The flu jab is given in the run-up to winter
People who underestimate the dangers of flu could be putting their lives at risk, says the man in charge of vaccinations in England.
A survey of more than 1,000 people found that a third of them thought flu was the same as a heavy cold.
Professor David Salisbury is urging over-65s and other "at-risk" groups to visit their GP for a flu jab.
The advice was backed by charities, which stressed complications could be serious or even fatal.
Because strains of flu viruses change every year, vulnerable people need to be vaccinated every 12 months.
The vaccine is available free to older people, and those with chronic conditions including heart disease, asthma and diabetes.
Similar arrangements are available in other parts of the UK.
Professor Salisbury, the director of immunisation for the Department of Health, said that last year, fewer than half of eligible people under 65 took advantage of the jab.
The survey also found that many people believed so-called "old wives' tales" about avoiding and treating the virus.
More than one in four believed that "feeding a cold" and "starving a fever" was appropriate advice, while one in three said vitamin C could cure it, despite no evidence this is the case.
One in 20 thought that carrying garlic could ward off flu, and a similar number thought that the flu jab itself could cause the illness.
The vaccine does not contain any live viruses, so this is impossible.
Professor Salisbury said: "The research shows that common colds are frequently confused with flu, but for some the flu virus can be potentially life-threatening.
"We are urging those at greater risk - including people suffering serious heart problems, asthma and diabetes - to get their flu jab from their GP. The flu jab can literally save lives."
Heart attack or stroke
Diabetes UK, Asthma UK and the British Heart Foundation (BHF) all backed the jab.
June Davison, from the BHF, said that flu could, in extreme cases, trigger a heart attack or stroke in those who already had heart problems.
And Vikki Knowles, from Asthma UK, said that people with asthma should discuss flu jabs with their GP before the "flu season" - normally at its height in midwinter - gets under way.
The survey results do not inspire any confidence that infected people will be able to keep the virus to themselves.
Almost half of those questioned did not know they should cover their mouths when sneezing, and wash their hands after coughing into them.
Even more people did not know that carrying used tissues or handkerchiefs was a good way to spread the illness.