A quarter of new HIV cases in England could be prevented by giving high-risk men prophylactic treatment, a study has shown.
The controversial programme could save the NHS millions as a result of fewer men getting infected and needing a lifelong supply of drugs to stop them succumbing to Aids, said researchers.
The study found the move was likely to become cost effective within four decades and over 80 years stood to make savings of up to £1 billion.
The findings are from a study funded by NHS England investigating the effectiveness of pre-exposure prophylaxis (PrEP) medication in 10,000 men.
All the participants were classified as men-who-have-sex-with-men (MSM) at high risk of HIV infection.
Each year in the UK, 3,000 more men from the MSM community are diagnosed as HIV positive, and their numbers have remained at a high level for several years.
Over a period of 80 years, an estimated one in four HIV infections - around 44,300 cases - would be prevented by prophylactic treatment introduced across England, the study found.
Dr Valentina Cambiano, from University College London, said: "There is no doubt about the effectiveness of PrEP. In addition to delivering a substantial health benefit, our work suggests that introduction of PrEP will ultimately lead to a saving in costs, as a result of decreased numbers of men in need of lifelong HIV treatment."
The PrEP treatment involved a pill containing a combination of two anti-HIV drugs, emtricitabine and tenofovir, taken daily or around the time of sexual activity.
Individuals recruited for the trial were from a very high risk group who accounted for just 5% of all men-who-have-sex-with-men.
NHS England initially refused to pay for PrEP, arguing that responsibility for HIV prevention lay with local authorities.
It agreed to fund the study at selected clinics after a High Court judge upheld a judicial review application challenging the legality of the original decision.
Writing in The Lancet Infectious Diseases journal, the authors estimate that between 8,400 and 12,200 British men aged 15 to 64 would have qualified for PrEP last year.
Although HIV incidence was predicted to reduce without the introduction of PrEP, the programme was likely to result in significant health and cost benefits, they said.
Co-author Dr Alison Rodger, also from University College London, said: "As anti-retroviral drug patents expire over the next few years, the emergence of generic drugs could result in large cost reductions for PrEP, and these reductions could help to limit the impact of PrEP on NHS budgets, making it cost-effective over a relatively short time."
Commenting in the journal, health economist Paul Revill from the University of York wrote: "These findings ... call for NHS policy makers to negotiate with manufacturers to get favourable deals on prices and to be far sighted: invest now and reap long-term gains.
"This approach is currently challenging when unmet needs across all areas of health care are so great and NHS resources are stretched so thinly. However, this study provides the definitive evidence to support such a decision.
"With a combination of frequent HIV testing, immediate treatment and PrEP availability, there is now the prospect of bending the curve of new HIV infections downwards in a way that did not seem feasible just a few years ago."
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