Nearly 1.5 million children, the vast majority of them in the developing world, die of diarrheal diseases each year. In April, Mahdi Saeed, an epidemiologist at Michigan State University, announced a new vaccine that could substantially reduce that number.

Although vaccines already exist for some causes of diarrhea, finding a fix for enterotoxigenic E. coli, the leading bacterial cause of diarrhea in children in the developing world, has proved to be difficult. The toxin produced by E. coli is too small to be recognized effectively by the human immune system, meaning that one round of infection does not provide immunity against future exposure.

As a result, a person can experience multiple bouts of diarrhea, which can lead to dehydration, malnutrition, and even death. Children are especially vulnerable because they have a higher density of chemical receptors susceptible to the E. coli toxin than adults do. That toxin is also a leading cause of traveler’s diarrhea, which annually affects millions of visitors to developing countries around the world.

In order to alert the immune system to the presence of the tiny toxin molecule, Saeed attached it to a larger molecule that did not alter its properties. Trials in mice showed that this piggyback approach increased the ability of the immune system to recognize the toxin.

A separate set of trials in rabbits, which concluded in April, demonstrated that the doubled-up molecule provoked the animals’ immune system to produce antibodies. And when these antibodies were tested in mice, researchers found that they made the mice immune to the effects of E. coli.

The new vaccine, 25 years in the making, could proceed to human clinical trials by the beginning of 2010, according to Saeed, who has been studying E. coli toxin since he was in graduate school. A vaccine could also reduce E. coli deaths among farm animals.

“E. coli is a killer,” Saeed says. “A vaccine would be a true lifesaver for children in the developing world.”