The paper describes some preliminary attempts to formulate simple mathematical models of the transmission dynamics of HIV infection in homosexual communities. In conjunction with a survey of the available epidemiological data on HIV infection and the incidence of AIDS, the models are used to assess how various processes influence the course of the initial epidemic following the introduction of the virus. Models of the early stages of viral spread provide crude methods for estimating the basic reproductive rate of the virus, given a knowledge of the incubation period of the disease (AIDS) and the initial doubling time of the epidemic. More complex models are formulated to assess the influence of variation in the incubation period and heterogeneity in sexual activity. The latter factor is shown to have a major effect on the predicted pattern of the epidemic; high levels of heterogeneity decrease its magnitude. Areas of biological uncertainty, future research needs, and public health implications are discussed.

The relation between the age at infection with hepatitis B virus (HBV) and the development of the carrier state is examined by using data from a number of published and unpublished surveys. A remarkably consistent relation was found. Infants infected perinatally (within the first 6 months of life) were found to have a high probability of becoming carriers (0.885; 95% C.L. 0.84-0.93). Over the infant and early childhood age classes there was found to be a sharp decrease in the proportion of infections which lead to the carrier state. By adulthood (over 15 years) the probability of developing the carrier status was found to be about 0.1. A model was fitted to the data by using maximum likelihood, which provides a good empirical description of the observed data and can be used to predict the expected probability of developing the carrier state given the age at infection. It is postulated that, as a result of this rapid decline in the probability of becoming a carrier during early childhood, a mass childhood immunization campaign, which will tend to postpone the average age at infection in the unvaccinated community, will have a disproportionately large impact on the rate of generation of new carriers.

Objective To evaluate the evidence for the effectiveness of isolation measures in reducing the incidence of methicillin resistant Staphylococcus aureus (MRSA) colonisation and infection in hospital inpatients. Design Systematic review of published articles. Data sources Medline, Embase, CINAHL, Cochrane Library, System for Information on Grey Literature in Europe (SIGLE), and citation lists (1966-2000). Review methods Articles reporting MRSA related outcomes and describing an isolation policy were selected. No quality restrictions were imposed on studies using isolation wards or nurse cohorting. Other studies were included if they were prospective or employed planned comparisons of retrospective data. Results 46 studies were accepted; 18 used isolation wards, nine used nurse cohorting, and 19 used other isolation policies. Most were interrupted time series, with few planned formal prospective studies. All but one reported multiple interventions. Consideration of potential confounders, measures to prevent bias, and appropriate statistical analysis were mostly lacking. No conclusions could be drawn in a third of studies. Most others provided evidence consistent with a reduction of MRSA acquisition. Six long interrupted time series provided the strongest evidence. Four of these provided evidence that intensive control measures including patient isolation were effective in controlling MRSA. In two others, isolation wards failed to prevent endemic MRSA. Conclusion Major methodological weaknesses and inadequate reporting in published research mean that many plausible alternative explanations for reductions in MRSA acquisition associated with interventions cannot be excluded. No well designed studies exist that allow the role of isolation measures alone to be assessed. None the less, there is evidence that concerted efforts that include isolation can reduce MRSA even in endemic settings. Current isolation measures recommended in national guidelines should continue to be applied until further research establishes otherwise.

Methicillin-resistantStaphylococcus aureus(MRSA) represents a serious threat to the health of hospitalized patients. Attempts to reduce the spread of MRSA have largely depended on hospital hygiene and patient isolation. These measures have met with mixed success: although some countries have almost eliminated MRSA or remained largely free of the organism, others have seen substantial increases despite rigorous control policies. We use a mathematical model to show how these increases can be explained by considering both hospital and community reservoirs of MRSA colonization. We show how the timing of the intervention, the level of resource provision, and chance combine to determine whether control measures succeed or fail. We find that even control measures able to repeatedly prevent sustained outbreaks in the short-term can result in long-term control failure resulting from gradual increases in the community reservoir. If resources do not scale with MRSA prevalence, isolation policies can fail “catastrophically.”