WASHINGTON (Reuters) - A needle-free vaccine protected more than 70 percent of visitors to Mexico and Guatemala from traveller's diarrhoea, popularly known as Montezuma's Revenge, researchers reported on Wednesday.

Even if travellers did get infected with the stomach bug, Iomai Corp.'s experimental vaccine patch prevented severe illness, the researchers reported in the Lancet medical journal.

"I think it's one of the most exciting new developments in travel medicine," said Dr. Herbert DuPont of the University of Texas in Houston, who helped test the vaccine.

"People could buy this and put it on themselves whenever they take a trip. It is the most convenient form of immunisation I have ever seen," DuPont said in a telephone interview.

The vaccine protects against Escherichia coli bacteria — specifically a strain known as Enterotoxigenic E. coli or ETEC.

It is the leading cause of diarrhoea in travellers to certain areas, causing four to five days of misery including nausea and cramps.

Iomai's team, along with DuPont's independent team and a group at Johns Hopkins University in Baltimore, tested the patches in a Phase II safety and efficacy trial. They got data back from 170 adults travelling to areas known to be hot spots of tummy trouble in Guatemala and Mexico.

During and after travel, 15 percent of the patients who got the vaccine developed diarrhoea of any type, and just five percent had ETEC-associated diarrhoea. This compared to 22 percent of travellers who got placebo, 10 percent of whom had ETEC diarrhoea.

Eleven percent of the travellers who got placebo had severe diarrhoea, compared to two percent of those who got the patch.

"It looked like it prevented more than 70 percent of the episodes of moderate or severe traveler's diarrhoea," DuPont said. "This vaccine is among the best we have for these kinds of diseases."

The vaccine also appeared to protect against non-ETEC causes of diarrhoea. DuPont said it may stabilise the intestine and prevent the reaction to infection that causes diarrhoea.

Austrian vaccine maker Intercell is in the process of buying Maryland-based Iomai, which also has a patch that boosts the effects of influenza vaccines.

DuPont, who said he receives no payments from Iomai, said the market potential could be large because "we have no vaccine for traveller's diarrhoea."

The needle-free approach could work against other infectious diseases, he said.