Children under two years are more likely to be admitted to hospital with influenza if they have an older sister or brother, according to UK researchers.
Those behind a new study have concluded that babies and young children can be better protected if parents take up the opportunity to get older siblings vaccinated.
“Children are very effective spreaders of respiratory viruses like flu”
The study, published today in the European Respiratory Journal, involved almost all children born in Scotland between October 2007 and April 2015, about 400,000 in total.
Researchers used hospital admissions and laboratory data to find children who had a record of a positive test result for flu.
They then compared the data against other routinely collected information, such as month of birth, whether the children were premature or had other conditions, and whether they had older siblings.
The results showed that children under six months old with older siblings were more than twice as likely to be admitted to hospital with flu than children 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.
“There is some evidence that maternal vaccination during pregnancy can protect young babies”
For those with two older siblings, there were two extra admissions for every 1,000 children. Almost half of flu hospital admissions in babies under six months old could be explained by older siblings.
In addition, the researchers found the risk was higher for babies born between July and December, who would be very young at the start of the flu season.
Rare chronic conditions, such as congenital heart disease or chronic lung disease, also raised the risk of hospitalisation, but only a minority of children who had been hospitalised had these conditions.
The researchers noted that in 2013 the UK began to introduce a new routine flu vaccination for children aged between two and 16 years.
They highlighted that this development could also have an important effect on babies and younger children.
The researchers said they hoped to continue to study the area to see if the introduction of the vaccine for children aged two years and older also brings down serious infections in the under twos.
The study was led by Dr Pia Hardelid, a lecturer in epidemiology at the UCL Great Ormond Street Institute of Child Health, and funded by the National Institute for Health Research.
Older siblings pose risk of serious flu for babies and toddlers
She noted that the lack of a vaccine approved for babies under six months meant there was a need to look at other ways to minimise the risk of infection.
“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 Dr Hardelid.
“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,” she said.
She added: “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.”