A study that estimates the risk that someone living with HIV and taking
antiretroviral therapy could transmit the virus reports that, on the basis of the few transmissions from
heterosexual partners on treatment that have been reported, it is not possible to
dismiss the risk of infection as zero.
The analysis by French researchers in Clinical Infectious Diseases estimates
that the highest-likely risk of HIV being transmitted is between 8.7 and 13
transmissions per 100,000 sex acts; in other words, from one in about 11,500 to
one in about 7700 acts. However, the researchers stressed to aidsmap.com that this is the highest-likely risk: the actual risk may be lower than this and could indeed be zero.
This implies that the accumulated highest-likely risk of HIV transmission
would rise to 1% after between 195 and 389 occasions of sex: a couple who have vaginal sex around six times a month would take two and a half years to have sex 195 times, or five and a half years to have sex 389 times.
is the second recent study to find that the long-term risk from a partner on
antiretroviral therapy (ART), while very much lower than from a partner not on
treatment, may not be negligible in the long term.
The other study, by the Centers for Disease Control and Prevention (CDC) in the USA, used a mathematical model to calculate the one- and ten-year risks of HIV infection in heterosexual and gay
couples. It then added in the mitigating effects of ART, condom use, circumcision and pre-exposure prophylaxis (PrEP). It used estimates of the likelihood of transmission, and the efficacy of
the different prevention methods, from various studies.
French researchers tackled the question by searching out the few actual reported
cases of HIV transmission within a heterosexual couple where the partner living with HIV was on ART, and where the virus was unequivocally shown to have come
from them. They then calculated the highest-likely probability of transmission
from someone on ART based on these cases.
The researchers found six studies that were set up sufficiently well to document such cases. They identified four cases of viral transmission from a partner on ART during 2773 person-years in 1672 heterosexual, serodiscordant
couples. (An additional 182 transmissions occurred when people were not taking ART.) Four of the studies took place in Africa and one each in Spain and
Brazil. Between 70 and 100% of study participants had an undetectable viral load at various time points. At the start
of the studies, sexual frequency in participants varied from three to twelve times
a month; the American model assumed an unvarying frequency of six times a
In three of these transmissions, which were
proven to come from the HIV-positive partner by genetic analysis, that partner
had been taking ART for less than six months. In the fourth transmission, the
person had been taking ART for less than a year. As the Swiss statement
says that people who have had an undetectable viral load and no sexually
transmitted infections for more than six months may be regarded as
non-infectious, the researchers did two calculations for the likelihood of
transmission risk, based on whether the transmission in that study had taken
place less or more than six months after the start of therapy. This explains
the two figures cited for the highest-likely risk of transmission of 8.7 or 13 transmissions per 100,000.
The researchers’ calculation that the
chance of transmission from a partner on treatment in a heterosexual couple could rise up to 1% after 195 to 389 occasions of sex allows a comparison with
the American model. The CDC estimated the ten-year risk of HIV transmission from a
partner on ART to be 2%. According to the French researchers, the highest-likely risk after 720 sex acts
(equivalent to ten years in the US model) was either 1.85% or 3.7% (depending on the timing of that one transmssion). This is compatible with the American estimates, though the CDC study computes an average risk of transmission from rather conservative assumptions about the efficacy of different prevention methods, while the French study computes a range of risk, from zero (the lowest-likely risk) to the uppermost-likely risk quoted.
researchers argue that we may never be able to get a more precise answer for the
long-term risk of transmission than this. Because transmission from someone on
treatment is so rare, if the highest-likely ‘true’ risk is, say, one in
100,000, it would have taken the HPTN 052 study, which provided an answer of "at least 96%" for the reduction in infections conferred by ART, 27
years to establish such a fact.
French study tells us nothing about the risk of transmission within a gay couple. The American model suggests that the long-term risks could be
very much higher for gay men simply because the risk of transmission via anal
sex (where the HIV-negative partner is the receptive one) is 18 times higher than
in vaginal sex. But we do not know if a partner on ART is 18 times more likely to transmit HIV – because no completely undisputed and verified transmission from a
partner on ART in a gay couple has been documented.
is important to reiterate that the true likelihood of a person on fully suppressive ART transmitting
the virus may be much closer to zero than these two studies suggest. Nonetheless, the
ten-year risk may not be negligible, and research into even
more effective prevention methods is still needed.