That's not to say the shot doesn't work -- it does, at least in the narrow goal of preventing severe and potentially deadly blood infections that often complicate pneumonia. But experts say the study underscores the need for new vaccines that can do a better job of blocking all sources of pneumonia, one of the country's most prolific infectious killers.
Pneumonia leads to between 350,000 and 620,000 hospitalizations of people 65 and older each year in this country. Many of those cases result from infection with Streptococcus pneumoniae, though others are caused by different bacteria, as well as fungi and viruses. In 1999, according to the U.S. Centers for Disease Control and Prevention (CDC), roughly 90,000 Americans died of pneumonia.
Regulators have approved two forms of pneumococcal vaccines. One targets the sugary outer membrane of the bacteria and is meant only for adults; the other, called a "7-valent protein-conjugated vaccine," is specifically designed for young children, in whom the germs can cause ear infections, meningitis and other diseases.
The CDC urges seniors to be vaccinated against pneumococcus. The latest study says the shot does prevent pneumonia that leads to serious bloodstream infections. However, it also suggests the shot isn't effective at blocking pneumonia that doesn't involve pneumococcus germs in the blood, a condition called bacteremia. Bacteremia can destroy heart valves and other organs.
"People should continue to get the vaccine as it's recommended because it prevents bacteremia," says study author Dr. Lisa Jackson, an adult immunization expert at the Center for Health Studies in Seattle. "But we need to work harder in developing new vaccine formulations in adults if we want to make any headway against pneumonia overall."
Jackson's study, which appears in the May 1 issue of the The New England Journal of Medicine, followed more than 47,000 Americans aged 65 and older for three years to see how getting vaccinated against pneumococcus affected their risk of infection.
Having the shot cut the odds of developing a blood infection with the germ by about 44 percent. But it was linked with a slight increase in the risk of being hospitalized with any form of pneumonia, and it didn't reduce the chances of being diagnosed with pneumonia acquired either in a hospital -- a common means of infection -- or in the community.
A second study in the journal tracked cases of pneumococcal illness -- not just pneumonia -- in children and adults before and after the approval of the conjugate vaccine in 2000.
In 1998 and 1999, there were about 24 cases of serious disease per 100,000 people each year in this country. In 2001, there were 17 cases of severe pneumococcal infections per 100,000 people. The drop was greatest among children under age 2, falling nearly 70 percent, from 188 cases per 100,000 to 59 per 100,000.
The incidence of serious pneumococcal disease also fell in adults, though less impressively. Among seniors, the rate dropped 18 percent. The combined vaccine also led to a significant decline in cases of pneumonia caused by drug-resistant strains of pneumococcus.
"What that vaccine does is it prevents the disease in children and through reduced transmission [of pneumococcus] adults are also getting less disease," says Dr. Cynthia Whitney, an epidemiologist at the CDC and lead author of the second study.