The first 2 years of a study looking into transmission rates of HIV between sero-discordant partners ( where one partner is negative and one is positive) , and where the HIV positive partner is on treatment and undetectable , has concluded that none of these patients transmitted HIV to their partners.
The study is ongoing and will not be completed till 2017, but researchers said that the risk of transmission during anal sex, for this group, is at most around 4%. The study looked at participants where one partner has HIV but is undetectable (viral load under 200 copies/ml), both in gay and straight couples. The included possible exposure through either vaginal or anal sex.

The researchers have concluded that the risk of getting HIV from an undetectable partner is virtually zero. During the two years of the study so far, none of the HIV negative partners became infected with HIV by their partners . The results were the same, even when taking into account the high levels of co-infection with other STI’s observed in the gay participants. This builds further on the Swiss Statement released in 2008, which declared that people who have HIV but are undetectable, did not transmit HIV, although this study did not look at the issue of co existing STI’s

I have to stress that this is an ongoing study that will not be finalised till 2017 and the best way to prevent catching HIV, is by having protected sex.