DALLAS, Feb. 1 – The flu vaccine may offer significant protection against stroke, especially for people age 75 or younger, French researchers report in the February issue of Stroke: Journal of the American Heart Association.
This is the first study to look at the influenza vaccine’s influence on stroke. Prior research has shown that infections are associated with stroke and heart attack, possibly because they destabilize atherosclerotic plaque and cause clots in arteries that supply blood to the brain and heart. Clots blocking blood flow to the brain can cause a stroke. If an artery to the heart is blocked, it causes a heart attack.
“Since subjects who have viral influenza can develop subsequent infections, we thought that flu vaccination may mean less infection overall and, therefore, less stroke,” says Pierre Amarenco, M.D., one of the study’s authors. “We found the reduction in stroke risk to be around 40 percent for those who were vaccinated, which would be a major advance in stroke prevention if further studies confirm these results.” Amarenco is a professor of neurology at Denis Diderot University and chairman of the department of neurology and stroke center at Bichat Hospital in Paris.
Researchers studied 270 people. They interviewed 90 patients age 60 and older who were admitted to a stroke clinic between December 1998 and March 1999 and between January and March 2000. For each stroke survivor, researchers also interviewed two age-, sex- and geographically-matched control subjects (180 total).
The researchers asked patients and controls whether they had been vaccinated against influenza during the most recent vaccination drive (which is October in France) and if they had been vaccinated every year for the previous five years. The subjects were asked whether they had been treated with antibiotics in the previous 3 months and if they suffered from chronic bronchitis.
Researchers also gathered participant data about risk factors for stroke including age, gender, hypertension, diabetes mellitus, high cholesterol, current smoking and body mass index. They asked subjects if they had taken at least one blood cholesterol test, if they had a history of stroke or heart disease, and about their occupations (to divide them into white- and blue-collar categories).
The researchers found that 59.4 percent of control subjects and 46.7 percent of stroke survivors had been vaccinated during the last vaccination period. Furthermore, 56.1 percent of controls and 41.1 percent of stroke survivors had been vaccinated for the previous five years. After adjusting for age, traditional risk factors and recent use of antibiotics, the researchers found that subjects who were vaccinated during the year of the study, and those vaccinated during the last five years had a lower stroke risk. Similar associations were observed for people in both groups who did not have a history of stroke or heart attack.
Researchers also found that people older than 75 were less likely to get this benefit from a flu vaccine. This held true for those vaccinated recently, as well as for those who had been vaccinated every year for the past five years. They concluded that this could be because with increasing age, the influence of the traditional risk factors, such as hypertension, is too great to detect a significant direct effect of influenza vaccination. They found a very significant interaction between age and hypertension, and age and history of stroke.
“These results support the view that infections are important risk factors for stroke as well as heart attack,” says Amarenco.
The authors suggest that influenza vaccination may help prevent stroke and other infection by an all-over stimulation of the immune system. They say further studies should explore whether other vaccinations similarly protect against stroke. They also questioned whether the association they observed between protection from stroke and flu vaccination was due to a direct effect of the vaccination or to the possibility that people who received the vaccine lived healthier lifestyles. This, they say, can be addressed only in a prospective randomized trial.
“We conclude that influenza vaccination may protect against stroke by reducing the occurrences of infections. However, we cannot exclude that vaccinated patients may have better lifestyles and, therefore, are less likely to have a stroke in the first place,” he says.
Amarenco and his colleagues are increasing the size of the study to 1,000 patients and 2,000 controls. They hope to confirm the role of influenza vaccination by doing a randomized control trial.
The information obtained in the study opens new avenues for stroke prevention, the authors say.
Each year, about 600,000 people suffer a new or recurrent stroke, according to the American Stroke Association. Stroke can affect people’s neurological functions, including speech, vision and coordination.
Philippa Lavallée, M.D., is the study’s lead author. Co-authors are Véronique Perchaud, M.D., Marion Gautier-Bertrand; and David Grabli, M.D.
Materials provided by American Heart Association. Note: Content may be edited for style and length.
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