The evidence that needle and syringe programs (NSPs) are effective in preventing HIV and hepatitis C virus (HCV) is weaker than acknowledged in the current scientific literature, a new analysis of research suggests. The new review of English-language literature to March 2007 included for analysis three high-quality "core" reviews and two supplementary ones.

The metareview found sufficient evidence that NSPs reduce self-reported injecting risk behavior (IRB), and tentative evidence that alternative pharmacy NSPs have an additional impact on IRB. UK researchers found only tentative evidence that NSPs are effective in preventing HIV and insufficient support for their preventing HCV. However, of the five reviews, three did not examine HCV "in any depth," noted Norah Palmateer, of Health Protection Scotland, and colleagues.

"Insufficient or weak evidence of an effect is not evidence of no effect," said Palmateer. "It is more a reflection of the studies and evidence available."

Part of the problem is that the studies have been observational, rather than randomized controlled clinical trials, Palmateer said. Observational studies are open to "selection bias," she said. For example, if those at greatest risk for HIV tended to use an NSP, a study might find higher HIV prevalence associated with the program. In addition, the NSPs studied had strict limits on the number of syringes and needles they could give clients, she said.

Of 10 HIV-focused NSP studies reviewed in 2004 by the World Health Organization, five had positive findings. However, four of the five had design weaknesses that limit the conclusions that can be drawn. Two of the reviews analyzed by Palmateer's team included many of the same studies in the WHO report. One research group agreed with WHO, while the other more guardedly found "modest" evidence that NSP's prevent HIV transmission.

"The main public health implications of the findings are that a higher level of coverage of interventions, including [NSPs], is likely required to reduce blood-borne virus transmission," said Palmateer.

The full meta-analysis, "Evidence for the Effectiveness of Sterile Injecting Equipment Provision in Preventing Hepatitis C and Human Immunodeficiency Virus Transmission Among Injecting Drug Users: A Review of Reviews," was published in Addiction (2010;doi:10.1111/j.1360-0443.2009.02888.x).