Complacency over the flu jab risks overwhelming the NHS, experts warn, as data reveals the scale of the challenge in expanding the vaccination programme.
Last month, the government announced plans to double the amount of people who receive the influenza jab.
But BBC analysis has found the take-up rate among those people in vulnerable groups eligible for a free jab has declined in recent years.
Academics say more needs to be done to explain the severity of the flu.
The government wants to increase the number of people vaccinated from 15 million to 30 million amid fears coronavirus cases will rise again in the autumn.
Local authorities in England saw an average 45% of people with serious health conditions under 65 take up the offer of a free vaccine last winter, data shows. That represents a drop from 50% in 2015.
The UK government has an ambition to vaccinate 55% of people in vulnerable groups, which includes people with multiple sclerosis (MS), diabetes or chronic asthma. The World Health Organisation (WHO) has previously said countries should vaccinate 75% of people in “vulnerable” categories.
Public Health England says the jab – which in recent years has been freely available to over 65s, those with long running health conditions and pregnant women, among others – is the most effective way to reduce pressures on intensive care units.
‘The vaccine is more vital than ever’
The prospect of a surge in flu cases over winter is also of concern to many people still suffering from what has been dubbed “long Covid”.
That is where coronavirus patients experience symptoms such as extreme fatigue and breathlessness long after two weeks.
Jeanne Jarvis-Gibson, a digital culture and communications student at the University of Liverpool, is one of many long Covid sufferers to join a growing online community.
The 27-year-old, from Washington in the US, used to be a keen runner – but now has to take a steroid inhaler and struggles to walk up stairs at her university halls.
She said: “I think getting the flu vaccine is vital now more than ever with the possible co-mingling of Covid and flu.
“I would think about it in the same way as wearing a mask when you go out. It’s about mutual respect for people.
“It’s more than just your own health – we have to think about everyone in our society.”
Dr Tonia Thomas, project manager at the Vaccine Knowledge Trust, which is part of the University of Oxford, said: “People think the flu is not that bad, that is even for people who are in the risk groups.
“They are leading healthy lives in terms of day-to-day living. I have spoken to patients who say they forgot they are in a risk group. It is only when they contract an infection that they realise their body responds differently to other people’s.”
The uptake rate among over-65s has remained consistent in recent years, however. Last winter, 73% took up the offer.
How will flu vaccines work this year?
By BBC Health Correspondent Smitha Mundasad
Health officials are ramping up efforts to ensure everyone who needs a flu vaccine gets one this year – with around 30 million people being offered them in England’s largest ever flu immunisation programme.
But how will it work? And will things be worse this year?
Flu, which can be deadly or need hospital treatment, poses additional threats during this year – as a big flu season combined with a fresh surge of coronavirus cases could overwhelm hospitals.
Between September 2019 and February this year, almost 8,000 people died from flu in England alone.
But the number has varied. In 2017 and 2018, more than 22,000 people died from the virus.
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During the 2019-20 flu season, 3.16 million people under the age of 65 with a serious health condition were vaccinated, 130,000 more than there were five years before.
But taking into account factors such as population growth, the proportion of vulnerable people receiving the jab has fallen.
Figures from the last winter flu season show local authorities in England averaged just 45%. In Scotland it was 43%, Northern Ireland 52% and Wales 44%.
London and the east of England are suffering from particularly low uptake rates among vulnerable patients and in schools.
Vaccine uptake across England
(%) Among under 65s in clinical risk category
Only 29% of people under the age of 65 with a serious health condition took up the jab in Hammersmith and Fulham last winter.
While the government is aiming for a "universal" uptake of the flu vaccine among health workers, it has also remained historically low in some parts of the NHS and, in particular, mental health care providers.
At West London NHS Trust, which looks after mental health patients in Hammersmith and Ealing, some 1,471 of its 2,880 frontline staff took the jab last winter.
"Our current position remains unacceptably low," said Stephanie Bridger, director of nursing and patient experience.
"This year - in the context of Covid-19 - we are redoubling our efforts to promote the critical importance of having the flu vaccination as early as possible."
Prof Heidi Larson, of the London School of Hygiene and Tropical Medicine, is a world-leading expert on attitudes towards immunisation and a director of the International Vaccine Confidence Project.
While many fear the jab or nasal spray can cause illness, she believes most underestimate the severity of flu.
"I find there is almost a complacency around the vaccine in the UK," she said.
"It doesn't have the same aggressive 'anti' sentiment we see against some of the childhood vaccines."
Early surveys carried out by the Vaccine Confidence Project show that, in Europe, there is growing support for the flu vaccine as people are concerned about a second Covid-19 wave.
However, Prof Larson said many do not take the vaccine for legitimate reasons, particularly as the nasal spray given to children contains porcine gelatine.
It is not suitable for vegetarians and, last year, the Muslim Council of Britain said it was not permitted in Islam except under exceptional health circumstances.
"There are also people that say it doesn't work enough, it's not effective enough," Prof Larson added.
"Some years it really isn't that effective against all strains.
"But I would certainly urge people to take it anyway as you wouldn't want the strains that it does protect against.
"It's also so important to limit the stress on the system.
"Two or three years ago the Red Cross had to come in to handle a bad flu season on its own."
Previous government flu campaigns have also failed to address the "myth" that the vaccine can cause illness, according to leading virologist Prof Wendy Barclay, head of the department of infectious diseases at Imperial College London.
She believes this puts many parents off allowing their children to take the nasal spray.
In the capital, only 50% of eligible schoolchildren had the free vaccine last winter.
Prof Barclay said: "My issue with Public Health England is if they don't make the message strong enough - so parents think: 'It's only the flu, my kids will be fine'."
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: "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."
A spokeswoman for the Department for Health and Social Care said the UK had some of the best flu vaccine uptake rates across Europe.
She said: "Flu vaccine uptake rates in Europe and the flu vaccination programme will be supported with a major new public facing marketing campaign to encourage take up amongst eligible groups for the free flu vaccine.
"With Covid-19 still likely to be circulating during flu season, it is more important than ever to have the flu vaccination and we urge anyone who is eligible to take up the offer of vaccination to help relieve pressure on the NHS and save lives."
Additional reporting by Anna Khoo and Daniel Wainwright.