Shift of percolation thresholds for epidemic spread between static and dynamic small-world networks

The aim of the study was to compare the epidemic spread on static and dynamic small-world networks. The network was constructed as a 2-dimensional Watts-Strogatz model (500x500 square lattice with additional shortcuts), and the dynamics involved rewiring shortcuts in every time step of the epidemic spread. The model of the epidemic is SIR with latency time of 3 time steps. The behaviour of the epidemic was checked over the range of shortcut probability per underlying bond 0-0.5. The quantity of interest was percolation threshold for the epidemic spread, for which numerical results were checked against an approximate analytical model. We find a significant lowering of percolation thresholds for the dynamic network in the parameter range given. The result shows that the behaviour of the epidemic on dynamic network is that of a static small world with the number of shortcuts increased by 20.7 +/- 1.4%, while the overall qualitative behaviour stays the same. We derive corrections to the analytical model which account for the effect. For both dynamic and static small-world we observe suppression of the average epidemic size dependence on network size in comparison with finite-size scaling known for regular lattice. We also study the effect of dynamics for several rewiring rates relative to latency time of the disease.Comment: 13 pages, 6 figure

Impact of Chlamydia trachomatis in the reproductive setting: British Fertility Society Guidelines for practice

Chlamydia trachomatis infection of the genital tract is the most common sexually transmitted infection and has a world-wide distribution. The consequences of infection have an adverse effect on the reproductive health of women and are a common cause of infertility. Recent evidence also suggests an adverse effect on male reproduction. There is a need to standardise the approach in managing the impact of C. trachomatis infection on reproductive health. We have surveyed current UK practice towards screening and management of Chlamydia infections in the fertility setting. We found that at least 90% of clinicians surveyed offered screening. The literature on this topic was examined and revealed a paucity of solid evidence for estimating the risks of long-term reproductive sequelae following lower genital tract infection with C. trachomatis. The mechanism for the damage that occurs after Chlamydial infections is uncertain. However, instrumentation of the uterus in women with C. trachomatis infection is associated with a high risk of pelvic inflammatory disease, which can be prevented by appropriate antibiotic treatment and may prevent infected women from being at increased risk of the adverse sequelae, such as ectopic pregnancy and tubal factor infertility. Recommendations for practice have been proposed and the need for further studies is identified

Role of concurrency in generalised HIV epidemics - Authors' reply

In response to our population-based study in which we followed up more than 7000 HIV-negative women over 5 years and failed to find evidence that concurrent sexual partnerships are an important driver of HIV incidence, James Shelton, Martina Morris, and Helen Epstein cite studies on HIV incidence in stable concordant HIV-negative partnerships as “direct evidence” for the concurrency hypothesis. However, these studies do not test the concurrency hypothesis (that concurrent sexual partnerships increase the rate of spread of HIV in a population) because they lack a meaningful counterfactual—ie, HIV incidence in people with the same total number of partners over the observation period but in serially monogamous partnerships. Furthermore, Morris and Epstein's claim that the concurrency hypothesis has already been tested and shown to hold runs counter to ongoing public debates on the topic,1, 2 other empirical data,3, 4, 5 and the results of a recent systematic review.6 The claim is also inconsistent with Morris and Epstein's call for a randomised controlled trial to test the hypothesis, which would not be ethically permissible if it had indeed already been shown that the hypothesis held true

Risk factors of sexually transmitted infections among migrant and non-migrant sexual partnerships from rural South Africa

In October 1998, cohorts of circular migrant men and their non-migrant sexual partners, and non-migrant men and their non-migrant sexual partners from rural South Africa were recruited and followed-up every 4 months until October 2001. At each visit, information on sociodemographic, sexual behaviour, sexually transmitted infections (STIs) and HIV was collected. In total, 553 individuals aged between 18 and 69 years were recruited. A man and his sexual partner(s) form a sexual partnership. Migration status, age, marital status, age at sexual debut, recent sexual partners and HIV status were found to be important determinants of STI. The risk of STI varies (s2=1.45, P<0.001) signi?cantly across sexual partnerships even after controlling for important determinants. The variance implies substantial correlation (0.59) between members of the same sexual partnership. Ignoring this correlation leads to incorrect inference. Migration contributes signi?cantly to the spread of STIs. Community interventions of HIV/STI should target co-transmitter sexual partnerships rather than high-risk individuals.

Heterogeneities in the transmission of infectious agents: Implications for the design of control programs

From an analysis of the distributions of measures of transmission rates among hosts, we identify an empirical relationship suggesting that, typically, 20% of the host population contributes at least 80% of the net transmission potential, as measured by the basic reproduction number, R(0). This is an example of a statistical pattern known as the 20/80 rule. The rule applies to a variety of disease systems, including vector-borne parasites and sexually transmitted pathogens. The rule implies that control programs targeted at the “core” 20% group are potentially highly effective and, conversely, that programs that fail to reach all of this group will be much less effective than expected in reducing levels of infection in the population as a whole