Study leaders Tim Clayton and Tom Meade of the London School of Hygiene & Tropical Medicine's Medical Statistics Unit did a clinical case-control study using a general practice database that contains details of data on some 2 million patients registered with some 500 primary care physicians.

The study, published in the European Heart Journal, found a doubling of risk of both heart attack and stroke in the week following respiratory infection, however, the risk lessened over time. There was little excess risk beyond one month, the study said.

Risk did not depend on age or gender, but there was also some evidence of an association between recent urinary tract infection and subsequent heart attack or stroke.

"We recommend that anyone with heart disease has the flu jab (flu shot)," Dr. Mike Knapton, of the British Heart Foundation, which funded the study, said in a study.

"Influenza is a serious infection, particularly in patients with heart disease such as heart failure, and it could even trigger a heart attack."