About 30 million people - nearly half the UK population - are being offered the flu vaccine, in the biggest winter vaccination campaign the NHS has seen.
For the first time, all primary school pupils can have the vaccination free.
Alongside children - so-called super-spreaders - the over-65s, pregnant women and those with existing illnesses will also be offered the vaccine.
Meanwhile, the government said it was confident the flu campaign would not be disrupted by a possible no-deal Brexit.
Manufacturers have been asked to ensure all supplies are in the UK by 31 October, when the UK is set to leave the EU. Normally, deliveries continue into November and December.
Currently, only one supplier - Sanofi - has indicated this deadline will be missed. And one delivery, of one million doses for people with long-term conditions, will not be shipped until November.
But Sanofi added it had contingency plans in place in case of problems using the Dover port - and was prepared to fly the doses in if necessary.
Ministers have also ordered extra supplies from another manufacturer in case of difficulties.
England's deputy chief medical officer, Prof Jonathan van Tam, said there had been "robust" planning and he was confident there would not be any problems for those who chose to be vaccinated.
"We do recognise it is an extraordinary year - that is why we have taken the steps we have," he said.
But Prof Van Tam also dismissed suggestions the UK was set for a bad flu season.
It has been reported Australia, which is coming to the end of its flu season, has struggled and, as a result, the UK could follow suit.
But Prof Van Tam said flu was "unpredictable" and the latest evidence suggested flu cases had peaked earlier than normal in Australia, sparking alarm.
It is only in the past six years the flu vaccine has been offered to healthy children, via a nasal spray, and this year marks the first time it will be available to all year groups at primary school across the UK.
The flu programme was extended to children because they are more likely to spread the virus between themselves and on to older, more vulnerable family members.
Primary pupils can have the vaccine in school, while the other groups can use pharmacies and GPs.
People not in one of the target groups can pay privately to be vaccinated if they wish.
Prof Yvonne Doyle, of Public Health England, urged people to come forward for the vaccine - the NHS was able to vaccinate less than half of those in some of the target groups last year.
"Some people think the flu is like the common cold. It's not. It can be a really serious illness and can be deadly for some," she said.
About 1,700 deaths last year were linked to flu.
Dr Jim McMenamin, of Health Protection Scotland, said: "Getting the vaccine only takes a few minutes and helps to provide protection from flu for around a year."