The study appeared in the July 27 issue of AIDS.

"Our review of the literature should blunt the claims of opponents of syringe exchange, but I'm not optimistic that it will," said lead author David R. Gibson, associate professor of infectious diseases at UC Davis and a senior scientist at UC San Francisco's Center for AIDS Prevention Studies. "Opponents of syringe-exchange programs can be quite data-resistant."

UC Davis researchers scoured the medical literature from 1989 to 1999 for studies examining the impact of exchange programs on HIV risk. The search turned up 42 published studies, most of them conducted in the United States, Canada, the United Kingdom and the Netherlands. Twenty-eight of the studies concluded that syringe-exchanges reduce HIV risk among injection drug users.

Of the remaining 14 studies, two found that the programs increase HIV risk, and 12 concluded the programs either have no effect or a mixture of both positive and negative effects.

Gibson and his co-authors found that the 14 negative and equivocal studies all looked at syringe-exchange programs in settings -- primarily in Canada, the United Kingdom and the Netherlands -- where injection drug users can legally purchase low-cost syringes at pharmacies. In contrast, only five of the 28 positive studies were conducted in such communities.

"If you exclude the studies that took place in communities where clean syringes are also available from pharmacies, 28 out of the 29 studies remaining show that syringe exchange is protective against either HIV risk behavior and/or HIV seroconversion," Gibson said.

Syringe-exchange programs in communities where clean needles can be obtained from other sources can be expected to appear less effective than programs that constitute a community's only source of clean syringes, Gibson argues.

Researchers call this effect "dilution bias." A program to provide free fluoride supplements to children who already drink fluoridated water, for example, might have no impact on tooth decay rates. But the same program, implemented in a population with no alternate source of fluoride, would decrease those rates.

Besides dilution bias, another factor complicates the evaluation of syringe-exchange programs in areas where clean needles are available from other sources. Researchers have found that in areas where drug users have a choice, the higher-risk drug users tend to gravitate to the syringe-exchange programs. This, too, can make syringe-exchange programs appear less effective.

Another possible explanation for the negative and equivocal study findings is that syringe-exchange programs, while helpful, may not be sufficient to prevent spread of HIV among injection drug users in all communities.

Among the 28 positive studies, beneficial effects were often substantial. Studies of syringe exchanges in San Francisco; Portland, Ore.; Tacoma, Wash.; and Baltimore all concluded that the programs decreased needle sharing among injection drug users ranging from 16 percent to 72 percent.

In a different type of study, researchers compared overall HIV seroconversion rates among injection drug users in cities with and without syringe exchanges. Seroconversion rates decreased 5.8 percent a year in the cities with the programs, but increased by 5.9 percent a year in the cities without them.

Perhaps the most direct evidence supporting needle exchange comes from studies in the early 1990's of an exchange program in New Haven, Conn. When injection drug users exchanged used syringes for new ones in that program, researchers tested the returned syringes for HIV. They found that as the volume of syringes exchanged grew, the time it took for the syringes to return to the exchange fell substantially -- meaning used, potentially infectious syringes spent less time in the community.

As a result, in the first three months of the program's operation, the percentage of HIV-infected syringes dropped by about a third, from 67 percent to 44 percent.

Injection drug use now accounts for nearly one-third of new AIDS cases in this country. When drug users' sexual partners are included, injection drug use accounts for up to three-quarters of new HIV infections. The infection spreads via shared use of injection equipment and other drug paraphernalia, as well as through unprotected vaginal and anal intercourse.

The world's first needle exchange program was established in Amsterdam in 1984 by the local Junky Union and was soon taken over by the Amsterdam Municipal Health Service. Other European countries, Great Britain and Australia soon followed suit.

In the United States, more than 100 syringe-exchange programs now operate in 30 states and 80 cities. The programs have been growing at a rate of about 20 percent per year. In 1997 alone, more than 17 million needles or syringes were exchanged through these programs.

Yet the programs have remained controversial. In the 1980s, recipients of grants from the National Institute on Drug Abuse were banned from conducting research into needle exchange. Such research may be awarded federal grants today, but a congressional funding ban still prevents any federal support of exchange programs themselves.

In recent years, critics of syringe exchange have seized on the two negative studies to bolster their opposition. Both studies were conducted in Canada, in settings where pharmacies also dispensed syringes.

One of the negative studies, published in 1997, followed a group of IV drug users in Montreal for about two years. The drug users who participated in a syringe-exchange program were 1.7 times more likely to become HIV positive during the study compared with those who did not participate.

However, the Montreal researchers reported last year at a San Francisco AIDS meeting that more recent data show no relationship between syringe exchange participation and HIV seroconversion.

The second negative study, also published in 1997, found that IV drug users who visited an exchange program in Vancouver, British Columbia, more than once a week were 10 percent more likely than non-participants to be HIV positive.

But a later study, conducted in the same setting, found that when additional factors were controlled for, this association disappeared.

Both the Montreal and Vancouver studies were conducted at sites where drug users had legal access to syringes both from pharmacies as well as syringe exchange, making it difficult to assess the impact of syringe exchange in the two cities.

In their review, Gibson and his co-authors did not attempt a formal meta-analysis of the 42 studies; the studies' methods and outcome measures differed too markedly. Instead, they appraised the strengths and limitations of the studies in such areas as adequacy of statistical controls, statistical power and other factors.

To settle the still-simmering debate over syringe-exchange programs, Gibson says future studies should more rigorously deal with confounding factors, including dilution bias.

Neil Flynn, professor of clinical medicine at UC Davis, and Daniel Perales, associate professor of public health at San Jose State University, are co-authors of the study.

This research was funded with grants from the National Institute on Drug Abuse, the National Institute of Mental Health and the United States Public Health Service.

30-Jul-2001