WEDNESDAY, March 19 (HealthDay News) -- Pneumonia caused by the Staphylococcus aureas bacterium and caught outside of the hospital environment may be more common in the United States than previously believed, preliminary research suggests.

"Over the last few years we have been receiving reports of a severe CAP [community-acquired pneumonia] caused by S. aureus. There are a lot of questions about this disease, but until now there have primarily been case studies which tend to highlight the severest of cases and may present a biased picture," Alexander Kallen, U.S. Centers for Disease Control and Prevention lead researcher, said in a prepared statement.

The CDC team looked for cases of S. aureus CAP in 2006-07 influenza season data from three Atlanta-area pediatric hospitals. The researchers found 53 such cases, which was more than they expected.

"No one really knows what the true incidence of S. aureus CAP is. People suspect that S. aureus causes 3 percent to 5 percent of all CAP cases, but the number of cases per month we found suggest that these rates of S. aureus CAP might be higher than previously estimated," Kallen said.

The CDC researchers also found that the fatality rate in S. aureus CAP cases was about 13 percent -- much lower than some previous estimates of between 30 percent and 50 percent.

In addition, they found that about half of the S. aureus CAP cases were caused by methicillin-resistant S. aureus (MRSA). The finding was "not unexpected but quite concerning," Kallen said.

The CDC team was alarmed that nearly 40 percent of the children with MRSA CAP didn't receive antibiotics that could combat the resistant strain.

"The fact that a lot of these kids who had MRSA were not treated with antibiotics that have activity against MRSA suggests that clinicians are not recognizing this organism as a cause of CAP during influenza season," Kallen said.

More research is needed to gain a better understanding of the patterns of S. aureus CAP, he noted.

The study findings were to be presented Wednesday at the International Conference on Emerging Infectious Diseases, in Atlanta.

More information

The American Lung Association has more about pneumonia.