New findings about the spread of HIV in the UK

Contrary to the prevailing belief that the HIV epidemic in the UK can be traced back to one source, a new study by Oxford and London scientists suggests that HIV spread via at least six independent virus introductions and subsequent transmission chains.

The findings also show that growth rates in HIV infection decreased substantially in the early 1990s. The timing in this slow-down of HIV transmission suggests that it is more likely that the spread was controlled by changes in sexual behaviour (such as increased condom use) rather than antiviral therapy, which was not widely introduced until the late 1990s.

The study, by University College London (UCL), the Health Protection Agency and the University of Oxford, found that HIV-1 subtype B spread through the UK via at least six large transmission chains of men having sex with men. It is published in the latest issue of the Proceedings of the National Academy of Sciences.

Dr Oliver Pybus in Oxford University’s Zoology Department, one of the authors on the paper, said: ‘This study shows that the structure of the HIV-1 epidemic in the UK is more complex than previously thought. There appear to be six established chains of transmission, each introduced in the early and mid 1980s, which have persisted in the UK to the present day. This goes against the prevailing belief that one initial entry of HIV-1 was responsible for the spread of the epidemic.’

The study statistically analysed the epidemic history of the HIV-1 subtype B strain from sampled gene sequence data. Molecular data on HIV-1 has become increasingly available since the introduction of routine HIV-1 gene sequencing for drug resistance. Scientists used this data to follow the changing number of infected individuals through time and estimate the demographic parameters shaping the epidemic. Dr Pybus said: ‘There is a genetic structure to the epidemic that hasn’t been looked at before – by studying genetic data we can see which particular strains are circulating, rather than just looking at how many people are infected.’

More than 57,700 people in the UK have been infected with HIV-1 since the first identification of AIDS in 1982. Despite a recent increase in heterosexually acquired infections within the UK, predominantly originating in sub-Saharan Africa, one of the most prevalent subtypes of virus within the country remains subtype B, which is mainly transmitted through sex between men.

Dr Pybus said: ‘A paper recently published in the British Medical Journal on HIV infections acquired through heterosexual intercourse in the UK, taken together with our paper, which is mainly looking at homosexual transmission, gives a good overall picture of HIV in the UK. The BMJ paper finds that 90 per cent of heterosexuals infected with HIV acquired the virus outside the UK, suggesting that onward heterosexual transmission in the UK is being controlled reasonably well. In contrast, our research shows that over the last 15 years at least six chains of onward transmission among men having sex with men have become established. So a substantial proportion of infections in men having sex with men are acquired in the UK, rather than abroad. However, a significant increase in condom use has been reported since 1990 and this could explain the decease in rates of transmission observed in the UK.’

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