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2005-03-15-Multiple toeholds for HIV in UK
The HIV epidemic in the UK cannot be traced back to just one source, research suggests.
Instead, work led by University College London suggests the virus gained a toe hold on at least six independent occasions.
The findings also suggest changes in sexual behaviour, rather than advances in drug therapy, have had most impact in slowing the spread of disease.
Details are published in Proceedings of the National Academy of Sciences.
The researchers examined genetic data on a widely found form of the virus called HIV-1 subtype B.
They found it spread through the UK via at least six large transmission chains of men having sex with men.
This, they said, suggests subtype B strains were introduced separately 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.
The researchers believe this was more likely to be due to changes in sexual behaviour than from reduced infectiousness resulting from antiretroviral therapy.
In the early stages, the number of people infected in each transmission chain a year doubled on average - similar to pattern thought to have occurred in the US during the 1980s.
Current infection rates seem to be highest in the most recent infection chain identified by the study - suggesting spread of the virus is being fuelled by people only recently infected themselves.
Researcher Dr Deenan Pillay said: "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."
Dr Pillay said that since 1990 condom use in the gay community had significantly increased. This might explain why infection rates in the transmission chains had slowed.
"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."
Michael Carter, of the HIV information service Aidsmap, said: "It seems pretty good sense to me that HIV was introduced into the UK by more than one individual in the 1980s.
"Behavioural change did make a huge impact to the instance of HIV and other sexually transmitted infections amongst gay men.
"Indeed, it shows how a community can stay sexually active and still reduce the risk of HIV, contrary to the arguments put forward by the US abstinence lobby."
Mr Carter highlighted the finding that the spread of HIV was probably fuelled by people who had only recently been infected. He said many probably did not know they were carrying the virus.
"The study underlines the importance of people who think that they may have been at risk of HIV of testing and obtaining treatment."
Ford Hickson, an expert in HIV transmission from Sigma Research, took issue with the idea that HIV in the UK originated from so few sources.
He said: "Health Protection Agency surveillance tells us that last year about 17% of gay men diagnosed with HIV in the UK had migrated to the UK with HIV (rather than having caught it here), and this proportion is always likely to have been substantial, so the idea that even six men are 'responsible' for introducing HIV to the UK is vaugely ridiculous."
He also stressed that both anti-retroviral drugs, and behavioural change were important factors in the effort to curb spread of HIV.
More than 57,700 people in Britain have been diagnosed with HIV-1 since the first identification of Aids in 1982.