A flu vaccine nasal spray is being offered to every two and three-year-old in England, Scotland and Wales for the first time.
All two and three-year-olds in England and Wales will be also offered the vaccine this winter. In Wales, children aged 11 to 12 will also be eligible, while children aged between two and 10 in certain areas of England will be offered protection.
In launching Scotland's largest ever immunisation campaign, the country's senior medical officer said the spray was more effective in children than injections, as well as simpler to administer.
For the first time, all two and three-year-olds in Scotland, about 120,000 children, will be offered the vaccine, as well as 100,000 primary school pupils in health board areas which are taking part in a pilot programme.
Better to be safe than sorry
Speaking after receiving his own vaccine to launch the campaign, First Minister Alex Salmond said it was better to be safe than sorry.
"As an asthmatic, I get my flu vaccination every year to make sure I'm protected and ready for the winter and I'm delighted to launch this national campaign," he said.
"It is hugely successful and the existing programme has seen 2,000 fewer hospitalisations and 25,000 fewer GP consultations.".