Merck (NYSE:MRK), Bristol-Myers Squibb (NYSE:BMY), Johnson & Johnson (NYSE:JNJ), and especially Gilead Sciences (NASDAQ:GILD) will live to see another day.

Data from a trial testing an HIV vaccine, a combination of products developed by sanofi-aventis' (NYSE:SNY) and VaxGen, looks potentially promising, but it still has a long ways to go. The vaccine reduced the infection rate by 31%, although the absolute numbers -- 74 infected on placebo, versus 51 infected on the vaccine, with 8,200 people in each group -- were so small that you have to wonder how statistically significant that reduction is.

Keep in mind that the infections happened as the participants went about their daily lives -- they weren't specifically exposed to the virus -- so there's potential that the difference is just due to the subjects on placebo acting in a more risky fashion, making them more likely to be infected. Or it could be just chance. Supporting that kind of interpretation is the apparent contradiction that vaccine recipients who were subsequently infected didn't have lower levels of virus in their bloodstream than their infected counterparts in the placebo group. If the vaccine were working, you'd expect that it could help those people fight the virus after they were infected.

Even if the infection rate reduction turns out to be real, the vaccine is, at best, a building block for something better.

Don't get me wrong. I'd like to see a HIV vaccine as much as the next person. But companies that make HIV drugs could lose billions of dollars in revenue if an effective vaccine is developed. All drug-company investors need to keep an eye on up-and-coming drugs from competitors that could take market share -- or, in this case, disintegrate the market.

Fortunately for HIV-drug makers, the virus is a tricky little bugger. VaxGen had a vaccine that looked promising, but failed back in 2003, and Merck's vaccine died in phase 2 trials in 2007. Scientists may eventually figure out how to get the immune system to attack the virus before HIV is able to infect the immune system, but it's going to take time. In the meantime, drugmakers can keep bringing in the revenue.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool has a disclosure policy.