A new study from Kaiser Permanente’s division of research finds that even mildly premature infants (gestational ages of 33 to 36 weeks) have an increased risk of medically attended respiratory syncytial virus (RSV) infection.

The study found that babies born at 37 weeks had 37% increased odds of RSV infection compared to babies 38 to 40 weeks, while babies born at 34 to 36 weeks had 70% increased odds. In contrast, babies born at 41 or more weeks had 14% decreased odds.

According to the researchers, the RSV infection risk is higher among infants exposed to supplemental oxygen or assisted ventilation during the neonatal period, but the need for oxygen is sometimes unavoidable for babies in intensive care. Even after controlling for prematurity, babies in the study who received supplemental oxygen during the birth hospitalization had a 50 % to 120% increased odds of medically attended RSV infection in the first year of life.

"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. We detected an increased risk even in babies born at 37 weeks," said lead investigator Gabriel J. Escobar, MD, a hospital-based pediatrician and research scientist with the Kaiser Permanente Division of Research in Oakland, Calif. "Further research is needed to determine whether strategies to prevent or mitigate RSV infections are indicated in late preterm infants."