Dr. Jacob Udell

'If we had a pill that could give that kind of heart benefits that flu vaccine seems to then in would be in the water,' says Dr. Jacob Udell. (Women's College Hospital/Canadian Press)

Flu vaccines may help reduce the risk of being hospitalized for heart attacks, strokes or unstable angina, particularly in heart patients, a Canadian cardiologist finds.

The study in Tuesday's Journal of the American Medical Association reviewed six clinical trials on heart health in people with an average age of 67. A third of the subjects had heart disease.  

Among the 3,238 patients treated with flu vaccine, 95 patients (2.9 per cent) died from cardiac diseases or were hospitalized for major cardiovascular events such as a heart attack, stroke, heart failure or fluid on the lungs.

In comparison, 151 of 3,231 patients (4.7 per cent) treated with placebo developed a major cardiovascular event within one year of followup.

"You give 58 people the flu shot, you protect one person from a major adverse cardiac event," said Dr. Jacob Udell, a cardiologist at Women’s College Hospital, who led the research.
 
"In the high-risk patients, those with a recent heart attack, the number needed to treat is eight. Eight people and I prevent a major cardiac event. That's pretty robust. If we had a pill that could give that kind of benefit, that stuff would be in the water."

Dr. Janet McElhaney is the medical lead for seniors care at Health Sciences North in Sudbury, Ont., where she's also reviewed studies on the effectiveness of flu vaccines.

"I think one of the really important concerns for me as a geriatrician is we're now talking about vaccine-preventable disability, not just preventing your risk of dying or having a heart attack, but really an impact on your overall health," McElhaney said.

The researchers speculate a series of steps are involved in how flu vaccines may offer protection to arteries.
 
First, flu infections lead to a major response by our immune systems, which includes inflammation that could trigger heart attacks.  
 
"We know from other research that inflammation plays a critical role in why plaque and buildup in coronary arteries and stroke in brain arteries eventually disrupts," Udell explained. "Perhaps … the flu and the resulting inflammation could trigger those heart attacks and so perhaps this vaccine would prevent those events from occurring" to prevent heart attacks.

Vaccinating gives a targeted immune response to the flu virus that may limit the amount of inflammation induced, McElhaney added.

Boosting vaccination rates

The researchers are organizing a clinical trial that will follow heart patients for up to 12 months after receiving the flu shot to try to confirm the safety and effectiveness of the influenza vaccine for reducing the risk of heart attack or stroke in people with heart disease.
 
The estimate for preventing cardiovascular events is plausible and would represent a significant public health benefit of flu vaccination, Dr. Kathleen Neuzil PATH's Vaccine Access and Delivery program in Seattle, said in an editorial accompanying the study.
 
Given that influenza sickens older adults with and without high-risk conditions and estimates of flu vaccine effectiveness in middle-aged and older adults varies between 40 per cent and 70 per cent, use of the vaccine is warranted, Neuzil said.  
Currently, less than half of people younger than 65 in North America with high-risk conditions like cardiovascular disease get the annual flu shot, studies suggest.
 
Udell's work was funded by the Canadian Institutes for Health Research and Canadian Foundation for Women's Health.

With files from CBC's Pauline Dakin