Public health experts gained another weapon in their battle against AIDS on Monday, when the U.S. Food and Drug Administration approved an HIV drug for preventive use. This will help pave the way for its authorization in developing countries.

The drug, called Truvada, is expensive, but if deployed smartly could do considerable good in Africa.

Made by California-based Gilead Sciences Inc., Truvada has been used to treat HIV since 2004, and now will be available by prescription in the U.S. to use as a prophylactic against infection. In three trials, the drug prescribed daily reduced the chances of acquiring HIV by 44 percent, 63 percent and 73 percent; in one trial, it failed to show efficacy.

Using HIV medication for prevention, a strategy called pre-exposure prophylaxis or PrEP, entails risks. The FDA required Gilead to include warnings that the new use of Truvada is only for people who are confirmed to be HIV-negative and who are tested every three months during use. The tests are needed because a person on the regimen who is unknowingly HIV positive could develop a drug-resistant strain of the virus, which could then be passed along to others.

Also, the strategy is expensive compared with the alternative, condoms. In Africa, where manufacturers offer HIV drugs at discounts, Truvada costs far less than the $24.67 per 200 mg tablet it sells for in the U.S. Still, by one estimate, the total annual cost there to use the drug preventively would be about $150. That’s a considerable sum in the poorest countries, though it’s a fraction of the roughly $600 a year cost of treating an HIV infection.

A PrEP regimen will require a significant shift in the way the fight against AIDS is waged in Africa. In the past, public health professionals have leaned on broad-based campaigns such as mass condom giveaways or roadside billboards. The costs and risks associated with the medication render this approach unsuitable. It makes sense to promote it only in the high-risk pool for which the FDA approved it in the U.S.: gay men, individuals in a sexual relationship with someone who is HIV positive, prostitutes and sexually active residents of high-incidence communities.

This will be a challenge for health workers in the developing world. They will need to learn more about often-ostracized populations, earn their trust, track the virus and get Truvada into the right hands. Those using the medicine will have to be regularly tested for HIV; they will also have to be counseled to reduce the chances that, once on Truvada, they become sexually reckless.

This will cost money. But mathematical models indicate that HIV drugs used for prevention can save money in the long run, if they are targeted to people in the greatest danger.

With AIDS assistance to the developing world hard-hit by the economic meltdown, falling 10 percent from 2009 to 2010 to $6.9 billion, it has become especially important to invest wisely. Truvada passes the test. Although still alarmingly high, the estimated number of people infected with HIV in 2010 (2.7 million) was 21 percent lower than the peak in 1997. This new option has the potential to hasten the trend.

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