known to reduce the transmission of STIs,there are a few quantitative population-based studies on the protective role of condom-use in reducing the incidence of STIs.The number of concurrent partners is correlated with their risk of being infectious by an STI such as chlamydia,gonorrhea,or syphilis.We develop a Susceptible-Infectious-Susceptible(SIS)model that stratifies the population based on the number of concurrent partners.The model captures the multi-level heterogeneous mixing through a combination of biased(preferential)and random(proportional)mixing processes between individuals with distinct risk levels,and accounts for differences in condom-use in the low-and high-risk populations.We use sensitivity analysis to assess the relative impact of high-risk people using condom as a prophylactic intervention to reduce their chance of being infectious,or infecting others.The model predicts the STI prevalence as a function of the number of partners of an individual,and quantifies how this distribution of effective partners changes as a function of condom-use.Our results show that when the mixing is random,then increasing the condom-use in the high-risk population is more effective in reducing the prevalence than when many of the partners of high-risk people have high risk.The model quantifies how the risk of being infected increases for people who have more partners,and the need for high-risk people to consistently use condoms to reduce their risk of infection.

Some models for epidemics of vector-transmitted diseases

Vector-transmitted diseases such as dengue fever and chikungunya have been spreading rapidly in many parts of the world.The Zika virus has been known since 1947 and invaded South America in 2013.It can be transmitted not only by(mosquito)vectors but also directly through sexual contact.Zika has developed into a serious global health problem because,while most cases are asymptomatic or very light,babies born to Zika-infected mothers may develop microcephaly and other very serious birth defects.We formulate and analyze two epidemic models for vector-transmitted diseases,one appropriate for dengue and chikungunya fever outbreaks and one that includes direct transmission appropriate for Zika virus outbreaks.This is especially important because the Zika virus is the first example of a disease that can be spread both indirectly through a vector and directly(through sexual contact).In both cases,we obtain expressions for the basic reproduction number and show how to use the initial exponential growth rate to estimate the basic reproduction number.However,for the model that includes direct transmission some additional data would be needed to identify the fraction of cases transmitted directly.Data for the 2015 Zika virus outbreak in Barranquilla,Colombia has been used to fit parameters to the model developed here and to estimate the basic reproduction number.