Most people in England, about 30 million, are to be offered a free flu vaccine this year, the government says.
It is to prepare for a winter that could see the annual flu season coincide with a surge in coronavirus.
The NHS flu programme will add all over-50s, people shielding and those who live with them and children in their first year of secondary school.
Prime Minister Boris Johnson said he wanted "everyone to have a flu jab in the run up to this winter".
He said it was important to "protect the NHS in the winter months because we've still got Covid and the threat of a second spike of Covid, and its vital therefore to keep that pressure off the NHS by everybody getting a flu jab."
How bad will flu and coronavirus be?
Flu, which can be deadly or need hospital treatment, poses additional threats during the pandemic:
- There is some evidence a double infection with coronavirus and flu is more deadly than either alone
- A big flu season combined with coronavirus could overwhelm hospitals
- If lots of NHS or care-home staff are sick with flu, then it may not be possible to respond to Covid-19 in the same way as during the peak in spring.
But it is impossible to predict how big a flu season we will have.
Australia, which is currently in its winter, is either having a very mild or very late flu season.
Social-distancing and hygiene measures in place to stop coronavirus should, in theory, also have an impact on the amount of flu going around.
Who will be offered the flu vaccine?
- people who were required to shield from coronavirus and anyone they live with
- people with some medical conditions including diabetes, heart failure and asthma
- pregnant women
- pre-school children over the age of two
- all primary school children, as last year, and, for the first time, Year 7 pupils
- initially all people over 65, before the programme is extended to the over-50s
The NHS will get in contact with those who are eligible.
It wants to ensure the highest risk patients are at the front of the queue when the immunisation programme starts in September.
Those aged 50-64 will be offered the jab later, with the full details still to be announced.
Last year, around 15 million people in England were vaccinated as part of the NHS campaign, far fewer than the 25 million who were invited to have it. This year more than 30m doses have been ordered.
The vaccine will also be available privately in pharmacies and some supermarkets to adults who are not eligible for an NHS jab. It usually costs around £20.
A Scottish government spokesperson said it was "examining the best options" for expanding eligibility for its flu vaccine programme which would be set out when it had "confidence that there is adequate supply of vaccines in order to delivery on any commitments we make".
Plans for Wales and Northern Ireland have not yet been announced.
How do flu and coronavirus symptoms compare?
The symptoms of flu include a fever and a dry cough.
By now this should sound familiar - as they are the key signs of Covid-19 too.
Other symptoms such as fatigue, aches, a sore throat or headaches are also shared by both infections.
And there are also other winter viruses, such as rhinovirus and RSV, that can also lead to similar symptoms.
This could create a problem come winter - as far more people would need testing to work out whether they have had coronavirus and their contacts needed tracing.
This is why the government has already announced plans to increase testing capacity to half a million a day.
Today's announcement on vaccines aims to suppress the flu season so numbers are as low as possible.
GP Dr William Bird, an expert on preventative health, told BBC Breakfast it was right to offer the flu jab more widely, but that logistics would be a big problem.
He said flu clinics were already run like a "military operation" with people "bunched together" - and social distancing would complicate matters.
Plans are also being put together for ensuring coronavirus does not disrupt vaccination by, for example, causing schools, which deliver the childhood programme, to shut.
Prof Chris Whitty, the chief medical advisor, said: "This winter more than ever, with Covid-19 still circulating, we need to help reduce all avoidable risks.
"Vaccinating more people will help reduce flu transmission and stop people becoming ill."
Children are "super-spreaders" of the flu virus, but the expansion of the childhood programme had not been anticipated before the pandemic.
The plan had been to assess the impact of vaccinating all primary school children, achieved for the first time last year, before deciding what to do next.
Meanwhile, there will be a renewed push to persuade health and care workers to have the vaccine.
Last year, 74% of staff were immunised.
Paulette Hamilton, from the Local Government Association, said: "Extending the free winter flu jab to as many people as possible is essential if we are to tackle two potentially deadly viruses head-on and prevent one seasonable and predictable virus creating the conditions for a second, more serious and unpredictable deadly second wave of Covid-19.
"It is absolutely critical that all our health and care workers get themselves vaccinated, to protect both themselves and the people they look after, including our elderly and most vulnerable, from a potentially devastating second wave of infections."
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