The real challenge which the US HIV/AIDS epidemic poses for the US public health services is not simply to achieve higher levels of diagnosis – but, far more than that, to improve linkage to, and retention in, care.  This claim is hardly controversial. But it is thrown into stark relief in a recent study by Skarbinski & Mermin, which estimates the number of HIV transmissions attributable to non-retention in care for 2009.

The authors employ the notion of a five-phase “care continuum”.  Using population data from the National HIV Surveillance System and medical data from the National HIV Behavioral Surveillance System and the Medical Monitoring Project, they estimate the number of HIV transmissions occurring at each phase.  The phases in the continuum are: (1) infected, but undiagnosed; (2) diagnosed, but not retained in care (attending at least one visit to a medical care provider Jan. – April 2009); (3) diagnosed, retained in care, but not given ART; (4) diagnosed, retained in care, prescribed ART, but not virally suppressed; (5) virally suppressed.

The reduction in attributable transmissions achieved for those diagnosed but not retained in care (phase 2), as compared with those who remain undiagnosed (phase 1), is 19%.  (It is probably due to a decrease in HIV-discordant unprotected sex).  But the reduction achieved for those who achieve viral suppression (phase 5), as compared with those who remain undiagnosed, is 94%.  In estimating the epidemiological impact of these reductions, we need to factor in the percentage of the infected population at each phase.  The large proportion (45.2%) of the HIV infected who are diagnosed but not retained (phase 2) explains the very high proportion of total transmissions (61.3%) attributable to this phase.  By comparison, only 30.2% are attributable to the undiagnosed (phase 1), and 2.5% to the virally suppressed (phase 5).  The low epidemiological impact of those at phases 3 and 4 is due to the relatively low proportion of those infected who remain in these phases.

The message, then, is that achieving greater success in retaining the HIV diagnosed in care may prove the key to combating the epidemic at population level.  Of course, diagnosis remains the indispensable first step.  But the potential gains of diagnosis will be only very partially experienced, so long as such a large proportion of those diagnosed are not retained in care.  Of course, improving retention in care may constitute a somewhat different – and perhaps more difficult – challenge for the US health services from diagnosis.  The specific problems of the US health system in this regard are discussed by Sherer (STI), and the characteristics of individuals “lost to follow up” by Haddow & Mercey (STI) and Lee & Gazzard (STI).  Local attempts to address these problems through a more “wrap-around” approach to health care in the US are described in my blog Bocour & Less (STI/blog) (see Bocour &  Less).  There has also been interest in the computer assisted self-interviewing in order to engage those lost to care (Dombrowski & Golden (STI)).