By Deborah Jones
Despite evidence that powerful anti-HIV drugs can help protect uninfected partners, the therapy is far from becoming routinely prescribed, scientists say.
Pre-exposure prophylaxis or PrEP means a healthy person who engages in sex with an HIV-positive partner takes anti-retroviral drugs daily as a preventive measure.
Experts say the regime is effective and carries minimal side effects.
"Pre-exposure prophylaxis is a game changer", co-chair Chris Beyrer told an International AIDS Society science conference being staged in Vancouver on Tuesday (local time).
"The data is overwhelming... it works when it's taken."
At the conference, scientists released results on PrEP in several small studies of people at high risk of contracting HIV, mostly due to sexual behaviour, in Brazil, the United States and Botswana.
The research showed that when PrEP is used correctly, it reduces HIV infection and is well-tolerated.
But several scientists said use of PrEP is far from becoming standard policy, and so remains an individual decision for people considered at high risk.
The therapy is also relatively new. The US Food and Drug Administration in 2012 approved use of the drug tenofovir for adults at high-risk, in combination with safe sex practices.
A San Francisco study of 557 men having sex with men, and transgender people, showed most followed the drug regime, while the two participants who did contract HIV had very low levels of PrEP drugs in their system, reported Albert Liu of the San Francisco Department of Public Health.
"Our results support strongly the scale-up of PrEP," he told reporters.
"The drugs appear to be well tolerated."
A Brazilian study of 509 men from 2014 to 2015 showed PrEP was well-received by recipients, "though more education is needed," said Beatriz Grinsztejn, Instituto Pesq Clinica Evandro Chagas.
She said the results, which are yet to be finalised for publication, will help Brazil decide whether to adopt PrEP as a standard public health measure.
Adverse reactions in one study included just three reports of nausea, a headache, and weight loss, among 200 young men studied in 12 American cities, said Sybil Hosek of Stroger Hospital, Cook County, Illinois.
Hosek noted the men most likely to adhere to the treatment regime were those with the most risky lifestyles.
A study in Botswana of 229 heterosexual people in high-risk lifestyles "observed no HIV transmission in the study," said Faith Henderson of the US Centers for Disease Control and Prevention.
About 87 percent of the women on Botswana adhered to the drug regime, she said, noting that is a high rate compared to other measures aimed at reducing HIV risk for women.
"Women and girls constitute half of all people with HIV worldwide, and PrEP is much needed," Henderson told reporters.
However, cost is an issue, and the pills can be too expensive for some. Another key barrier is making sure people take their pills daily.