A childhood vaccine against pneumonia-causing bacteria introduced in 2000 hasn't made a visible dent in the disease, U.S. researchers.
In the first study to provide national estimates of childhood pneumonia, they found rates of the lung infection had stayed more or less constant between 1994 and 2007.
At the beginning of that period, 19 in 1,000 children got a pneumonia diagnosis at the doctor's office or at an emergency department, compared to 22 in 1,000 at the end.
But that doesn't mean the vaccine -- Pfizer's Prevnar, or PCV7 -- has been useless.
For instance, earlier work found the number of kids who had to be treated for pneumonia at the hospital dropped by more than half after the vaccine became available.
"It's possible that the vaccination has had a major impact on the more serious complications of pneumonia," said Dr. Samir S. Shah of the University of Pennsylvania School of Medicine in Philadelphia, who led the work.
Prevnar protects against a type of bacteria called Streptococcus pneumoniae, or just pneumococcus, which causes several kinds of infections -- including pneumonia, meningitis and middle ear infections.
"If you look at how effective the vaccine was in reducing meningitis and blood infections, it has done a phenomenal job," said Shah, whose study was supported by the National Institutes of Health.
While Pfizer said Prevnar is not licensed to prevent pneumonia in the U.S., it stressed the new study's design might be shrouding possible effects of the vaccine on the disease.
Because pneumonia can be caused by many microbes other than pneumococcus, including viruses, looking at all cases of the disease will overshadow any potential effect Prevnar might have.
"What is not available in this study is what is causing the pneumonia that is being captured in these databases," said Dr. Kim Center of Pfizer. "Part of that difficulty in interpreting studies such as these... is that the results can be subject to a lot of dilution"
More than a million Americans end up in the hospital every year with pneumonia, and in 2007 alone some 52,000 people died from the disease, according to the U.S. Centers for Disease Control and Prevention (CDC).
Among kids under age five, pneumococcus bacteria lead to nearly 5,000 cases a year of meningitis, bloodstream infections and other serious illnesses.
The new report, published in the journal Pediatrics, also found that most doctors used broad-spectrum antibiotics to treat pneumonia, although more targeted drugs such as penicillin are more effective.
"The problem is that if you prescribe a lot of these antibiotics you end up seeing higher rates of drug resistance in the community," Shah said. "If you can shift prescribing away from broad-spectrum antibiotics to narrow antibiotics, that may improve public health."
In the meantime, Shah said Pfizer's new pneumococcus vaccine, PCV13, which replaced PCV7 on the childhood immunization schedule this year, is likely to afford better protection than its predecessor.