Children should be given the flu vaccine before Christmas to prevent them putting older relatives at risk of infection, NHS bosses have warned.
Doctors say the virus can spread more easily at schools and nurseries, which puts grandparents and others at risk of getting ill over the festive season.
Those with heart or lung conditions and pregnant family members can be especially vulnerable, the NHS warned.
Dr Paul Cosford said the vaccine was "quick, easy and painless".
The children's flu vaccine is offered as a yearly nasal spray to young children to help protect them against flu.
Children aged two and three are able to get the vaccine free on the NHS, via GP practices.
An expansion of the scheme means children in reception class and primary school years one, two, three and four are also all eligible for the vaccine.
According to the latest NHS figures, just 18% of school-age children have had the nasal spray immunisation.
Dr Cosford, Public Health England's medical director, said flu causes 8,000 deaths a year in England and Wales.
"The vaccine is the best protection there is against flu," he added.
"The nasal spray vaccine last year reduced children's risk of flu by 65% meaning they were less likely to spread it to relatives and others they come into close contact with.
He called for parents to give consent for eligible school-aged children to receive the vaccine in school.
Prof Keith Willett, NHS England's medical director for acute care, said children were "super-spreaders" and the flu season "traditionally reaches its peak" at Christmas.