To the Editor: Law and Batey1 rely on a flawed study for their conclusion that needle/syringe programs (NSPs) have saved lives and money.

The study in question2 compared the incidence of HIV and hepatitis C virus (HCV) infections in cities round the world and concluded that cities with NSPs had achieved reductions in HIV incidence that were not seen in cities without NSPs. However, interestingly, no similar reduction in HCV incidence was reported.

Detailed reading of the study shows that a third of the cities (22/67) without NSPs were in Thailand — a country in which, unfortunately, there are many other reasons why HIV incidence is increasing rapidly. Given the large proportion of Thai cities included in the study, it is plausible that the rapid rise in HIV incidence in these cities biased the overall results of the study, leading to an erroneous conclusion that NSPs themselves were associated with a reduction in HIV incidence in cities worldwide.

Further reading of the study shows that HCV incidence was not measured in any studies in Thailand. So, the reported lack of effect of NSPs on HCV incidence depends on comparisons between cities with and without NSPs from other parts of the world, perhaps less affected by some of the problems in Thailand.

In conclusion, if Thai cities had been excluded from the study, it seems likely that no change in the incidence of either HCV or HIV might have been found in association with NSPs. The original study needs urgent re-analysis to see if this is in fact the case.

In reply: Copeman's criticism of the report Return on investment in needle and syringe programs in Australia1 is essentially that the comparison of the effectiveness of needle/syringe programs (NSPs) is confounded by other factors. This point, and its implications for the results, was extensively discussed in that report. Copeman suggests that the estimated reduction in HIV due to NSPs might largely be attributable to the inclusion of data from many cities in Thailand that do not have NSPs. This criticism is not supported by the data. A sensitivity analysis including only cities from developed countries was performed at the time of the report (see Methods, Section 3.1.2, page 131), but was not included among the report's results because of space constraints and because it didn't alter the main findings. The analysis of cities in developed countries showed an overall mean reduction in the annual rate of change in HIV seroprevalence of –30.0%, compared with –24.7% based on all cities, albeit with lower statistical significance (P = 0.105 v P = 0.057), reflecting the loss in power through exclusion of cities.

Copeman's assertion that the report indicated that NSPs had no effect on rates of hepatitis C virus (HCV) infection is incorrect. The report estimated that, following the introduction of NSPs, HCV prevalence among injecting drug users declined by 2% per annum, compared with no introduction of NSPs (P < 0.001).

The report is freely available on the Internet,1 and we encourage readers to look at it for themselves.