Dr. Marie Griffin, a professor of preventive medicine at Vanderbilt University, and colleagues analyzed data from 1997-2009 in a national database of hospitalizations.
The vaccine was meant to prevent blood and ear infections from pneumococcus bacteria, but pneumonia also results from pneumococcus infection, Griffin said.
Griffin said older people, who have weaker immune systems, could be infected by children who carry the bacteria but are healthy.
"This vaccine not only decreased pneumonia in children, it also decreased pneumonia hospitalizations in older adults," Griffin said in a statement. "Pneumonia is a leading cause of hospitalization in the United States. The protective effect we saw in older adults, who do not receive the vaccine but benefit from vaccination of infants, is quite remarkable. It is one of the most dramatic examples of indirect protection, or herd immunity, we have seen in recent years."
The study, published in the New England Journal of Medicine, suggested this herd immunity is an even more effective prevention for the elderly than the vaccine currently recommended to prevent pneumonia in older adults.