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For the past 20 years, evidence has been building up to show that the likelihood of passing on HIV is linked to the amount of the virus in the blood.

The landmark PARTNER 1 study (2014) looked at over 58,000 instances of sex without a condom, where one partner was HIV positive and one was HIV negative. There were zero cases of HIV transmission in couples where the HIV positive partner was on effective treatment (‘undetectable’).

The statistical certainty of the PARTNER 1 study was slightly lower for receptive anal sex with ejaculation than it was for vaginal sex though. The PARTNER 2 study (2018), which looked only at instances of condomless anal sex, showed zero transmissions for both same-sex male couples and heterosexual couples.

With the results of both those studies, scientists have concluded that the chance of any HIV-positive person with an undetectable viral load transmitting the virus to a sexual partner is scientifically equivalent to zero.

Combined studies PARTNER 1 and PARTNER 2, with 2017’s Opposites Attract study, together make up about 126,000 occasions of condomless sex between serodiscordant partners, with no transmissions.

This gives us the robust evidence to say, with confidence, that people on effective HIV treatment can’t pass on the virus.