Most people have never heard of rotavirus, but it infects nearly every child at a young age, and is the most common cause of severe childhood diarrhea.

The vaccines, designed to protect children against a widespread germ called rotavirus, produced a strong immune response that kept many children from getting sick and protected others enough to keep them out of the hospital.

And the studies -- which enrolled 131,263 children, the largest vaccine tests since the polio trials of the 1950s -- offered reassuring findings about the safety of the vaccines. That was a critical issue because a similar vaccine was withdrawn from the market six years ago after apparently injuring some children.

Hospitalizations Reduced Dramatically

In findings that startled many experts, a rotavirus vaccine developed by Merck & Co. and tested largely in industrialized countries cut hospitalizations for all types of diarrhea by 63 percent in young children. Another vaccine developed by GlaxoSmithKline PLC and tested in a more vulnerable population -- poor children in Latin America -- cut hospitalizations for all types of diarrhea by 42 percent. The companies sponsored both studies.

Experts had previously said that rotavirus causes one third to one half of severe childhood diarrhea cases worldwide, but the new studies suggest those estimates may be low, and that rotavirus may be killing more children than the 440,000 a year estimated by the United Nations.

The studies weren't large enough to calculate precisely how much the vaccines could cut the death rate from rotavirus infection, but several experts said it was clear they could have a dramatic impact.

"Both of these vaccines, if put into use, have the possibility of saving tremendous numbers of hospitalizations and deaths," said Stanley Plotkin, inventor of the rubella vaccine and the dean of American vaccinologists. He was a leader of early research that led to the Merck vaccine but praised both products yesterday as public-health breakthroughs.

Accelerated Global Deployment

Most people have never heard of rotavirus, but it infects nearly every child at a young age, and is the most common cause of severe childhood diarrhea. Thousands of American children are hospitalized with the virus every year, but they get good medical care, so few die of it. Both companies intend to market their vaccines in the United States, and the Merck vaccine could be licensed here within months.

The situation is far worse overseas, where babies whose parents lack access to good medical care die of rotavirus diarrhea in substantial numbers. More than 20 countries have already approved the GlaxoSmithKline vaccine, and some -- notably Brazil, Panama and others in Latin America -- are moving rapidly to offer it to poor children in publicly funded immunization campaigns.

However, efforts to deploy the vaccines in the world's poorest countries have lagged amid logistical and financial difficulties. The Rotavirus Vaccine Program, a Seattle organization set up to get the vaccines into broad global use, praised the studies yesterday and said it was moving ahead with more research designed to accelerate deployment. Even by optimistic estimates, though, it is likely to be several years before the vaccines reach the poorest villages.

Experts Remain Cautious

The products could face sharp regulatory scrutiny in the United States and Europe. An earlier rotavirus vaccine sold by Wyeth, a Madison, NJ, pharmaceutical company, appeared on the US market in the late 1990s but was withdrawn after apparently causing intestinal blockages in a handful of children.

Merck and Glaxo both mounted massive tests of their vaccines designed to detect even unusual side effects.

The studies being published today show little cause for concern: Rates of intestinal blockage and other problems were similar between children who got the vaccines and children who got dummy versions. However, many experts remain cautious about the vaccines until they have been on the market for a while.

GlaxoSmithKline of London developed its vaccine with an explicit goal of selling it in both rich and poor countries at different prices. Merck of Whitehouse Station, NJ, initially tested its vaccine mainly in developed countries but recently announced plans for tests in poor countries in Asia and Africa.

Penny M. Heaton, a company scientist who led research on the vaccine, said yesterday: "Certainly, Merck is committing to providing this vaccine for the developing world."