NEW YORK (CNNMoney.com) - Imagine a world without HIV. If that seems farfetched, then try to imagine there's an HIV vaccine.

Of course, there is no HIV vaccine on the market -- yet. But a successful HIV vaccine, if it's ever developed, could bring in blockbuster sales for the companies behind it, save lives, and possibly even improve Big Pharma's battered image, all at the same time.

Merck (up $0.24 to $34.75, Research) and Sanofi-Aventis (up $0.08 to $11.90, Research) are currently leading the way in developing AIDS vaccines; both are at work on their own versions. Analysts who cover the companies said that HIV vaccines have blockbuster potential, but the drug makers said they are years away from completing their trials.

"I hope they're successful, but the odds are long," said Robert Hazlett, an analyst for Suntrust Robinson Humphrey who covers Merck, the New Jersey-based drug giant.

"These are high-risk, high-reward [vaccines]," said Bernstein analyst Gbola Amusa, who covers Paris-based drug giant Sanofi-Aventis. "The chances of their success are remote, but if they do work, we would expect them to be blockbusters if they produced enough to supply developed nations and undeveloped nations."

Vaccine sales would be less vulnerable to pressure from generic drug makers and counterfeiters than drugs that are based on simple molecular compounds because they are harder to manufacture, said Amusa. This is particularly important in the AIDS industry: Brazil, for instance, ignores patents and produces its own AIDS drugs to supply them to its impoverished population, siphoning sales away from Big Pharma.

Merck has spent the last two decades developing an HIV vaccine, and is now conducting Phase 2, or late-stage, human trials. Assuming that Phase 2 is successful, Merck still has to conduct the third and final phase of testing with thousands of patients before submitting the vaccine to the Food and Drug Administration, which will take years to do.

Sanofi-Aventis is conducting Phase 3 tests in Thailand, but said it won't release data any earlier than 2009. And even if these tests are successful, the company would have to run a whole new series of tests to meet FDA specifications.

If these companies do successfully develop a vaccine, achieving blockbuster sales will be a challenge. Amusa said that vaccines are notoriously difficult to produce in large quantities.

HIV, or human immunodeficiency virus, is the virus that causes AIDS, also known as acquired immunodeficiency syndrome. HIV is transmitted sexually or via infected syringes and breaks down the infected patients' natural defenses. AIDS does not kill its victims directly; they die when their ravaged immune systems succumb to pneumonia or other illnesses.

Some 25 million AIDS patients have died from illnesses related to AIDS since it was first identified in 1981, including 3.1 million deaths in 2005, according to the United Nations. There are 40.3 million people living with AIDS worldwide, including 4.9 million new infections in 2005, says the U.N. Two-thirds of the world's AIDS patients live in sub-Saharan Africa, where many people are too poor to afford food, let alone medicine.

GlaxoSmithKline (up $0.23 to $54.23, Research) and Bristol-Myers Squibb (up $0.25 to $22.95, Research) are the industry leaders in producing AIDS anti-virals, which are treatments taken by patients after they've been infected to prolong and improve their lives. GlaxoSmithKline has nine HIV anti-virals on the market totaling $2.7 billion in 2005 sales. Bristol-Myers has five HIV anti-virals totaling $446 million in 2005 sales.

There is no cure for AIDS. But in theory, a vaccine could phase out AIDS by preventing new infections.

"There is no AIDS vaccine on the market, and that's the urgency," said Merck spokeswoman Janet Skidmore. "A vaccine will be necessary to end the epidemic. We're pleased that we have our lead candidate in Phase 2, but it will be years before an AIDS vaccine gets to market even if the trials work out the way we want it to work out."

Developing an AIDS vaccine is particularly difficult because the constantly mutating virus has developed resistance to existing treatments, and often seems one step ahead of the researchers.

"It's a wild card," said Barbara Ryan, an analyst for Deutsche Bank North America who covers Merck, referring to a vaccine's prospects for success. "If it works, great, but I don't think there's a tremendous amount of visibility right now. It's been a long time in the making at Merck and there have been disappointments along the way. The HIV virus is crafty, and there are many different strains."

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None of the analysts interviewed for this story own shares in the companies they cover here, though Deutsche Bank North America does own shares of Merck.

To read about the industry for home test AIDS kits, click here.

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Merck slashes AIDs drug prices. Click here.