Small children are significantly more at risk of serious illness from influenza if they have older brothers or sisters, new research has shown.
Babies and toddlers are more likely to be admitted to hospital with flu complications if they are not the first born in their family, the study found.
Children are “effective spreaders” of respiratory infections and can easily pass viruses onto their vulnerable younger siblings, said the researchers.
Parents were told they can help protect their young children by getting older offspring vaccinated.
Small children risk lung infections and breathing difficulties if they catch flu. They may also develop a very high fever, leading to fits.
Between 3% and 11% 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, from 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 - some 400,000 in total.
Of the whole group, more than 1,000 children were admitted to hospital after catching flu.
Compared with being first born, children under six months who had one older sibling were twice as likely to be admitted. For those with two or more older siblings, the risk tripled.
Rates of hospital admission 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.
In 2013, a new flu vaccine nasal spray for children aged two to 16 was introduced in the UK. The vaccine could be important for protecting babies and younger children, said the researchers.
Dr Hardelid said: “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.
“The nasal spray vaccine, which is now being offered in GP surgeries and primary schools in the UK, provides a good opportunity to protect the children who receive it, as well as their younger siblings.
“There is not much parents can do about the time of year their baby is born, but women can also help reduce the risk of serious flu for their newborns by taking up the invitation to have a vaccine when they are still pregnant.
“There is some evidence that maternal vaccination during pregnancy can protect young babies from flu infection.”