New research shows more promise for using AIDS treatment drugs as a prevention tool, to help keep uninfected people from catching HIV during sex with a partner who has the virus.
There were no infections among gay men who used a two-drug combo pill either daily or just before and after sex with someone with HIV, one study found.
In a second study, no uninfected men caught the virus if they had sex only with a partner whose HIV was well suppressed by medicines.
Both studies were discussed on Tuesday at the International AIDS conference in Amsterdam.
The United States' top AIDS scientist, Dr Anthony Fauci, called the results "very impressive" and "really striking."
About 36 million people worldwide have HIV and 1.8 million new infections occur each year, said Fauci.
"The only way you're going to end the epidemic is by preventing additional cases of transmission," he said. The treatment drugs are "tools that, if widely implemented, theoretically could end the epidemic."
Expanding access to them is smart policy, Fauci added.
"You get a twofer: You save the life of the person who's infected ... and you're making it virtually impossible for that person to transmit that infection to their sexual partner."
Until there's a vaccine, condoms are the best way to prevent HIV infection, but not everyone uses them, so other options are needed.
A two-drug combo used to treat people with HIV, sold as Truvada by Gilead Science and in generic form in some countries, has been shown to help prevent infection when one partner has the virus and one does not.
A new study was designed as a real-world test in about 1600 gay men in the Paris region who were at high risk of getting HIV because of many sex partners, reluctance to use condoms or other reasons.
They were offered the preventive pills either for daily use, as is recommended in the US, or "on demand" - before and after unprotected sex. A little more than half chose on demand, and have been tested every three months to see if they had caught HIV.
"Since we started a year ago, we have not seen a single infection," said study leader, Dr Jean-Michel Molina of Saint Louis Hospital in Paris. "On-demand seems to be at least as effective as daily when it's used in real life."
The second study tested a different approach - keeping an infected partner's virus severely suppressed with HIV medicines, which is known to greatly reduce the risk of spreading it.
Dr Alison Rodger of University College London led a study of 779 gay male couples in 14 European countries where one partner was uninfected and the other was taking drugs to suppress HIV. They were tested every six to 12 months to see if the infected partner still had the virus under control, and whether the other partner had caught it.
After a median of 18 months, none of the infected men spread HIV to their partner, despite about 75,000 sex acts without condoms.
