U.S. researchers say they have developed a painless method of immunizing people without needles, a development with a potential big impact in Third World countries.

The method, called transcutaneous immunization, can deliver a vaccine via a skin patch that contains a bacterial product called cholera toxin (CT), commonly used as an adjutant to enhance the immune system.The technique has proved effective on mice, and trials on humans are due to start at the end of next month. If all goes well it could be available commercially within the next five to 10 years.

"We can take an off-the-shelf vaccine, mix it with cholera toxin and get a very nice immune response from that off-the-shelf type vaccine. The implications are that one could eliminate needles," Gregory Glenn, of the Department of Biochemistry at the Walter Reed Army Institute of Research in Washington, told Reuters.

The technique targets the immune system in a new way -through the skin. The cholera toxin is the key.

By applying a simple and safe mixture of CT and vaccine components to the skin through a patch the scientists found that it stimulated an immune response to vaccine components such as diph-theria or tetanus toxins.

"The adjutant concept isn't new and CT isn't new. What is new is that we are able to use it through the skin," said Glenn.

"You can take a vaccine like tetanus or diphtheria and add it to the CT. It activates the immune system, and the immune system gives you antibodies to diphtheria or tetanus."

The technique could eliminate the need for trained personnel and sterile needles, a huge major boost in developing nations where large-scale immunization against disease is a major problem.

"We're opening up a new paradigm of vaccine delivery," said Glenn.

"Scientists are excited because we're accessing the skin immune system, a very powerful immune system. Those cells that are in the skin immune system are the darlings of the vaccine world."

Not only would the immunization be painless, it could cause less of a reaction in the body, and multiple vaccines and boosters could be delivered in a way that would not be possible with needles.

In a letter to the scientific journal Nature, Glenn and his colleagues described how they tested the technique on mice. There were no inflammations on the skin where the vaccine was administered and the mice produced antibodies.

Glenn also believes the technique could be useful in developing a vaccine against cholera.