The vaccine offers protection from heart attacks for people with narrowed arteries, according to a study in the journal Heart.
Those with chronic heart disease, such as heart failure, are already offered the yearly jab on the NHS.
But researchers said UK authorities should now consider extending the programme to everyone aged 50 to 64.
A team from Australia set out to examine whether flu is an unrecognised factor in increasing the risk of heart attacks.
They examined hospital data for 559 patients over the age of 40, around half of whom had suffered a heart attack.
Nose and throat swabs and blood samples were taken when the patients were admitted and again four to six weeks later.
The results showed around one in eight (12.4%) of the heart attack patients had recently had flu, compared with just under 7% in the comparison group. Half of all the patients had had the flu jab that year.
Flu had not been diagnosed in around one in 10 of those who had the infection, suggesting it may be missed in hospital patients who turn up with other health problems, the experts said.
While the research showed that flu did not increase the risk of a heart attack, having the jab appeared to be protective. It cut the chance of having a heart attack by 45%.
Previous research has shown that flu may encourage blood to thicken or prompt an inflammatory response in arteries that are already narrowed, which could lead to a blockage and a heart attack.
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The authors, from the University of New South Wales in Sydney, said: "Even a small effect of influenza vaccination in preventing (heart attacks) may have significant population health gains.
"The potential population health impact of influenza vaccination, particularly in the age group 50-64 years, who are at risk for (heart attack) but not targeted for vaccination, should be further explored.
"Our data should inform vaccination policy and cardiologists should be aware of missed opportunities to vaccinate individuals with ischaemic heart disease against influenza."
Thembi Nkala, senior cardiac nurse at the British Heart Foundation, said: "The flu can lead to serious illness, so getting your flu jab is vital if you have, or are at risk of, heart disease.
"It's why many heart patients who are deemed at high risk are already offered the vaccination as a matter of routine."
Dr John Watson, head of the respiratory diseases department at Public Health England, said: "We welcome this interesting research on the flu vaccination's effects in middle-aged people with underlying conditions.
"In addition to being offered to all those aged 65 years and older, and to pregnant women, the flu vaccination in the UK is offered to all those from six months to under 65 years with underlying chronic conditions that place them at risk of complications from flu.
"This includes those with existing heart conditions.
"The study results are a timely reminder that susceptible patients should have their flu jab this winter, and it will be interesting to see further research into this field as advocated by the authors of this study.
"Consequently, the results of this study support the use of flu vaccination as recommended in this country."