TUESDAY, May 5 (HealthDay News) -- Babies born just a few weeks premature still have a much greater chance of developing a respiratory tract infection, a new study has found.

Researchers for Kaiser Permanente found an increased risk for the infection -- known as respiratory syncytial virus, or RSV -- especially among premature infants who are put on oxygen or ventilation shortly after birth.

RSV is the most common cause of inflammation of the small airways in the lung, called bronchiolitis, and pneumonia in children in the United States younger than 1 year, according to the U.S. Centers for Disease Control and Prevention.

Babies born at 37 weeks, one week shy of what is considered full term, had a 37 percent greater risk of developing an RSV infection than full-term infants; babies born during weeks 34 to 36 had a 70 percent greater risk.

Babies put on supplemental oxygen while still in the hospital after birth had a 50 percent to 120 percent increased chance of developing a medically attended RSV infection during their first year of life.

Though the risk is higher among infants exposed to supplemental oxygen or assisted ventilation during the neonatal period, the researchers said that the need for oxygen is sometimes unavoidable.

"Although extreme prematurity is a known risk factor for severe RSV infection, this study helped us to learn more about risk factors for RSV infection among mildly premature infants," Dr. Gabriel J. Escobar, a pediatrician and research scientist with the Kaiser Permanente Division of Research in Oakland, Calif., and the study's lead investigator, said in a news release from health-care company. "We detected an increased risk even in babies born at 37 weeks."

The results of the study, which involved 108,794 babies discharged from six hospitals during an almost seven-year period, were to be presented Tuesday in Baltimore at the annual meeting of the Pediatric Academic Societies.

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• Respiratory Syncytial Virus Infections