The patients also faced a higher risk shortly after coming down with other respiratory infections, the findings showed.
Philippa Hobson, senior cardiac nurse at the British Heart Foundation, commented on the findings: "There is evidence that heart attacks happen more often during or immediately after an acute inflammatory illness, such as flu".
Older adults, people who have never had a heart attack, and patients infected with the influenza B virus are especially at risk, the researchers found.
"We know that influenza vaccines aren't 100% effective", he added.
"What we were also surprised about is that we found that there was an increased risk with other respiratory viruses as well", said Kwong, a scientist at Public Health Ontario and the Institute for Clinical Evaluative Sciences in Toronto.
The heart attack rate was 20.0 admissions per week during the seven days after diagnosis of the flu, versus 3.3 per week during the 52 weeks before and 51 weeks after that seven-day window. "All of these can increase the chance of having a heart attack", Dr Kwong said.
Even so, people aged 65-plus - as well as those with underlying medical conditions or compromised immune systems - are urged to get the flu shot as they are more susceptible to complications if they do come down with the infection.
The study in the New England Journal of Medicine found a person is six times more likely to suffer a heart attack during the week after being diagnosed with the flu compared to a year before or after getting sick.
The researchers cautioned that the people in their study were not suffering from mild flu symptoms.
Kwong explains that infections cause inflammation, which can lead to damage of the inner lining of the blood vessels that serve the heart and can increase the chances of blood clots forming there.
In recent days, Udell has seen five confirmed cases of influenza causing a heart attack or heart failure, and another five patients whom he suspects also have the flu.
It comes amid an outbreak of strains known as Aussie flu and Japanese flu.
Flu rates this year are at their worst in seven years and official figures released today will confirm whether the number of cases is continuing to increase. It's safe, you take it just one time and it has minimal side-effects. Out of that group, 332 were hospitalized for a heart attack within a year.
"If someone does have the flu, unfortunately there is no cure, but patients can assist their own recovery through taking plenty of rest and drinking lots of fluids as it is easy to become dehydrated".
The findings have reinforced the need for those at high risk to be immunised.
Dr. Kwong also stressed the importance of staying home when sick and regularly washing hands to avoid spreading the flu.
People who are at risk for heart disease can include those with high blood pressure, high cholesterol levels, diabetes or obesity, as well as those who smoke, have a family history of heart disease or are age 65 or older, according to the American Heart Association (AHA).