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Modern Medicine Image Gallery Studies show anywhere from a 25 percent to a 50 percent decrease in mortality rates for heart patients who get the flu vaccine. See more modern medicine pictures.

Photo courtesy CDC

For years, studies have drawn a link between influenza and heart disease. As early as 2003, a study published in the "New England Journal of Medicine" involving people with heart problems -- either prior heart attacks, angioplasty or stent placement (to open arteries) -- found that those who got the flu shot were 20 percent less likely to end up in the hospital with a heart-related problem and 50 percent less likely to die in the year following vaccination. The results are pretty amazing, but also fairly logical: Any sort of viral infection, which includes the flu, makes the heart work harder, and people who have had heart attacks or surgeries to prevent them have weakened hearts to begin with.

But that's not all these studies are reporting. Researchers are finding that the increase in survival rate isn't only related to preventing the flu; they believe the flu vaccine (the shot, not the nasal spray, which is not approved for heart patients) may have additional effects that actually protect heart patients from having a major heart-related incident in the year following vaccination. While details are spotty, the implication here is that the people who did not receive the vaccine and did die of heart disease did not necessarily have the flu.

The studies show fairly consistent results, although it should be noted that these are observational studies, not laboratory studies. This means researchers have less control over the conditions. Still, at least four studies since 2003, the most recent one conducted in Poland in 2006, show that heart patients who receive the flu vaccine are hospitalized less and have lower fatality rates in the year following vaccination than heart patients who do not receive the vaccine. The data shows anywhere from a 25 to 50 percent decrease in mortality and hospitalization rates from heart-related events for vaccinated patients. One study found that heart-attack deaths rise in flu season and fall in the off-season like clockwork.

The most obvious explanation for the findings is that the flu is really, really dangerous for people with heart problems. It follows that preventing flu-related complications like lung infections, which decrease oxygen intake and make the heart pump harder to get oxygen to all the parts of the body, would decrease heart attacks in people who have weakened hearts. Plus, when the immune system is actively fighting off a spreading infection like the flu, one thing it does is release chemicals that cause affected tissue to become inflamed (see How the Immune System Works to learn more). This inflammation affects blood clotting and vascular efficiency, which can land heart patients in the hospital or the morgue. The relationship between the flu and heart complications is well-documented.

What's less well-understood is why the flu vaccine seems to protect heart patients beyond the obvious "prevent the flu, prevent heart complications" angle. Researchers have set forth a few hypotheses on why the vaccine may have this protective effect. They mostly center around the immune system. One possibility is that any activation of the immune system (as occurs with vaccination) helps heart patients more fully recover from the effects of a heart attack or heart surgery. In this case, with a more complete recovery, another heart attack or heart-related hospitalization would be less likely. The other main discussion has to do with inflammation, which is closely linked to heart attacks, as mentioned above. It could be that the vaccine's triggering of the immune system helps it to more easily fight off all sorts of infections, and infections make it more likely that someone with a heart problem will be hospitalized or die. By strengthening the immune system's ability to fight infection before it really takes hold, the vaccine ends up preventing the immune system's inflammatory response.

The lesson here is this: If you have a history of heart disease, you should get the flu shot every year unless your cardiologist tells you otherwise. The U.S. government is aiming to get 90 percent of heart patients over the age of 65 vaccinated by 2010. Right now, the number is somewhere around 70 percent, and it's even lower for younger people with cardiovascular disease. Estimates are that hundreds or even thousands of heart-disease-related deaths could be prevented every year if the 90 percent vaccination mark were reached.

For more information on the flu, heart disease and related topics, see the next page.