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The United Nations (UN) recently released population projections based on data until 2012 and a Bayesian probabilistic methodology. Analysis of these data reveals that, contrary to previous literature, the world population is unlikely to stop growing this century. There is an 80% probability that world population, now 7.2 billion people, will increase to between 9.6 billion and 12.3 billion in 2100. This uncertainty is much smaller than the range from the traditional UN high and low variants. Much of the increase is expected to happen in Africa, in part due to higher fertility rates and a recent slowdown in the pace of fertility decline. Also, the ratio of working-age people to older people is likely to decline substantially in all countries, even those that currently have young populations. [ABSTRACT FROM AUTHOR]

Background: Every 2 years, the Joint United Nations Programme on HIV/AIDS (UNAIDS) produces probabilistic estimates and projections of HIV prevalence rates for countries with generalised HIV/AIDS epidemics. To do this they use a simple epidemiological model and data from antenatal clinics and household surveys. The estimates are made using the Bayesian melding method, implemented by the incremental mixture importance sampling technique. This methodology is referred to as the 'estimation and projection package (EPP) model'. This has worked well for estimating and projecting prevalence in most countries. However, there has recently been an 'uptick' in prevalence in Uganda after a long sustained decline, which the EPP model does not predict., Methods: To address this problem, a modification of the EPP model, called the 'r stochastic model' is proposed, in which the infection rate is allowed to vary randomly in time and is applied to the entire non-infected population., Results: The resulting method yielded similar estimates of past prevalence to the EPP model for four countries and also similar median ('best') projections, but produced prediction intervals whose widths increased over time and that allowed for the possibility of an uptick after a decline. This seems more realistic given the recent Ugandan experience.