Efforts to control blood-borne infections have dramatically reduced HIV incidence among injection drug users (IDUs), but declines in hepatitis C virus (HCV) infection have been less substantial, prompting health officials to call for better prevention and treatment strategies.

Incidence of HIV infection decreased from 5.5 cases per 100 person-years in a cohort of injection drug users recruited from 1988 to 1989 to zero cases per 100 person-years in a cohort recruited between 2005 and 2008.

In contrast, HCV infections declined from 22 cases per 100 person-years in the 1988 to 1989 cohort, to 17.2 per 100 person-years in a cohort recruited in 1994 and 1995. A slight increase to 17.9 cases was reported in another group recruited in 1998, before infections rates declined to 7.8 cases per 100 person-years from 2005 to 2008 (P=0.07).

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The disparity in HIV and HCV reduction rates may be due to the fact that HCV is 10-times more transmissible than HIV, according to study researcher Shruti H. Mehta, MD, of the Johns Hopkins Bloomberg School of Public Health, in Baltimore, and colleagues.

Because HCV can be acquired after just one instance of needle-sharing, hazard reduction interventions such as needle exchange and substance abuse treatment programs that have been effective at reducing HIV, may have less of an impact. CDC estimates indicate that approximately one-third of IUDs share needles.

“Efforts need to be intensified on both the prevention and treatment fronts to reduce the reservoir of HCV-infected IDUs,” the researchers wrote in the March 1 issue of the Journal of Infectious Diseases.

To get a better understanding of HCV incidence in IDUs, they analyzed data from four cohorts of IDUs during a 20-year period (1988-1989, n=2,946; 1994-1995, n=391; 1998, n=244; 2005-2008, n=875). 

The researchers found that compared with the 1988-1989 cohort, HCV incidence significantly declined among patients younger than 39 years in each subsequent cohort (adjusted prevalence ration=0.73; 95% CI: 0.65-0.81).

However, the same decreases were not observed in patients older than 39 years — among that age group HCV infection rates only declined in the most recent cohort (adjusted prevalence ratio=0.87; 95% CI: 0.77 to 0.99).

“For many persons, the interval between initiation and injection simply remains too brief for prevention strategies to be successful,” the researchers wrote.

They emphasized the importance of targeting prevention efforts towards very young IDUs and drug users who have not yet begun injecting. The other strategy for reducing the HCV reservoir is treatment, the efficacy of HCV treatment regimens is often limited among certain subpopulations including blacks with genotype-1 HCV and those with HIV coinfection, the researchers noted.

“With new, more efficacious therapeutics on the horizon, it is critical that strategies to improve uptake and completion of HCV infection treatment of IDUs be implemented,” they wrote.

Jason Grebely, PhD, and Gregory J. Dore, MBBS, PhD, of the University of New South Wales in Sydney, Australia, offered several suggestions for improving treatment and prevention.

“The development and implmentation of national harm-reduction strategies including broader coverage, enhanced early access and intesnifcation and combination of interventions are probably all needed,” they wrote.

Grebely and Dore also emphasized the importance of conducting randomized clinical trials to evaluate new HCV interventions to ensure that they are successful.