updated 12/16/2005 6:35:21 PM ET 2005-12-16T23:35:21

An experimental vaccine provided some protection against that scourge of tourism, traveler’s diarrhea, when put to a rigorous test among U.S. students in Mexico and Guatemala, scientists reported Friday.

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The research provides important evidence that a vaccine is possible against the hard-to-avoid germs, instead of merely urging travelers to guard against risky food and water. Now the challenge is to strengthen that protection.

“This is a very encouraging first step,” said microbiologist A. Louis Bourgeois of Johns Hopkins University’s Bloomberg School of Public Health, who led the study of the Swedish-developed vaccine.

Traveler’s diarrhea is the leading cause of illness among visitors to developing countries, striking an estimated 20 million international travelers a year. While there are numerous causes, the chief culprit is bacteria called enterotoxigenic E. coli, or ETEC. It is spread through contaminated food and water, and while rarely life threatening to the otherwise healthy traveler, it can cause up to a week of misery.

ETEC is becoming more resistant to antibiotics, making it harder to treat the seriously ill and increasing the need for a vaccine, said Dr. David Hooper of Massachusetts General Hospital, who helped organize the American Society for Microbiology meeting where the data was unveiled.

Scientists are hunting a variety of ways to make an anti-ETEC vaccine. This one came about almost accidentally: During tests of a cholera vaccine in Bangladesh, some recipients also seemed temporarily protected from ETEC-caused diarrhea. So researchers at the University of Goteborg in Sweden and that country’s SBL Vaccines created a vaccine especially for traveler’s diarrhea.

They used part of that cholera vaccine, because a toxin produced by cholera is very similar to an ETEC-produced toxin. The new vaccine also contains killed E. coli cells — designed to stimulate the immune system to attack the germs before they dock in the intestine and start making toxin.

Some 1,406 U.S. students swallowed the vaccine or a placebo before spending up to a month studying Spanish in Guatemala or Mexico. (The vaccine is considered so safe that students received it by mail, swallowing it at home as researchers supervised by telephone. )

At first, the vaccine appeared only to lessen disease severity, Bourgeois said.

Then researchers used blood tests to determine if the vaccine in fact stimulated an immune response. It didn’t “take” in everybody.

But among responders, it was 84 percent effective at blocking severe diarrhea, and 63 percent effective at blocking even mild cases, too. Vaccine responders who did get sick had half as many days of illness as placebo recipients.

The big question is why only about half of recipients have a strong immune response. Bourgeois said the vaccine’s developers are working to improve that.

“Obviously this is not the final product, but this needs to be pursued,” Bourgeois said, noting that a workable travelers’ vaccine might help the harder goal of protecting children in developing countries who die from the germ. “We hope to ... further rekindle and re-energize people to develop vaccines against ETEC.”

© 2012 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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