With flu season underway, many people will choose to be vaccinated, in the hopes of warding off a debilitating bout of fever, sneezes, coughs and aches.
But a new British study suggests they may end up getting a bonus protection they hadn't even considered: a drop in their short-term risk for stroke.
According to a team of scientists from the University of Lincoln, the flu shot seems associated with a reduced risk for stroke by about one-fifth over two months. However, the study did not prove that the flu vaccine causes a drop in stroke risk.
Exactly why there seems to be an association between the two remains unclear.
Study co-author Niro Siriwardena said that evidence of the flu vaccine's impact on stroke risk does not come as a complete surprise, since earlier research has shown a similar link.
"The main surprise about the finding," he said, "is that the flu vaccination is given every year to prevent respiratory complications of flu like pneumonia, rather than to prevent stroke."
Siriwardena, a professor of primary and pre-hospital health care with the Community and Health Research Unit at the University of Lincoln, and his team reported the finding recently in the journal Vaccine.
The U.S. Centers for Disease Control and Prevention estimates the number of flu-related deaths at between 3,000 and 49,000 each year. An estimated 80 percent to 90 percent of those occur among people 65 and older.
To explore the possible association between the flu vaccine and stroke risk, the investigators pored over the health records of nearly 18,000 adult patients in the United Kingdom who had experienced a stroke sometime between 2001 and 2009.
All the patients had received a flu vaccine. But because the vaccine has a maximum effectiveness of just six months, the study team was able to compare the number of strokes occurring within the 180 days following inoculation to the number of strokes that occurred after 180 days.
The team observed an immediate 55 percent plunge in stroke risk over the first three days following vaccination. And though stroke risk rose gradually over the following days, it remained 36 percent lower between days four and seven; 30 percent lower between days eight and 14; 24 percent lower between days 15 and 28, and 17 percent lower between days 29 and 59.
What's more, patients who were vaccinated relatively early in the flu season -- between Sept. 1 and Nov. 15 -- garnered the most benefit in terms of stroke risk reduction, the researchers said.
The upshot: Those for whom the flu shot is recommended should get it, and get it sooner rather than later. Since a move toward "universal vaccination" in 2010, that basically means all Americans over the age of 6 months are recommended to get the shot.
Siriwardena noted that it remains to be seen whether the benefit of the flu vaccine might be lesser or greater, depending on a patient's particular risk profile for stroke.
Dr. Gregg Fonarow, a professor of cardiology at the University of California, Los Angeles, said that "while those at higher risk for stroke may derive the greatest benefit in terms of stroke risk reduction, given the devastating impact that strokes can have, even modest reductions in stroke risk [are] worthwhile."
