The findings, published in the latest issue of the Proceedings of the National Academy of Sciences, also suggest that antiviral therapy has not had a significant impact on the growth of the epidemic and that changes in sexual behaviour have been more effective in slowing the spread of the disease.

The collaborative study led by University College London (UCL) scientists found that HIV-1 subtype B spread through the UK via at least six large transmission chains of men having sex with men, suggesting separate introductions of subtype B strains into the UK in the early-to-mid 1980s. After an initial period of exponential growth in infection rates, the spread generally slowed in the early 1990s, more likely from changes in sexual behaviour than from reduced infectiousness resulting from antiretroviral therapy.

The study by UCL, the Health Protection Agency and the University of Oxford 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.

During the exponential growth phase, the transmission chains had an average growth rate of a doubling of the number of people infected each year, similar to that estimated for the US subtype B epidemic during the 1980s. The average number of infections across each chain was 445, approximately 2.5% of the infected population at the time. This trend is remarkably similar to the values for the US epidemic, where the number of transmitted infections and prevalence in 1995 reached 5000 and 200,000 infections respectively.

The most recent transmission chain identified by the study shows a faster doubling time in 2003 than the other five. Current surveillance data shows a recent increase in infections amongst homosexual men in the UK, which may partly have come through this chain.

Dr Deenan Pillay of UCL ™s Centre for Virology says: Our study suggests that the HIV-1 subtype B epidemic currently circulating the UK is made up of at least six established chains of transmission, introduced in the early and mid 1980s. This goes against the prevailing belief that one initial entry of HIV-1 was responsible for the spread of the epidemic.

Since 1990 there have been important changes in Britain ™s social attitudes and awareness of HIV-1 and AIDS. Despite a very recent increase in high-risk behaviour among men having sex with men, a significant increase in condom use has been reported since 1990, which could explain the equilibrium reached for the number of infections.

Antiretroviral therapy may also have impacted on transmission rates, but our evidence does not demonstrate this. You would expect growth rates to decrease in the late rather than early 1990s around the time that potent therapy became widely used if this was the case. Instead, we see little correlation between widespread availability of treatment and reduction of transmission. This is highly pertinent to the recent increase again in new HIV-1 diagnoses within the UK.

Our study also contradicts assumptions that the HIV-1 epidemic is composed of smaller, independent epidemics defined by risk group, where we have found evidence for at least six larger sub-epidemics, which HIV monitoring, prevention and treatment programmes may want to take into account when developing new initiatives.

More than 57,700 people in Britain have been infected with human immunodeficiency virus type 1 or 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 clades (subtypes) of virus within the country remains subtype B, which is mainly transmitted through sex between men. Very little is known about how subtype B successfully invaded the British population, and more importantly, how the virus has subsequently spread and evolved.

However, given that the first UK cases of AIDS reported in 1982 were probably infected within a window of 10 years prior to that time, the currently circulating strains may not represent the first HIV-1 lineages within the UK. If earlier strains existed they may have been unsuccessful in sustaining transmission to the present, although the absence of older strains could also reflect a sampling bias in this study.

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