Immunizing children with Prevnar to guard against life-threatening meningitis, pneumonia and blood infections can also protect adults who have not been vaccinated, a new study shows.

The vaccine, made by U.S. drug company Wyeth, is recommended for children under the age of 2, and in 2004 alone it prevented 13,000 infections, said Cynthia Whitney, chief of the respiratory disease branch of the Centers for Disease Control and Prevention in Atlanta, who helped write the study.

One reason is that the vaccine produced what biologists call the “herd effect,” where it becomes harder for disease to spread if enough people are vaccinated.

With toddlers vaccinated with Prevnar, the risk of others developing a pneumococcal illness was reduced, and the likelihood of being infected by a strain resistant to antibiotics was cut in half, the Whitney team calculated.

“The amount of disease it prevents is amazing,” Whitney said. “Most of the cases we ended up preventing were in adults, because there are so many more of them in the population.”

The study, published in this week’s New England Journal of Medicine, also found that since Prevnar hit the market in 2000, the number of infections resistant to penicillin or multiple antibiotics dropped by 57 percent or more from 1999 to 2004.

“It was almost like a pleasant side effect that the resistant strains went away,” Whitney said.

The effect was even more dramatic in children, where the number of resistant cases declined by at least 80 percent.

But the researchers also warned that less-common strains of the bacteria, ones equally capable of developing resistance to antibiotics, may be taking the place of the strains the vaccine was designed to prevent.

They found that forms of the bacteria not covered by Prevnar, particularly a strain known as serotype 19A, were becoming more prominent. Such strains are also capable of developing resistance to antibiotics.

In 1999, 19A was responsible for just 2 percent of the antibiotic-resistant pneumococcal strains seen in children under the age of 2; by 2004, it was seen in 35 percent.

“This replacement disease has the potential to reduce the overall benefit of the vaccine against resistant infections,” the researchers wrote.

Therefore “it is important to continue to not overuse antibiotics,” Whitney said.

Because of these results, other countries have begun moving toward recommending or requiring the vaccine for infants and toddlers.