Anti-retroviral therapy for HIV patients seems to have had a secondary benefit in addition to slowing the progression of symptoms: it has cut the number of HIV diagnoses, a Canadian study has found.

The findings come as the18th annual AIDS conference begins Sunday in Vienna and runs until July 23.

Highly active anti-retroviral therapy (HAART) — essentially a cocktail of drugs used to prevent the spread of HIV in the body — halved the number of HIV diagnoses between 1996 and 2009, researchers at the British Columbia Centre for Excellence in HIV/AIDS have found.

For every 100 HIV patients in B.C. who received the therapy, new diagnoses of the disease fell by three per cent.

Individuals receiving HAART were tracked — their numbers growing to 5,413 in 2009 from 837 in 1996. During this time, new HIV diagnoses dropped to 338 in 2009 from 702 in 1996.

The authors believe the decrease in cases cannot be explained by fewer HIV patients having risky sexual encounters. During the study period, rates of sexually transmitted diseases actually rose.

Instead, they believe that because HAART reduces a HIV patient's viral load, meaning it becomes almost undetectable, the presence of the virus is also much reduced in semen and vaginal secretions. Thus, the lowered viral load leads to fewer transmissions and fewer new HIV cases.

The researchers also witnessed a decrease in cases among intravenous drug users, also potentially indicating that when a person's viral load is low, the spread of the disease via blood is also lowered.

"Our results show a strong and significant association between increased HAART coverage, reduced community viral load and decreased number of new HIV diagnoses per year in the population of a Canadian province," the researchers write.

"Thus, taken together, the available evidence strongly suggests that community viral load is a key driving force of new HIV diagnoses and can be successfully modulated through effective expansion of HAART coverage within medical guidelines."

The study was published Sunday in the Lancet.