For patients with ischemic heart disease, a flu shot may be one way to lower the risk of a heart attack.

A study published online Aug. 21 in the Heart found that undiagnosed influenza was present in about 10 percent of hospitalized patients and that patients who received flu shots had a 45 percent lower risk of an acute MI (AMI).

While the vaccine did have a protective effect against AMI, the study did not find influenza to be a predictor of all-cause mortality and cardiovascular and respiratory hospitalizations as other research has suggested.

“Studies show an increase in rates of AMI and death during the annual influenza season,” wrote the authors, led by C. Raina MacIntyre, PhD, of the University of New South Wales in Sydney, Australia.

The study included 559 participants 40 years of age and older from Sydney. There were 275 case subjects who all had an AMI and were hospitalized during flu season between 2008 and 2010. The 284 controls were orthopedic or ophthalmic clinic outpatients treated during the same time period.

Of the controls, 19 tested positive for influenza infection compared with 34 cases. A significantly higher number of controls were vaccinated in the year the participants were recruited (184 controls vs. 92 cases).

In addition to the presence of influenza, the researchers also determined the presence of a self-reported acute respiratory tract infection (ARTI), which prior research had also shown to be associated with a risk of adverse outcomes.

More case subjects reported a baseline ARTI (24.8 percent vs. 18.6 percent of controls). After multivariate adjustment, ARTI, like influenza, was not a significant predictor of AMI. As single variables, however, both influenza and ARTI doubled the risk of MI.

The fact that about 10 percent of hospitalized patients had undiagnosed influenza, the authors said, indicates it is often missed in patients with other comorbidities. 

Combined with the finding that vaccination may be protective against AMI, they argued that their study should raise awareness among physicians.

“The potential population health impact of influenza vaccination, particularly in the age group 50-64 years, who are at risk for AMI but not targeted for vaccination, should be further explored,” they wrote. “Our data should inform vaccination policy and cardiologists should be aware of missed opportunities to vaccinate individuals with ischemic heart disease against influenza.”