The risk of heart attack increases sharply after a respiratory infection, a new study finds.

But the absolute risk that any one episode will cause a heart attack is low, the Australian researchers added.

The researchers looked at 578 people who suffered a heart attack and found that 17 percent had experienced symptoms of respiratory infection within seven days before the heart attack, and 21 percent within the prior month.

The risk of a heart attack is 17 times higher in the week after a respiratory infection, the University of Sydney team concluded.

In a second analysis, the researchers focused on upper-respiratory tract infections, such as the common cold, sore throat, hay fever and sinus infections.

"For those participants who reported milder upper-respiratory tract infection symptoms, the risk increase was less, but was still elevated by 13-fold," study author Lorcan Ruane said in a university news release.

"Although upper-respiratory infections are less severe, they are far more common than lower-respiratory tract symptoms. Therefore, it is important to understand their relationship to the risk of heart attacks," he explained.

The study was published May 15 in the Internal Medicine Journal.

According to study senior author Dr. Geoffrey Tofler, "Possible reasons for why respiratory infection may trigger a heart attack include an increased tendency towards blood clotting, inflammation and toxins damaging blood vessels, and changes in blood flow." Tofler is a cardiologist from the University of Sydney, Royal North Shore Hospital and Heart Research Australia.

"Our message to people is while the absolute risk that any one episode will trigger a heart attack is low, they need to be aware that a respiratory infection could lead to a coronary event. So consider preventative strategies [such as flu and pneumonia vaccines] where possible, and don't ignore symptoms that could indicate a heart attack," he advised.

"The next step is to identify treatment strategies to decrease this risk of heart attack, particularly in individuals who may have increased susceptibility," Tofler concluded.

More information

The U.S. National Heart, Lung, and Blood Institute has more on heart attack prevention.