Having older brothers and sisters puts infants at higher risk for being hospitalized with the flu.

Researchers studied 1,115 hospital admissions of children under 2 born in Scotland from 2007 to 2015.

Compared to a firstborn child, a second-born child under 6 months old was more than two and a half times as likely to be hospitalized. With two older siblings, an infant under 6 months was three times as likely to be hospitalized as one with none. The study is in the European Respiratory Journal.

Maternal smoking, maternal age under 30, and being born in the autumn flu season were also associated with hospital admissions of infants, but only birth in the flu season increased the risk more strongly than having an older sibling.

In the United States, the Centers for Disease Control and Prevention estimates that 26,000 children under 5 are hospitalized every year with influenza, with 37 to 171 deaths yearly since 2004.

There is no flu vaccine approved for use in children younger than 6 months, but the C.D.C. recommends that everyone older than that get a flu shot every year.

“Young children are very good at spreading infections,” said the lead author, Pia Hardelid, a lecturer at University College London, “so by vaccinating them, you are protecting infants and other members of the household as well.”