The PARTNER study, which two
years ago created headlines by establishing that the chance of an HIV-positive
person with an undetectable viral load transmitting their virus was very low
and quite possibly zero, released new data at the 21st International AIDS Conference (AIDS 2016) that further refined
In 2014 the study found no HIV transmissions between 767 serodiscordant
couples, who between them had condomless sex an estimated 44,400 times, where
the HIV-positive partner had a viral load under 200 copies/ml.
The latest estimate includes 888 couples, 38% of them gay male
couples, who were followed for 1238 couple-years altogether (1.6 years per couple
on average), which encompassed an estimated 58,213 sex acts – 31% more than in
The study involves 75 clinics in 14 European countries. Blood
tests and behavioural data are collected every 4 to 6 months. In order to
be eligible for the study during each 4 to 6 month period, the couple has to
report condomless sex with each other, without using post-exposure prophylaxis (PEP) or pre-exposure prophylaxis (PrEP), at least once
during this time, and the HIV-positive partner has to maintain a viral load
below 200 copies/ml. Fifty-five (6.2%) of the 888 HIV-positive partners
reported a detectable viral load at some point in the study.
load tended to underestimate viral suppression: 6% of gay men, 13% of heterosexual
men and 16% of women thought they had a detectable viral load or did not know
it when they were, in fact, undetectable.
The lack of transmissions obviously means that the
lowest-likely estimate for the chance of an undetectable HIV-positive partner
transmitting HIV is zero. But the longer time span and larger number of sex
acts included means that the highest-likely estimate of transmission falls,
simply because the precision of the estimate rises – the picture portrayed by
the data gets less fuzzy, so to speak.
Thus in 2014 the highest-likely estimate for the chance of between-couple
transmission via any sex was 0.5%; the new data narrows this ‘upper confidence interval
bound’ to 0.3%. But it is important to keep in mind that, as presenter Alison
Rodger said, zero transmissions means that “the estimated rate of
transmission is zero”.
The highest-likely chance of transmission via anal sex similarly
narrows down from 1.0% to 0.71% a year in everyone and 0.89% in gay men (incidentally,
20% of the 522 couple-hours of anal sex in the study were contributed by
heterosexuals). And it does not mean that transmission via anal sex is riskier
in gay men than heterosexuals: it is simply that as the number of couple-years
providing the data gets smaller, the statistical picture gets fuzzier.
Similarly, the upper limit for the estimate for transmission
via anal sex where the negative partner was the man in heterosexual couples and
the insertive partner in gay couples, was 0.88% in all couples and 1% in gay
Finally, the upper limit for the estimate for transmission
during the sex act that, if the HIV-positive partner did have a detectable viral load, would be the riskiest – namely,
anal sex where the receptive partner is the HIV-negative one and there is ejaculation
– is 2.23% in all couples and 2.7% in gay men, down from 4% in 2014. But this
is because there were only 166 instances of this in all couples during the
study and 137 in gay men. It does not mean that it is riskier if the
HIV-positive partner is virally suppressed. Again, it is most likely that if
they are, the chances of transmission even in receptive anal sex with
ejaculation is zero. It is just that, with fewer data points, it is a less assured zero.
There were eleven new HIV infections in the HIV-negative
partners during the study, ten in gay men and one in heterosexuals. But genetic
sequencing showed that in every case, the virus acquired by the HIV-negative partner was quite different from their partner’s virus – in two cases, it was a
completely different HIV subtype. Eight out of the eleven people who were newly
infected reported that they had recently had condomless sex with someone
outside the main relationship.
The higher number of infections in gay men is no surprise;
33% of them reported condomless sex with people outside their relationship versus
4% of heterosexuals, and 17.5% of gay men were diagnosed with an acute sexually transmitted infection in
the last reporting period compared with 6% of heterosexuals.
“On the basis of this data,” presenter Alison Rodger of London's Royal Free Hospital commented, “We
can fairly safely say that the chance of transmission from a virally-suppressed
HIV-positive person during heterosexual sex is negligible."
“However,” she added, “We need to collect more data on gay
men before saying this with the same degree of certainly”.
PARTNER will continue till 2017 and its final data will be
presented in 2018; unless an unexpected transmission happens, this should
further reduce our uncertainty about the true chances of transmission by
someone with an undetectable viral load.