Small children are significantly more at risk of serious illness from influenza if they have older brothers or sisters, research shows.
Babies and toddlers are more likely to be admitted to hospital with flu complications if they are not the first born in their family.
Children are “effective spreaders” of respiratory infections and can easily pass viruses to their vulnerable younger siblings, researchers say.
Parents can help protect their young children by getting older offspring vaccinated.
Between three per cent and 11 per cent of children under two in developed countries acquire flu-associated illnesses each year, placing a heavy burden on health services.
Lead scientist Dr Pia Hardelid, of University College London’s Great Ormond Street Institute of Child Health, said: “Flu can be a serious infection in very young children but at the moment there is no vaccine approved for babies under six months.
“This means we need to look at other ways to minimise the risk of infection.”
The study examined health and birth records of almost all children born in Scotland between October 2007 and April 2015 – about 400,000.
Compared with being first born, children under six months who had one older sibling were twice as likely to be admitted.
The risk tripled for those with two or more older siblings.
Hospital admission rates were also higher for children aged six to 23 months with older siblings but the birth order association for this age group was much weaker.
Babies born between July and December, who would be very young and vulnerable at the start of the flu season, were also at greater risk of hospitalisation.
The findings are reported in the European Respiratory Journal.