Meningitis is an infection of the protective membranes that surround the brain and spinal cord.  The illness is often fatal, and when it's not, children who get meningitis frequently are left with severe brain damage.

In 2001, pharmaceutical companies introduced a new vaccine - PCV7 - to target seven common forms of meningitis-causing pneumococcus bacteria. Since then, most children in the United States have received this vaccine.

Dr. Lee Harrison from the University of Pittsburgh led a team of researchers from all over the United States to evaluate PCV7's effectiveness after seven years of use.

"We had about 18.5 million people under surveillance," Harrison says. "Within that population, we had 1,379 pneumococcal meningitis cases."

Harrison and his colleagues found that overall, those 1,379 cases represented about a 30 percent decline in pneumococcal meningitis cases, compared to a similar time period when the vaccine wasn't available.

"In the targeted population, for example, children under two years of age, there was a 64 percent decline in pneumococcal meningitis," Harrison reports.  "And remarkably, we showed that among adults 65 years of age and older, there was a 54 percent decline in pneumococcal meningitis? so this is a group that is not even receiving this vaccine."

Harrison explains that because there were fewer children with pneumococcal disease, these older people were less likely to catch it from the children. This is referred to as "herd immunity."

"We suspected that there was going to be some of this herd immune effect, but we were surprised by how large of an impact the vaccine has on the unimmunized population," he says.

Harrison says there were a disturbingly large number of meningitis cases caused by pneumococcal strains that were not targeted by the vaccine and were also resistant to antibiotics . He says many of those cases should be addressed by a broader version of the vaccine that is due out in several years.

Harrison's research is published in the New England Journal of Medicine.