Since anti-retroviral therapy (ART) for
HIV can extend life by 50+ years, providing ART to all who need it worldwide
is an ethical imperative.  But is treatment also an effective prevention
strategy as well?  Can we "treat our way out of the epidemic"
as some at the highest levels of government and science argue? 
The efficiency of HIV transmission depends primarily on the concentration
of virus in an HIV+ person. Effective ART substantially reduces the
viral burden and thus would be expected to correspondingly reduce infectiousness.  

Observational studies in several places,
beginning with the famous Rakai study in Uganda, have shown that in
serodiscordant heterosexual couples the likelihood of the negative partner’s
becoming infected is very low when the positive person’s viral load
is very low.  However, preliminary conclusions from observational
studies do not always prove true in more rigorous study designs or in
real world experience. This underscores the importance of a large, multi-site
clinical trial (HPTN Protocol 052, Dr. Myron
Cohen, Protocol Chair) of
1750 heterosexual serodiscordant couples in six countries to see whether
or not early vs. later initiation of ART will reduce transmission of
HIV to the uninfected partners. Results of the study, conducted by the
NIAID HIV Prevention Trials Network, will likely not be available until
sometime after 2013.  

Is the treatment effect significant
at a population or public health level?

It would be wise to remain skeptical.
As many as 50% of new HIV infections come from individuals who are in
the acute or very early stage of HIV infection when they are likely
to be highly infectious and unlikely to know of their infection. 
Drug resistant strains of HIV emerge and may be transmitted.  Many
individuals have difficulty tolerating ART indefinitely and others have
problems in adherence for a variety of reasons.  

The strongest argument for skepticism
is the experience of much of the US and parts of Western Europe, where
treatment is relatively widely available and yet HIV incidence remains
stubbornly high and even rising.  At the very least, the relationship
of treatment to prevention is not simple or automatic, even under the
best current conditions. 

If this is the case in the wealthiest
countries, how much farther away then are we from being able to "treat
our way out of the epidemic" in developing nations, home to the majority
of people living with HIV and those at highest risk of contracting it? 
There were 3 new infections with HIV-1 for each person started on HAART
in 2006.  

Treatment is a basic human right. 
It may also contribute to lessening transmission and thus be an important
part of effective prevention strategies. Nonetheless, no magic bullet,
no universally effective prevention strategy exists, and none is likely
for the foreseeable future. Meanwhile, less than one in five persons
at substantial risk of infection with HIV has access to even basic prevention
services, let alone a broad evidenced-based package of prevention tools.   

We have work to do.