”If someone is infected and takes PrEP (pre-exposure prophylaxis) this would be really bad,” he said, explaining how that person would promote the spread of resistant HIV strains.

An expert on resistance, Mellors said some drug resistance among people taking antiretroviral therapy was inevitable.

About 80% of people currently on treatment have suppressed their virus. Most (4/5) of the 20% who have not, have some resistant virus.

Mellors expressed concern at the “substantial overlap in the drugs being used for treatment and those being studied for prevention”.

He said in a worst-case scenario widespread resistance at a population level to these drugs could end up rendering them ineffective for both treatment and prevention.

But he added: “It is very rare to become infected with a virus we can’t treat today.”

Dr Susan Schader from McGill University said in reality resistance wasn’t an “all or nothing” problem.

Patients could increases doses, and could switch therapies if the higher levels proved toxic to them, she said.

But in South Africa and other African countries the latest, sophisticated drugs are relatively inaccessible because they cost far more than the first line treatments.

Dr Ume Abbas from the Cleveland Clinic Foundation said she had done a mathematical model on drug resistance if PrEP were to be rolled out at a population level.

“We found if an individual was already infected at baseline (in other words the person wanted PrEP not realizing it was too late since they had HIV) that would be a major factor in fuelling the spread of drug resistance,” she said.

Dr Regina Osih from Wits University listed the public health concerns if PrEP were to be rolled out as adherence, resistance, the capacity of the health care system, identifying the ideal target populations and ethical choices.

blogs.timeslive.co.za/hiv/2010/05/24/prep-needs-to-be-well-controlled-to-limit-hiv-drug-resistance/