NEW YORK (Reuters Health) - It may one day be possible to receive vaccines now given as shots in the form of a pill that contains bacterial spores, new study findings suggest.

European researchers successfully immunized mice against potentially deadly tetanus using spores of bacteria attached to a toxin produced by the tetanus bacterium. When exposed to the immune system, this toxin confers immunity to the disease without causing illness.

These findings represent a "proof of principle" that a new type of vaccine is possible for other diseases besides tetanus, study author Dr. Simon M. Cutting of the Royal Holloway University of London in the UK told Reuters Health.

"If used in humans, spores might be able to replace the classical booster vaccine which we all take," Cutting said.

"But instead of an appointment at the clinic and an injection, you could take a tablet orally," he added.

Using bacterial spores to bestow immunity to disease holds many advantages over existing vaccines, Cutting said.

Many vaccines consist of inactivated bacteria or viruses that, if alive, would cause disease, Cutting said. These vaccines can produce side effects, and researchers have to take extra precautions to ensure that they are safe, he said.

Furthermore, most vaccines have a limited shelf life and must be refrigerated, Cutting noted.

In recent years, researchers have sought to extract the portion of the inactive bacterium or virus contained in vaccines -- a snippet that is harmless to the body but confers immunity to disease -- and somehow couple it to a substance that deposits it in the body.

As described in the current study, which appears in the journal Infection and Immunity, Cutting and his team attached a portion of the tetanus toxin to bacterial spores, which are reproductive cells that have thick walls and can withstand harsh environments.

The researchers gave those vaccine-containing spores to mice, orally and through the nose. When the mice were exposed to a dose of the tetanus toxin that should have been fatal, mice given the spores survived.

"Thus, they are vaccinated," Cutting declared.

Administering vaccines using bacterial spores could circumvent issues of refrigeration and people's unwillingness to get shots, the researcher said.

"Dirty needles are a major problem in developing countries, but it is also known that in the West, many people will 'forget' or fail to take the necessary booster vaccines because of the time and effort in getting the jab, as well as the dislike of having the injection," Cutting said.

The researcher cautioned, however, that this technique has not yet been shown to be safe and effective in humans. It will likely be many years before a new vaccine could become available, he said.

"The development of new vaccines appears to be a slow and step-by-step process," Cutting said.

SOURCE: Infection and Immunity 2003;71:2810-2818.