In the largest study
of its kind to date, a sub-study of the Partners in Prevention trial undertaken
by Baeten and colleagues,1 cervico-vaginal and seminal viral load were measured in 2521 HIV-positive
participants. They calculated
the impact of these viral loads on the risk of HIV transmission and compared
this to actual observed transmissions in the study.
from 1805 women, including 46 who apparently transmitted HIV to their partner,
and semen samples from 716 men, including 32 who apparently transmitted HIV to
their partner, were tested. Each tenfold increase in genital viral load was
associated with a 2.2-fold increase in the likelihood of transmission by women,
and a 1.8-fold increase in the likelihood of transmission by men.
After adjusting for
blood plasma viral load, genital viral load was found to be independently
associated with the risk of transmission. Irrespective of the influence of
viral load in the blood, there was still an additional 70% increased risk in
the chance of transmission per tenfold increase in genital viral load. This
suggests that measuring genital viral load would be a better guide to the
likelihood of transmission than viral load in blood.
However, the study
also found that genital viral load was more often undetectable than blood
plasma viral load, and that there were seven female-to-male and four
male-to-female HIV transmissions from people with undetectable genital viral
load but detectable HIV in their blood. Conversely, there were no transmissions
from participants with an undetectable viral load in blood and detectable
genital viral load.
These conflicting results are probably due to the time lag
between the time of infection and taking the viral loads in the genital fluids
of people who transmitted. It is likely that genital viral load varies more
quickly or unpredictably than blood plasma viral load, so that the association
between viral load and infectiousness did not show up in the study.
recent study suggests that women not
taking treatment who have subtype C virus have significantly higher viral
load in their cervico-vaginal fluid than women with subtype B. It also found
that women taking treatment
who have either subtype
B or C are more likely than men to have detectable virus in the genital tract
even when viral load is undetectable in the blood.2
C is found primarily in sub-Saharan Africa, India,
and in people who have moved from these regions. Subtype B is found primarily
in west and central Europe, the Americas, Australia, South America, and several
southeast Asian countries (Thailand, and Japan), northern Africa and the Middle
East, as well as in people who have moved from these regions.
study included 158 men and 170 women in seven different countries (Brazil, India,
Malawi, Peru, South Africa,
United States and Zimbabwe)
who started treatment with three different combinations of antiretroviral drugs,
and had viral load in blood plasma and genital fluids measured at baseline and
again after 48 and 96 weeks of treatment.
with subtype C infection had the highest genital tract viral load at baseline,
a median of approximately 125,000 copies/ml. This compared to a median baseline
genital tract viral load of 10,000 copies/ml for women with subtype B
infection; 10,000 copies/ml for men with HIV subtype C and 6000 copies/ml for
men with subtype B infection.
antiretroviral therapy was highly effective at suppressing viral load in blood
in women and men with both subtypes, there were significant gender differences
in genital tract viral load with a lower proportion of women (84% versus 97% of
men) sustaining an undetectable genital tract viral load by week 96. The
researchers believe that this might be explained by differences in genital
tract drug penetration associated with physiological differences between men