The world's oldest successful vaccine, used to immunize against smallpox, has been altered genetically and is one of several leading candidates to become a vaccine against acquired immune deficiency syndrome (AIDS).

Although no such vaccine is expected to be ready for human use within the next few years, several approaches to making one are being tried.

The latest known to be ready for tests in monkeys and apes is made from vaccinia virus. Vaccinia causes cowpox and, more than 150 years ago, Edward Jenner proved the concept of vaccination by using the cowpox virus on humans to prevent another pox, the deadly smallpox.

National Institutes of Health scientists, led by a pioneer researcher on new uses of vaccinia, Dr. Bernard Moss, have spliced an AIDS gene into the vaccinia virus, trying to fool the immune system into believing that the vaccinia is AIDS.

If the body's defenses perceive vaccinia as AIDS, antibodies against AIDS will be formed.

The researchers have tried the vaccinia-AIDS combination in mice and in cells in culture dishes and reported that the mice and several types of cells do react to the vaccinia as if it were an AIDS virus and so produce antibodies against AIDS.

The report was published yesterday in the British journal Nature.

Moss said that the vaccinia-AIDS virus is being tried in African green monkeys and that, if successful, he hopes to try it in chimpanzees before the end of this year.

But at present "this is not a vaccine, nor will it be used as a vaccine in humans," Moss said.

By definition, vaccines must protect humans or animals against disease, and no vaccine candidate has shown that it can prevent AIDS infection in either. In only one case has it been shown that a vaccine candidate can protect against AIDS infection in cells on culture dishes.

The idea of a vaccine begins with the body's way of defending itself against such invaders as the AIDS virus. The body identifies invaders by "sensing" their surface composition. If a foreign surface is spotted, the body makes molecules called antibodies that bind to those surfaces and disable the invader.

A vaccine is a harmless invader whose surface mimics a real invader. In this case, the vaccinia virus with surface molecules from an AIDS virus is the mimic. When put into mice, it tricks the system into making antibodies to attack surface molecules of the AIDS virus.

Thus, in theory, after a vaccine-like vaccinia has entered the system and tricked the body into making large numbers of antibodies, an actual AIDS invader would immediately be disabled by the swarming antibodies.