Children should have the flu vaccine before Christmas to prevent them putting relatives at risk of infection, NHS bosses in England have warned.
Doctors say the virus can spread more easily in 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, officials said.
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.
In England, 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.
In Scotland, the flu vaccine is offered to all primary school children, as well as children aged two to five years of age who are not yet in primary school.
However, children of all ages with a health condition will still be offered the flu vaccine from six months.
In Wales, the vaccine is recommended for children from six months of age.
All children aged two to eight on 31 August 2017 will be offered the nasal spray flu vaccine routinely this year.
And in Northern Ireland, children are offered the flu vaccine if they were born between 2 July 2013 and 1 September 2015. Children at primary school are also offered the immunisation if they were born between 2 July 2006 and 1 July 2013.
According to the latest NHS England 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.
Dr Cosford said the nasal spray vaccine last year substantially reduced children's risk of flu, "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.
Update 6 December 2017: This story now includes details of the situations in Scotland, Wales and Northern Ireland.