A new study just published in the Canadian Medical Association Journal concluded that influenza immunization reduces the risk of heart attack (acute myocardial infarction) by 19% in people over the age of 40. The study covered seven flu seasons, from 2001 to 2007, and included 78,706 patients living in England and Wales, a huge population that was possible to study because of the national healthcare system database in the United Kingdom.
The main conclusion is not very surprising; other studies have shown similar effects. Influenza attacks the lungs and makes your heart work much harder to pump your blood through the congested tissue. If you are going to have a heart attack, it is more likely you will have it when your lungs are damaged from influenza. Conversely, protection from lung infection would remove this risk factor for heart attack. This is why people with cardiac diaease have always been considered a high-risk group that should get their flu shots every year.
In digging deeper into the results, you discover that the authors found a few other facts that I think are important but the popular press will probably miss. The study found that timing was important. People who got their immunization early in the season (from September to mid-November) were more protected from heart attacks (21% versus 12%) than people who got their vaccine later (from mid-November on). Flu is always around before the official flu season starts, and it seems like it is better to be protected early than to wait until the epidemic starts.
Another little fact reported by the authors was that you were not protected against heart attack if you were immunized the previous year but not the current year. Only the current vaccine protected you for the current flu season. You had to be immunized every year to be protected.
The authors did not extend the study to a financial analysis, but it seems clear to me that preventing a very expensive thing like a heart attack makes the flu shot even more cost effective.
This large population-based study reinforces things that we have been saying about influenza prevention: flu shots are cost-effective and cost-avoiding, get immunized every year, and get immunized early.