WASHINGTON — A vaccine against rotavirus, the leading cause of diarrhea in infants, has led to a dramatic drop in hospitalization and emergency room visits since it came on the market two years ago, doctors reported Saturday.

A bonus: the vaccine seems to be preventing illness even in unvaccinated children by cutting the number of infections in the community that kids can pick up and spread.

"We're a little surprised by the degree of impact given the coverage we've achieved," said Jane Seward of the federal Centers for Disease Control and Prevention. Only about half of young children had received the vaccine and very few had received all three doses when the studies were done.

Results were reported Saturday at an infectious diseases conference in Washington.

Before the vaccine, more than 200,000 U.S. children were taken to emergency rooms and more than 55,000 were hospitalized each year with rotavirus, which causes vomiting and diarrhea, mostly from January through May. Worldwide, the virus kills 1,600 young children each day.

Since Merck & Co.'s Rotateq came out in 2006, hospital visits and stays due to the virus have dropped 80 percent to 100 percent, studies by the CDC and several other groups show.

Last winter, rotavirus cases started and peaked two to three months later and were much less extensive than in previous years, CDC scientists report. Hospitals in a network that tracks these cases for the CDC saw more than an 80 percent drop in admissions from them, one study showed.

Another study, by Merck, found a 100 percent drop in hospitalizations and ER visits during the 2007 and 2008 rotavirus seasons compared to previous ones. The study was based on a review of health insurance claims for about 61,000 infants and diagnoses by doctors in routine clinical practice.

Rotateq is an oral vaccine given at two, four and six months of age. In June, a second rotavirus vaccine came on the market — GlaxoSmithKline's Rotarix. It requires only two doses, completed by four months of age.

Also at the conference, scientists reported that a new version of Wyeth's Prevnar vaccine seems to better protect kids against germs that cause pneumonia, meningitis and ear infections, but whether it makes it onto the market before dangerous strains become a big problem remains to be seen.

Scientists have been retooling Prevnar, which came on the market in 2000 and is advised for children under age 2. It protects against the seven strains of Strep bacteria that were causing the most serious infections at the time. Since then, new strains have become more of a threat and increasingly are resistant to common antibiotics.

The experimental new vaccine adds six of these to the original seven. Scientists from Wyeth and from Johannes Gutenberg University in Mainz, Germany, compared immune responses to the new vaccine, given to 293 babies, versus those of an equal number of babies given the old one.

The new vaccine did about as well as the old one on six of the seven original strains and well on the six new ones, including the one causing the most worrisome, hard-to-treat infections now.

The company has said it plans to seek federal approval for it in early 2009, and review can take a year or more. British-based GlaxoSmithKline has a similar vaccine in final-phase testing that targets 10 strains common in Europe and other regions.

In the meantime, parents should continue to have their toddlers get the existing Prevnar, and to use antibiotics only when needed because they don't work against the common cold and overuse worsens the bacteria resistance problem, said Dr. Cynthia Whitney, a pneumonia expert at the CDC.