More than a million people in England put their health at risk every year by not taking up the offer of a flu jab, the chief medical officer has warned.
The flu jab is usually available from about October each year
Some 58% of the at-risk under 65s, which include those with asthma and diabetes, did not receive the vaccine last year, figures show.
Launching the Winter flu campaign, Professor Liam Donaldson urged eligible individuals to get vaccinated.
Young people are offered the jab but do not believe they are risk, GPs said.
Vaccine campaigns in Northern Ireland and Scotland are set to be launched next week.
It is estimated that around two million people in England under the age of 65 fall into a high-risk category making them eligible for a flu vaccine.
All those over the age of 65 automatically qualify for the free vaccination - and there is a high uptake in this group.
High-risk groups are people with serious heart or respiratory conditions, kidney, or liver disease, diabetes, lowered immunity due to disease or treatment, multiple sclerosis, or conditions of the nervous system, and those who have had a stroke.
In 2006, 58% of younger at-risk people did not get a flu jab, leaving them particularly vulnerable to the side effects of the virus, which can lead to bronchitis, pneumonia, hospitalisation or even death.
As an extra protection against a possible flu pandemic those working in the poultry industry are now also recommended for vaccination.
According to Department of Health figures, flu contributes to over 25,000 excess winter deaths every year and thousands of people are hospitalised due to serious complications.
The past few years campaigns have been overshadowed by shortages and delays of flu vaccine but the Department of Health said there are no reported problems with supplies.
"It is a common misperception that it's only older people who suffer the most when they get flu," said Professor Donaldson.
"Many children and adults under the age of 65 are putting themselves at just as much risk to the effects of the virus.
ARE YOU ELIGIBLE?
Over the age of 65
Resident in a long-stay residential care home
Suffer from diabetes, chronic respiratory conditions or serious heart, kidney or liver disease
Undergoing cancer treatment
Have lower immunity due to HIV or medication, such as steroids
Have had a stroke
"It only takes a minute to get the flu jab, but this will protect you, your child or grandchild for 12 months," he said.
Professor Mayur Lakhani, chair of the Royal College of GPs said prevention was better than cure.
"As a practising GP, I have noticed that young people are not coming forward for the flu jab, mistakenly believing that they're not at risk.
"We would urge young people with chronic illnesses to come forward."
Cathy Moulton, care advisor at Diabetes UK, said having flu can cause blood glucose levels to fluctuate in people with diabetes.
"This can leave people with diabetes open to many health problems including complications of flu such as pneumonia and bronchitis."
Erica Evans, care development manager at Asthma UK, said: "Colds and flu trigger the symptoms of 90% of the millions of people with asthma in the UK and while it is almost impossible to avoid catching the common cold, having a jab can help to prevent the flu virus taking hold."