New HIV strains spread slowly
In most countries, the local HIV epidemic is still dominated by the strain of HIV which first entered that specific population. There are several different strains of HIV, organised into types, groups and sub-types; however the global mixing of these different strains has so far been slow. New research in PLOS Computational Biology explains that this slow spread is caused by the first comer advantages, making it very difficult for an invasive strain to enter that same population.
Outside Africa, the spread of different HIV types is characterised by confined local epidemics usually dominated by a single subtype. A hypothesis for this phenomena is that competition of viral strains at the epidemic level may involve an advantage on the part of the first strain to colonise a population. To test this hypothesis, researchers used a computer model to understand how different HIV strains spread within a population, and how they compete and interfere with each other. This showed that as soon as one strain of HIV has established a stable epidemic, it slows down the invasion of a secondary strain in the same population. This effect can be explained by the fact that individuals infected by the first strain of HIV, survive for a relatively long period of time and are resistant to infection from a secondary strain, and therefore the invader strain cannot be spread within the sexual network of the individual.
This research does not imply that the HIV strains currently dominating the epidemic are the most transmissible strains, rather, that they were lucky to have arrived first and therefore have the competitive advantage. The research does suggest that the interference mechanisms of competition between two different HIV strains slows down the adaptation of HIV at the population level. This can explain why there has been no new major HIV types or groups to emerge since the mid-20th century. The first comer advantages does not mean that there are no strains which are more transmissible than the current dominate strains. It also does not imply that these strains cannot be created by mutation and recombination. It is possible that a new strain will outgrow the current variants, although will happen very slowly.