The results of this study appear in the November 2006 issue of Hepatology, the official journal of the American Association for the Study of Liver Diseases (AASLD). Published by John Wiley & Sons, Inc., Hepatology is available online via Wiley InterScience at http://www.interscience.wiley.com/journal/hepatology.
HCV, a major public health threat affecting over 170 million people worldwide, is primarily acquired through injection drug use (IDU). IDU accounts for over 75 percent of HCV cases and HCV is seen in up to 90 percent of IDUs, with most of these individuals going on to develop chronic infection. Recent advances in effectively treating HCV have led to a "cure," (meaning no virus is detectable) in many cases, however there is a concern that treatment is not as beneficial to IDUs because they are continually exposed to the virus.
Through a collaborative effort between Jason Grebely and Brian Conway of the University of British Columbia, Mark Tyndall of the BC Centre for Excellence in HIV/AIDS, the BC Centre for Disease Control, and Vancouver Coastal Health in Vancouver, a large community-based study was conducted comparing 926 individuals who tested negative for HCV during the recruitment period of the study between January 2003 and June 2004 with 506 individuals who had HCV, 152 of whom had spontaneous clearance of the virus. Clearance was considered to be the presence of HCV antibodies followed by at least one negative test for HCV. Using medical records, they then looked at the incidence of HCV infection between 1992 and 2005 in those who had antibodies but no detectable virus and those who tested negative in order to evaluate the effect of previous infection on reinfection rates. Although the two groups were similar in terms of the proportion of individuals engaging in illicit drug use, those previously infected were more likely to be engaged in frequent illicit drug use and IDU. Individuals who had tested positive for HCV, but subsequently tested negative were followed for an average of five years, compared to almost three years for those without previous HCV infection.
The results showed that those with previous HCV infection and viral clearance were four times less likely to develop infection again than those infected for the first time, despite the fact that they had higher rates of HIV coinfection, illicit drug use and injection drug use. In fact, 90 percent of those reinfected continued to engage in illicit drug use, including 50 percent who reported IDU.
"Our data lend support to the hypothesis that previous exposure to HCV may be protective, possibly on an immunologic basis, despite repeated exposure to HCV," the authors state. They propose two potential explanations for their results: those with HCV clearance are genetically predisposed to resist HCV infection and reinfection, or those previously exposed to HCV may be more experienced and have safer injection routines, which would have some protective value. The authors acknowledge some limitations in their study, such as the fact that HCV antibody tests have become more sensitive in recent years compared to the period from which the study looked at results and the fact that the study was retrospective, with testing being performed only by physician request, not systematically. However, they note that these limitations could easily be addressed in future prospective studies with systematic testing for HCV.
The authors point out that treatment for HCV infection is often withheld from IDUs because of the supposed high risk of reinfection. "However, our data suggest that spontaneous clearance may confer some protection against re-infection," they write. "If protection against HCV infection extends to those who have cleared their viremia following antiviral therapy, it could provide a stronger rationale for expanding treatment programs for IDUs, including those who continue to be at risk for HCV exposure." Although further research is required, the present study indicates that since IDUs play such an important role in HCV transmission, strategies that address this group could have a significant impact on the HCV epidemic.
David Greenberg | EurekAlert!
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