Children younger than 5 years old, especially those under the age of 2, are at greater risk of suffering serious influenza-related complications, says the Centers for Disease Control and Prevention (CDC). Study findings published in the European Respiratory Journal now suggest that infants with an older sibling may have a higher risk of being hospitalized for the flu, reports The New York Times.

For the study, researchers examined 1,115 medical records of children under 2 years old who were born in Scotland between 2007 and 2015 and admitted to the hospital. Scientists found that second-born infants younger than 6 months were more than two and a half times as likely to be hospitalized because of the flu than the first born child, while those with two older siblings were three times as likely as an infant with no siblings to end up in the hospital.

In addition, researchers noted that being born during autumn or spring, being born to a mother younger than 30 or being born to a mom who smoked were associated with infant hospitalizations for the flu. (Interestingly, being born during flu season was the only factor that increased a child’s risk of being admitted to the hospital for influenza more than having an older sibling.)

“Targeting vaccination programmes to high-risk children will not prevent the vast majority of influenza admissions,” wrote the study’s authors. “Parents of children aged [less than] 2 years should be advised that vaccination of older siblings will protect younger children against influenza infection.”

While there is no approved vaccine for infants younger than 6 months, the CDC currently recommends annual flu vaccines for anyone older than 6 months. Flu seasons vary in severity each year. Since 2010, the agency estimates that from 7,000 to 26,000 children under 5 were hospitalized in the United States for influenza infection.

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