PARTNER 1 was conducted between September 2010 and May 2014, and PARTNER 2 from May 2014 to April 2018. There were 888 couples in PARTNER 1, 337 of them (38%) gay couples. In PARTNER 2, another 635 gay couples were recruited, making a total of 972 gay couples and 516 heterosexual ones in the whole study.
Not all those couples provided what’s called “couple-years of follow-up” (CYFUs) throughout their time in the study. Couples only contributed to the data if they had had condomless sex since the last data were collected, if the HIV-positive partner had maintained a viral load under 200 copies/ml throughout, and if the HIV-negative partner had not used pre-exposure prophylaxis (PrEP) or post-exposure prophylaxis (PEP).
This meant that 783 male/male couples ended up providing 1596 couple-years of data, with an average eligible time within the study of 1.6 years. Couples reported on average 43 acts of condomless sex a year.
At baseline, the average age of the HIV-negative partners was 38 and of the HIV-positive ones, 40. They reported having had condomless sex with each other for an average of a year before joining the study. The HIV-positive partners had been on antiretroviral therapy for an average of four years. Ten per cent of the negative partners and 14% of the positive ones were diagnosed with an STI during the study.
The 1596 couple years, containing an estimated total of 76,991 condomless sex acts, produced no transmission between partners. There were 15 new infections – but three-quarters of them reported recent condomless sex with a different partner, and genotyping of the HIV transmitted showed that not one infection came from the regular partner; six had a completely different subtype of HIV. Altogether, 285 of the HIV-negative men (37%) reported condomless sex with other men.
These new data allowed the researchers to estimate much narrower confidence intervals. The upper bound of the 95% confidence interval for all condomless sex in the whole of PARTNER is now 0.23%. And the upper bound for receptive anal sex with ejaculation is 0.57% – fairly close to the upper bound for all sex in PARTNER 1.
What this means is: if you ran the PARTNER study 20 times, there is a more-than-even chance that you’d miss a real transmission (or see a false one) in 435 couples followed for one year – or one transmission in one couple followed for 435 years.
And if you repeated the study only using the 19,836 estimated occasions of the HIV-negative partner being receptive, and the HIV-positive partner ejaculating, you’d miss a real transmission (or see a false one) once in 175 couples in a year – or once in 175 years in one couple.
These, to repeat, are very unlikely to reflect the true likelihood of transmission – they are statements about how likely it is that the zero transmissions observed might not reflect reality. It remains the case that the most likely probability, by far, that an HIV-positive person with a viral load under 200 copies/ml can infect their partner is zero.
The researchers were also able for the first time to compute the upper limit of the 95% confidence interval for the risk of transmission where one partner had an STI. This is higher, at 2.74%, or a 5% chance of seeing (or missing) one transmission per couple per 35 years – but again, the most likely risk was zero, and the only reason this zero is ‘fuzzier’ is because there were only an estimated 6301 occasions of condomless sex (of any sort) where one partner had an STI.
PARTNER is not the only study about viral load and infectiousness. Last year the Opposites Attract study also found no transmissions in nearly 17,000 acts of condomless anal sex between serodiscordant gay male partners, meaning that no transmission has been seen in about 126,000 occasions of sex, if you combine this study with PARTNER 1 and 2.
What has changed is that we can now state that U=U with at least as much confidence for gay men as we already could for heterosexuals or, as the researchers say, “PARTNER2 provides a similar level of confidence for gay men as for heterosexual couples in PARTNER 1.”
“We looked so hard for transmissions,” Alison Rodger told aidsmap.com. “And we didn’t find any.”
The last line in the last slide she presented today was “Undetectable = Untransmittable.”