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