The vaccine, Prevnar, targets infections caused by pneumococcus bacteria, including pneumonia, blood poisoning, meningitis and ear infections. Approved in 2000, it is the only pneumonia vaccine for infants and toddlers and is recommended for all children under 2.
During its first full year in use, the rate of blood infections and meningitis dropped by nearly 70 percent in children under 2, the researchers reported. There were also declines in those diseases in unvaccinated adults.
"The vaccine is working. It is not only preventing diseases in high-risk children but also in their families," said Dr. Cynthia Whitney, who led the study for the Centers for Disease Control and Prevention.
The findings appear in Thursday's New England Journal of Medicine. A second study in the journal looked at another pneumonia vaccine, which is recommended for those over 65. In that three-year study of 47,365 people, the vaccine cut the risk of serious blood infections almost in half, but didn't prevent pneumonia in the elderly.
The researchers said that vaccine is beneficial for its protection against blood infections.
"It's a very safe vaccine. It's not very expensive, and in general, people only need to get it one time. It's worth getting," said Dr. Lisa A. Jackson, who led the CDC-funded research at the Group Health Cooperative, an HMO based in Seattle.
Pneumococcus bacteria is carried in the nose and throat of healthy people, and is spread from person to person. The very young and the elderly are most vulnerable, as well as people with medical conditions that weaken their immune system and their ability to fight the bug.
Until the infant vaccine was introduced, there were up to 135,000 annual hospitalizations for pneumonia and 60,000 cases of blood infections, including 3,300 cases of meningitis, according to the CDC.
To measure the vaccine's impact, the CDC study tracked blood poisonings and meningitis in seven parts of the country. How many babies have been immunized isn't known yet, but the vaccine has been widely adopted. Four doses are needed at $61 each, according to the vaccine maker Wyeth Pharmaceuticals.
From 1998 to 2001, the findings show, the rate of blood infections and meningitis dropped by 69 percent in children under 2 from 188 cases to 59 cases per 100,000. The rate fell 44 percent for 2-year-olds but there was no change for older children.
The vaccine had previously been shown to be effective in controlled studies.
"It's nice to see in real life it's the same," said Whitney.
In adults, the disease rate dropped as much as 32 percent in those 20 to 39, suggesting that there was less transmission between children and adults, the researchers said.
There were also 35 percent fewer infections caused by strains resistant to penicillin.
"These are very important findings and if they can be confirmed more broadly and more directly ... that's really powerful stuff," said Dr. Greg Poland, director of the Mayo Clinic Vaccine Research Group, who was not involved in the studies.
He said doctors need to explain to elderly patients that the vaccine used in adults might not protect them against pneumonia but does prevent more lethal diseases.
Jackson said a different vaccine may be needed for the elderly, and one possibility is the infant's vaccine. It protects against seven types of pneumococcal bacteria but works differently than the older vaccine, which is designed to guard against 23 types.
The medical director of adult vaccines for Merck & Co., which makes the Pneumovax 23 vaccine, said it targets the most common but not all the strains that cause pneumonia. Nor does it guard against other causes of pneumonia which were included in the study, said Dr. Joan Benson. The vaccine costs about $16.