Of all the mysteries posed by AIDS, perhaps the deepest and most damaging is a human one: why have we failed so utterly to stop its transmission? Most people with H.I.V. in the world, including a vast majority of the 22 million who are infected in sub-Saharan Africa, caught it from a sexual partner.
Despite billions of dollars spent to slow this form of transmission, only a few countries have had significant success -- among them Thailand, Uganda and Zimbabwe -- and their achievements have been unreplicable, poorly understood and short-lived. We know that abstinence, sexual fidelity and consistent condom use all prevent the spread of H.I.V. But we do not yet know how to persuade people to act accordingly.
Then there is another way that H.I.V. infects: by injection with a hypodermic needle previously used by an infected person.
Outside Africa, a huge part of the AIDS epidemic involves people who were infected this way. In Russia, 83 percent of infections in which the origin is known come from needle sharing.
In Ukraine, the figure is 64 percent; Kazakhstan, 74 percent; Malaysia, 72 percent; Vietnam, 52 percent; China, 44 percent.
Shared needles are also the primary transmission route for H.I.V. in parts of Asia. In the United States, needle-sharing directly accounts for more than 25 percent of AIDS cases.
Drug injectors don't pass infection only among themselves. Through their sex partners, H.I.V. is spread into the general population. In many countries, the H.I.V. epidemic began among drug injectors.
In Russia in 2000, for example, needle-sharing was directly responsible for more than 95 percent of all cases of H.I.V. infection.
So virtually all those with H.I.V. in Russia can trace their infection to a shared needle not many generations back. Though it has been scorned as special treatment for a despised population, AIDS prevention for drug users is in fact crucial to preventing a wider epidemic.
Unlike with sexual transmission, there is a proven solution here: needle-exchange programs, which provide drug injectors with clean needles, usually in return for their used ones. Needle exchange is the cornerstone of an approach known as harm reduction: making drug use less deadly.
Clean needles are both tool and lure, a way to introduce drug users to counseling, H.I.V. tests, AIDS treatment and rehabilitation, including access to opioid-substitution therapies like methadone.
Needle exchange is AIDS prevention that works.
While no one wants to have to put on a condom, every drug user prefers injecting with a clean needle.
In 2003, an academic review of 99 cities around the world found that cities with needle exchange saw their H.I.V. rates among injecting drug users drop 19 percent a year; cities without needle exchange had an 8 percent increase per year. Contrary to popular fears, needle exchange has not led to more drug use or higher crime rates.
Studies have also found that drug addicts participating in needle -- -exchanges are more likely to enter rehabilitation programs. Using needle exchange as part of a comprehensive attack on H.I.V. is endorsed by virtually every relevant United Nations and United States-government agency.
All over the world, however, solid evidence in support of needle exchange is trumped by its risky politics.
Harm reduction is thought by politicians to muddy the message that drug use is bad; to have authorities handing out needles puts an official stamp of approval on dangerous behavior.
Consider the United States. In 1988, Congress passed a ban on the use of federal money for needle exchange; President Clinton said he supported needle exchange but never lifted the ban, and it remains in effect.
It not only applies to programs inside the United States but also prohibits the U.S. Agency for International Development from financing needle-exchange programs in its AIDS prevention work anywhere in the world.
The administration of George W. Bush made the policy more aggressive, pressuring United Nations agencies to retract their support for needle exchange and excise statements about its efficacy from their literature. (Today, U.N. agencies again recommend that needle exchange be part of H.I.V.-prevention services for drug users.) Despite Barack Obama's campaign pledge to overturn the ban, his first budget retained it. The House of Representatives recently passed a bill that would lift the ban -- but it includes a provision that would make using federal money for needle exchange virtually impossible in cities, where it is needed most.
There are some parts of the world -- Western Europe, Australia, New Zealand -- that do widely use harm-reduction strategies, including needle exchange.
And programs have begun even in Iran, of all places, which offers needle exchange and methadone; its program of giving prisoners methadone is now the world's largest.
China is now taking AIDS seriously, beginning to institute government-sponsored harm reduction nationwide. But the overwhelming majority of drug injectors around the world still have no such access.
Because government financing is so politically unpopular, in most of the 77 countries that offer needle exchange, the programs are run by nongovernmental groups. As a result, these efforts are small, isolated and often undermined by uncooperative police and health departments. The world is casting aside the single most effective AIDS prevention strategy we know.
Russia needs needle exchange more than any other country: its H.I.V. epidemic is large, one of the fastest-growing in the world, and perhaps the most dominated by injecting drug use. Yet the needle-exchange efforts that do exist are scarce, small and under siege. I traveled there recently to see what lessons they hold. At 9 p.m. on a May night, in a tough neighborhood in Moscow's north, I joined two young men as they climbed the stairs from the Metro. Arseniy and David were in their late 20s, wearing jeans and baseball caps. They had arrived to give out clean needles and promote harm reduction -- but theirs was a guerrilla effort.
Needle exchange is legal in Russia -- sort of. It must follow federal regulations. The catch is that these regulations don't exist: the Federal Drug Control Service, whose top officials have called needle exchange "nothing more than open propaganda for drugs," has been sitting on them for five years.
As a result, no new harm-reduction programs have started during that time. Old ones continue where local authorities tolerate them, but Moscow's government disapproves of needle exchange.
So like their clients, Arseniy and David avoid the police. One of their clients was Masha, who, like every other drug user I interviewed, talked about police extortion.
It is every addict's main fear, but avoiding police shakedowns means only more dangerous injecting: if you fear being caught walking around with a needle, you use the community needle your dealer provides.
Arseniy told me that he started doing harm reduction as a volunteer with an organization working with the homeless. "Most of my clients used drugs, and I understood we couldn't do anything without needle exchange," he said. So the workers began buying needles with their own money and giving them out. Now he, David and another pair of outreach workers get financing from another Russian group.
The city of Moscow, then, has only a handful of people doing needle exchange. An extremely conservative estimate of Moscow's drug injectors puts the number at 240,000.
Moscow's drug policy could be called harm augmentation: discourage drug use by making it as dangerous as possible.
Arseniy and David, for example, can't direct addicts to methadone clinics, since methadone -- the global gold standard rehabilitation method -- is illegal in Russia. Nor can they bring users into the health system: beyond the most basic health services, public health care in Moscow is only for officially registered residents; many drug injectors are homeless or from other cities and are unregistered in Moscow. The only thing Arseniy and David can do is give out the card of a drop-in center, called Yasen, across the city from the clients they were serving.
When I visited Yasen, staff members told me stories of ambulances refusing to pick up an overdosing drug user and hospitals turning away people who come in with the afflictions of a violent life on the streets. Russia does have free detoxification clinics, but they use harsh, outdated methods, and less than 10 percent of their clients stay drug-free for a year. Checking in lands an addict on the official list of drug users -- a designation that can affect opportunities for jobs, housing and privileges like driver's licenses.
While the city of Moscow treats drug users purely as potential criminals, St. Petersburg is different.
The main reason is the work of Humanitarian Action, among Russia's first needle-exchange programs, which started its work in 1997. The heart of Humanitarian Action is its mobile clinic, a blue bus that visits 10 neighborhoods a week on a regular schedule.
On a Friday afternoon when I visited, the bus was parked on the side of a busy street in front of a block of apartment towers.
There was a line out the back door of people returning bags of used needles and getting clean ones.
Lena Porechenkova, a skinny, grizzled woman with tinted glasses, was running the bus's needle-exchange counter.
She spoke to a fresh-faced woman of 22, also named Lena. She was planning to quit, Lena said. "But I don't want to get onto the state list of addicts and have problems getting a job later." She said she might consider buying methadone (often sold illegally by heroin dealers) and trying to quit on her own.
"Well, it's possible to overcome this on your own," Porechenkova said, but she added that it is possible to pay $200 and be anonymous. "Why don't you talk to our psychologist?" She called over Nikolai Yekimov, who took Lena into a tiny office in the bus. Yekimov has a database of rehabilitation centers.
The bus also offers a case manager, who helps the client assemble the necessary papers and test results and will even pick her up and accompany her to the clinic. When Lena left, I asked her where she would go for advice if the bus didn't exist. "Nowhere," she said.
Humanitarian Action is a model program.
It has everything harm reduction needs -- save the most important thing: size. The group estimates that it has 4,000 repeat clients -- a tiny proportion of St. Petersburg's drug injectors, who number as many as 150,000.
In a few Russian cities needle-exchange programs run by the municipal or regional governments have kept H.I.V. rates among drug users relatively low. But most of the country's 75 harm-reduction programs -- -- almost all of which do needle exchange -- are run by Russian nongovernmental groups with money from the Global Fund to Fight AIDS, Tuberculosis and Malaria. These programs run on $20,000 to $30,000 a year, which is far too small to have an impact.
And they are imperiled. Russia is now too rich to accept Global Fund grants for H.I.V. prevention, so these programs will lose their financing over the next two years.
Russian officials are resisting requests from international AIDS advocates to keep needle exchange alive.
The future of harm reduction does not reside in small programs carried out by internationally financed groups, however stellar their quality. Such programs have proved that harm reduction works, but they cannot make it epidemiologically relevant.
Only a government can ensure that the police and hospitals will respect drug users' rights to health care. Only a government can do needle exchange on a wide scale. This is what is needed to reduce H.I.V. rates -- not just for drug users, but for us all.
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