Early next year, 30 college students in Boston will add an unusual test to their curriculum. From January through March, they will travel to the Forsyth Dental Clinic to drink a liquid protein produced by the bacteria that cause tooth decay. Then they will have their blood and saliva tested for the presence of special antibodies that ultimately fight tooth decay.

If the results are positive, a vaccine against cavities may be available within the next decade to be given routinely alongside vaccines for diptheria, measles, mumps, tetanus and whooping cough.

Tooth decay afflicts some 95 percent of the population, costs billions of dollars to treat annually and is the leading cause of tooth loss in adults 35 years and younger.

Two advances -- fluoridation and dental sealants -- are capable of temporary protection against tooth decay. But the real breakthrough, a vaccine that could immunize people against dental cavities, has remained elusive.

There are two promising leads. Some researchers, like immunologist Daniel Smith of Forsyth Dental Clinic, are concentrating on the protein produced by the bacteria that cause tooth decay. Others, including biologist Roy Curtiss III of Washington University's School of Dental Medicine, are attempting to control the bacterium itself.

Known as streptococcus mutans -- or s. mutans -- the bacteria that produce cavities gradually colonize the mouth during tooth development. Infants "don't seem to have the bacteria in their mouths until they develop teeth," says Washington University's Curtiss. "Studies suggest very strongly that infants acquire the bacteria from other family members."

Once the bacteria take hold in the mouth they reproduce slowly but persistently. "Only 50 percent of 3-year-olds have detectable levels of s. mutans in their mouths," says Forsyth's Smith. But "s. mutans grows in the mouths of some 95 percent of teen-agers."

"If we could devise a scheme where we could immunize kids before they have s. mutans and then keep antibodies high against the bacteria during the cavity-prone years, we think we could have a more complete system for protection from tooth decay."

S. mutans is one of the more benign members of the large bacterial family that causes strep throat and rheumatic fever. This germ erodes teeth in a three-step process, explains Washington University's Curtiss.

First, it attaches to a thin layer of saliva that bathes the teeth with a filmy protein.

Next, s. mutans takes advantage of sugar in the diet. The bacteria break down sugar for their own energy use and also to produce a gluey substance that enable them to stick to the tooth and to other bacteria. This buildup is known as dental plaque and can be removed only by professional dental cleaning, Curtiss says.

Finally, cavities develop as s. mutans produces lactic acid, which gradually eats away tooth enamel.

Curtiss and his colleagues are attempting to produce a vaccine that will exploit the bacteria's use of a protein called Spa-A. This protein gives the bacterial surface a fuzzy coat and seems to be a crucial part of all three stages of tooth decay. Curtiss is working to prompt the immune system to make antibodies to Spa-A, which would -- he hopes -- interfere with plaque build-up and beat tooth decay.

Despite some promising advances for a vaccine, Curtiss believes that routine immunization for tooth decay is still "three to 10 years away." One problem that has hampered researchers is an unexpected complication that arose in animal testing and has forced investigators to concentrate on more difficult methods of vaccine production.

Injection of s. mutans into animals -- to prompt the production of antibodies -- occasionally has caused the animal's immune systems to mistake heart cells for the bacteria. In this case, the antibodies could attack the heart. So instead of promoting a reaction to the bacteria, researchers are trying to prompt an attack on substances necessary for the bacteria's survival -- such as Spa A. The problems have been worked out sufficiently to gain Food and Drug Administration approval for human testing in Boston.

Even if a vaccine against tooth decay were available today, experts caution that it still would be only "one of the weapons" in the dental arsenal, Smith says. Other key weapons include "good dental hygiene, use of fluoride and regular trips to the dentist."