In middle-aged people with narrowed arteries, a flu jab appears to halve the heart attack risk, say researchers.
The researchers wanted to find out if flu is an unrecognised, but clinically important, contributing factor to increased heart attack risk.
Scientists assessed 559 patients over the age of 40 who were referred to a tertiary hospital during consecutive winters in 2008-10. Some 275 of these patients had sustained a heart attack and 284 had not.
Nose and throat swabs and blood samples taken at admission and 4-6 weeks later showed that around one in eight (12.4 percent; 34) of the heart attack patients had recently had flu, compared with just under 7 percent (18) in the comparison group. Half of all the patients had had the flu jab that year.
Flu had not been diagnosed in around one in 10 of those who had the infection, indicating that it may be missed in hospital patients with other clinical problems, say the authors.
A recent respiratory infection was more common among those patients who'd had a heart attack and doubled the risk.
But after taking account of other influential factors, such as age, high cholesterol, and smoking, flu did not increase heart attack risk. But vaccination against the infection did seem to be protective, decreasing the risk of a heart attack by 45 percent.
The researchers said that extending the flu vaccination programme to 50 to 64 year olds has been mooted before, but not considered to be cost effective.
The study has been published in the journal Heart.