The vaccinations cut heart attacks by up to 85 percent – and halve the number of strokes, say scientists.
A study of high risk hospital patients also found deaths from any cause fell by almost three quarters among protected over 50s. It was based on more than seven million patients – one of the largest of its kind.
Most people in England are to be offered it free this year because of the pandemic.
Lead author Roshni Mandania, a medical candidate at Texas Tech University, Dallas, said: “The results we found are staggering. It’s hard to ignore the positive effect the flu vaccine can have on serious cardiac complications.
“Some people don’t view flu vaccinations as necessary or important, and many may face barriers accessing health care including receiving the flu vaccine.”
Being immunised against flu helps prevent chest infections – which weakens the immune system. But take up among high risk groups – such as over 50s and care home residents – is extremely low.
So Ms Mandania’s team compared its impact between the 168,325 participants who got vaccinated, and the vast majority who did not.
Immunised over 50s were 85 and 28 percent less likely to suffer a cardiac arrest or heart attack – caused by electrical and circulatory problems in the organ, respectively.
They also had a 47 percent lower risk of a mini stroke, or TIA (transient ischaemic attack). Overall, mortality rates fell by 73 percent.
The stress flu puts on the body is well known – and can cause a heart attack or stroke, explained the researchers.
But a virtual American Heart Association meeting was told this group was much less likely to be vaccinated compared to the general population – 1.8 versus 15.3 per cent.
Vaccination rates for HIV/AIDS patients, care home residents and the obese was also around two percent – compared to roughly nine percent for others.
Ms Mandania said: “These groups should have the highest vaccination rates because they are the most at risk. However, our findings show the opposite – flu vaccinations are under-utilised.”
“As health care providers, we must do everything we can to ensure our most vulnerable populations are protected against the flu and its serious complications.”
She used the 2014 National In-patient Sample to assess the rate at which the jab was administered to those considered high risk for flu and its complications.
Dr Eduardo Sanchez, the American Heart Association’s chief medical officer for prevention, said most people should be vaccinated each year.
He said: “We have partnered with the American Lung Association and the American Diabetes Association to collectively deliver a message to providers and to the general public that all adults and all children, by and large, should be getting influenza vaccinations year after year.
“In particular, for patients who have chronic diseases like high blood pressure, diabetes or emphysema, it’s critically important to get the annual flu vaccine. The potentially serious complications of the flu are far, far greater for those with chronic diseases.”
The NHS flu programme is to add all over-50s, people shielding and those who live with them and children in their first year of secondary school.
This will involved about 30 million people. It is to prepare for a winter that could see the annual flu season coincide with a surge in coronavirus.
Heart and circulatory diseases cause more than a quarter (27 per cent) of all deaths in the UK – nearly 170,000 each year. Flu claims around 10,000 lives annually, although this can rise to over 30,000 in a bad year.