Babies born between July and December were at a high risk as they would be very young at the start of the flu season.
Babies with an older sister or brother are more than twice as likely to be admitted to hospital with influenza as compared to children without older siblings, according to a study published today.
Flu can be serious in very young children because it can cause lung infections and breathing difficulties. It can also cause a very high fever, leading to fits called febrile convulsions.
Researchers from University College London (UCL) in the UK studied almost all children born in Scotland between October 2007 and April 2015, about 400,000 in total.
They used anonymised hospital admissions and laboratory data to find out which children had a record of a positive laboratory test result for flu.
The researchers then compared this with other routinely collected information, such as month of birth, whether they were premature or had other health conditions, and whether the children had older siblings.
The results, published in the European Respiratory Journal, showed that children under six months old with older siblings were more than twice as likely to be admitted to hospital with flu than those who did not have older siblings.
Among children under six months who had one older sibling, there was around one extra hospital admission for every 1,000 children compared to children who did not have any siblings, researchers said.
For those with two older siblings, there were two extra hospital admissions for every 1,000 children. Almost half of flu hospital admissions in babies under six months old could be explained by older siblings, they said.
Researchers noted the risk was also higher for babies born between July and December, who would be very young at the start of the flu season.
"Children are very effective spreaders of respiratory viruses like flu. Our study suggests that older siblings pose a risk of serious infection for their baby sisters and brothers," said Pia Hardelid, a lecturer at UCL.