Having older brothers and sisters puts infants at higher risk for being hospitalised with the flu. Researchers studied 1,115 hospital admissions of children under two born in Scotland from 2007 to 2015. Compared to a firstborn child, a second-born child under six months old was more than two and a half times as likely to be hospitalised. With two older siblings, an infant under six months was three times as likely to be hospitalised as one with none. 

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 five are hospitalised every year with influenza, with 37 to 171 deaths yearly since 2004. There is no flu vaccine approved for use in children younger than six months, but the CDC 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.”