It is nearly 30 years since the discovery of the Aids virus and a vaccine seems as far away as ever. In its absence, education and condoms were considered the only viable way of preventing the spread of HIV.
And yet, each year, some 2.7 million more people are infected with HIV. Far from being quelled, the Aids epidemic is growing, particularly in Southern Africa where a pernicious strain of the virus leads to "hypertransmitters".
Scientists are increasingly turning to antiretroviral drugs – the one great success story of Aids research – as a prophylactic against the spread of HIV. The Harvard study at Mochudi in Botswana tries to target HIV-carriers in the "acute" phase of the infection, when they have high levels of virus in their bloodstream.
Another study, involving the South African Centre for Epidemiological Modelling and Analysis, is proposing to give antiretroviral drugs to all HIV-positive men and women to prevent or lower the risk of viral transmission.
Researchers say anti-retrovirals decrease the concentration of the virus in the body by 10,000 times, which probably translates into a 25-fold reduction in infectiousness. Some scientists believe that if everyone with HIV in Africa took antiretrovirals, the epidemic could be gone within a generation, because each infected person would infect on average less than one other person, instead of the additional five or six infected at present.
The cost, estimated to be £2.5bn a year for South Africa alone, may appear high. But the cost of not doing it, and having to deal with an ever-growing number of Aids cases, could turn out to be a lot greater, financially and in human misery.