It is like riding a bicycle to avoid the traffic and accidentally getting fit.

People who get the flu vaccine are less likely to have a heart attack.

A study by University of NSW researchers has found that people who have been vaccinated against influenza are 29 per cent less likely to have a heart attack - representing a greater protective effect than ceasing smoking and nearly as much as taking statins.

They are now advocating that the flu vaccine be provided free of charge to everybody aged over 50, extending the current scheme that makes it available to people over 65.

Senior author Raina MacIntyre said the role of the flu vaccine in preventing heart attacks had not been taken into account in previous cost-benefit analyses of broadening the national flu vaccination program to people aged between 50 and 65.

But giving universal access to the flu vaccine to anybody aged over 50 was likely to save many more lives.

People who had heart attacks were twice as likely to have recently suffered the flu.

"The debate needs to be reframed around the fact that coronary heart disease is the leading cause of death in the world and in Australia and we've already got good preventions," Professor MacIntyre said.

"This is just low-hanging fruit. To me it's a no-brainer."

The association between heart attacks and flu has been established since the 1930s, when it was observed that the number of heart attacks peaked at the height of the flu season.

Previous research indicates that the flu may cause the heart to beat faster and the blood to clot more easily, causing blockages in already partially obstructed arteries.

People whose arteries are 80 to 90 per cent blocked will show symptoms of coronary artery disease, but people whose arteries are only 50 or 60 per cent blocked may show no signs of ill health until they are struck by the flu.

University of Sydney infectious disease specialist Professor Robert Booy said many vaccines had happy unintended consequences, including the hepatitis B vaccine which protects against liver cancer and the measles vaccine which protects against related complications including dysentery, bacterial pneumonia and malnutrition.

The HPV vaccine protects against cancer of the mouth and throat.

"I also have a theory that it might even be an indirect protection," Professor Booy said.

"The baby of a mother who is vaccinated may benefit because it maybe you acquire the virus at birth through the birth process, and it's certainly feasible to think that babies are prevented from acquiring an infection that in future may cause cancer of the mouth and throat."

Professor MacIntyre's research was a meta-analysis of 16 studies that explore the link between influenza and heart attacks and it is published in the BMJ's Heart journal.

One of the studies included in the meta-analysis, also conducted by Professor MacIntyre, was funded by GlaxoSmithKline which manufactures a flu vaccine.