Several past studies have found that media reports of suicides and homicides appear to subsequently increase the incidence of similar events in the community, apparently due to the coverage planting the seeds of ideation in at-risk individuals to commit similar acts. Here we explore whether or not contagion is evident in more high-profile incidents, such as school shootings and mass killings (incidents with four or more people killed). We fit a contagion model to recent data sets related to such incidents in the US, with terms that take into account the fact that a school shooting or mass murder may temporarily increase the probability of a similar event in the immediate future, by assuming an exponential decay in contagiousness after an event. We find significant evidence that mass killings involving firearms are incented by similar events in the immediate past. On average, this temporary increase in probability lasts 13 days, and each incident incites at least 0.30 new incidents (p = 0.0015). We also find significant evidence of contagion in school shootings, for which an incident is contagious for an average of 13 days, and incites an average of at least 0.22 new incidents (p = 0.0001). All p-values are assessed based on a likelihood ratio test comparing the likelihood of a contagion model to that of a null model with no contagion. On average, mass killings involving firearms occur approximately every two weeks in the US, while school shootings occur on average monthly. We find that state prevalence of firearm ownership is significantly associated with the state incidence of mass killings with firearms, school shootings, and mass shootings.

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.

Background. The potential impact of antiretroviral therapy (ART) and pre-exposure prophylaxis (PrEP) with overlapping and nonoverlapping antiretrovirals (ARVs) on human immunodeficiency virus (HIV) transmission and drug resistance is unknown. Methods. A detailed mathematical model was used to simulate the epidemiological impact of ART alone, PrEP alone, and combined ART + PrEP in South Africa. Results. ART alone initiated at a CD4 lymphocyte cell count <200 cells/µL (80% coverage and 96% effectiveness) prevents 20% of HIV infections over 10 years but increases drug resistance prevalence to 6.6%. PrEP alone (30% coverage and 75% effectiveness) also prevents 21% of infections but with lower resistance prevalence of 0.5%. The ratio of cumulative infections prevented to prevalent drug-resistant cases after 10 years is 7-fold higher for PrEP than for ART. Combined ART + PrEP with overlapping ARVs prevents 35% of infections but increases resistance prevalence to 8.2%, whereas ART + PrEP with nonoverlapping ARVs prevents slightly more infections (37%) and reduces resistance prevalence to 7.2%. Conclusions. Combined ART + PrEP is likely to prevent more HIV infections than either strategy alone, but with higher prevalence of drug resistance. ART is predicted to contribute more to resistance than is PrEP. Optimizing both ART and PrEP effectiveness and delivery are the keys to preventing HIV transmission and drug resistance.

A classical epidemiological framework is used to provide a preliminary cost analysis of the effects of quarantine and isolation on the dynamics of infectious diseases for which no treatment or immediate diagnosis tools are available. Within this framework we consider the cost incurred from the implementation of three types of dynamic control strategies. Taking the context of the 2003 SARS outbreak in Hong Kong as an example, we use a simple cost function to compare the total cost of each mixed (quarantine and isolation) control strategy from a public health resource allocation perspective. The goal is to extend existing epi-economics methodology by developing a theoretical framework of dynamic quarantine strategies aimed at emerging diseases, by drawing upon the large body of literature on the dynamics of infectious diseases. We find that the total cost decreases with increases in the quarantine rates past a critical value, regardless of the resource allocation strategy. In the case of a manageable outbreak resources must be used early to achieve the best results whereas in case of an unmanageable outbreak, a constant-effort strategy seems the best among our limited plausible sets.