Study Claims Flu Shots Associated With Lower Risk of Heart Attack
Experts dispute statistical result, but note the finding falls in line with previous research
MONDAY, Sept. 20, 2010 (HealthDay News) -- Annual flu shots may protect against heart attacks, and the earlier in the season a shot is given the greater the protection, a new British study claims.
"The risk of getting a heart attack was reduced by 19 percent for those who had a vaccination in the past year," said A. Niroshan Siriwardena, a professor of primary and prehospital health care at the University of Lincoln.
The study, which drew some criticism from U.S. experts, was based on a review of the medical records of more than 78,000 people in England and Wales; roughly 16,000 of them were heart attack patients, and almost 8,500 of the heart attack patients had been vaccinated.
Dr. Kirk Garratt, associate director of the division of cardiac intervention at Lenox Hill Hospital in New York City, said the study found there were 19 percent fewer heart attack patients vaccinated in the previous year, not that there was a 19 percent reduction in heart attacks among the vaccinated.
If getting a flu shot could prevent 19 percent of heart attacks, it would have been noticed before now, Garratt added.
Another expert, Dr. Harlan Krumholz, director of the Yale-New Haven Hospital Center for Outcomes Research and Evaluation at Yale Medical School, cited similar flaws in the finding.
Referring to previous research about heart attacks and flu vaccine, Krumholz said it was already believed that a connection existed between vaccination and heart attack prevention. But the study's claim of a 19 percent reduction in heart attacks is not justified by the data, he added.
When asked about the criticism, Siriwardena said the study looked at "the likelihood of heart attack in vaccinated and unvaccinated patients, adjusting for other risk factors for heart attack."
"We expressed our results as risk of heart attacks," Siriwardena added, rather than vaccination rates among those who did or did not have a heart attack.
But Garratt said that statement was incorrect.
"This study did not measure risk of heart attack in vaccinated and non-vaccinated people. It measured rates of vaccination among heart attack patients and those without heart attack," said Garratt, noting it was a useful way to look for a connection between heart attacks and flu shots, but not to draw the conclusion drawn by the author.
Siriwardena stressed, however, that, "I think it is important again to say again that we found an association rather than proving cause-and-effect. We also found a greater association between reduction of heart attack and early vaccination."
Krumholz, who is also a cardiology professor at Yale, said that the U.S. Centers for Disease Control and Prevention is recommending everyone be vaccinated in any event.
"The study is timely because we're trying to encourage people to get vaccinated," said Krumholz. "The findings reinforce the recent clear evidence that vaccinations are beneficial."
The American Heart Association and the Association of American Cardiologists both recommend flu shots for people who have heart disease or have a high risk of developing cardiovascular problems, Krumholz added.
The main theory about the benefits of flu shots, Garratt said, is that an infection can trigger a rupture of plaque inside arteries, causing a heart attack. Plaques are hardened fats and other substances that can build up on artery walls and cause blockages.
Flu also raises inflammation levels in the body, possibly setting the stage for a heart attack, Krumholz noted.
For the study, published in the September issue of the Canadian Medical Association Journal, information was taken from a medical record database covering about 5 percent of the population in England and Wales. Both study cases (heart patients) and controls were at least 40 and had at least five-and-half years of medical records before the study began.
The study matched about 16,000 heart attack patients, or cases, to about 63,000 controls by age, sex and other data that could affect the results. Each heart attack case had about four controls. The data was then analyzed after adjusting for other factors such as having additional cardiovascular risks, and how often the subject visited the doctor.
The study further looked at whether vaccinations helped prevent pneumococcal pneumonia, but found no protective benefit.
Krumholz said that was not surprising because pneumococcal infections are rare, compared to flu infections, and any impact of vaccination would be "diluted" by the small numbers.
For more on flu shots for those with heart disease, go to American Heart Association.